Identify the correct formula to calculate rate of reaction: [UHS 2024]
A
Change in concentration of substance ÷ Time taken for the change
B
Time taken for the change ÷ Change in concentration of substance
C
Time taken for the change ÷ Change in concentration of substance \( \times 100 \)
D
Time taken for the change \( \times \) Change in concentration of substance
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The rate of a chemical reaction quantifies the speed at which reactants are consumed or products are formed.
Formula:$$ \text{Rate} = \pm \frac{\Delta C}{\Delta t} $$
Solution:- Mathematically, a rate is always a change in a measurable quantity divided by the time it took for that change to occur.
- In chemistry, the quantity is the concentration of a substance.
- Therefore, the correct verbal formula is: Change in concentration of substance ÷ Time taken for the change.
Why other options are incorrect:Dividing time by concentration gives an inverted unit (s dm\(^3\) mol\(^{-1}\)). Multiplying them yields a nonsensical unit of concentration-time.
Consider the hypothetical equation:
\( a\text{A} + b\text{B} \longrightarrow c\text{C} + d\text{D} \)
Which of the following represents correct rate equation? [UHS 2024]
A
Rate = \( k[\text{A}][\text{B}] \)
B
Rate = \( k[\text{A}]^a[\text{B}]^b \)
C
Rate = \( k[\text{A}]^a \)
D
Rate = \( k[\text{B}]^2 \)
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:According to the classical Law of Mass Action (often assumed for theoretical elementary reactions unless experimental data dictates otherwise), the rate is proportional to the product of active masses of reactants raised to their stoichiometric coefficients.
Formula:$$ \text{Rate} = k[\text{Reactant 1}]^{\text{coef}_1}[\text{Reactant 2}]^{\text{coef}_2} $$
Solution:- The reactants are \( \text{A} \) and \( \text{B} \).
- Their respective stoichiometric coefficients in the balanced theoretical equation are \( a \) and \( b \).
- Assuming an elementary reaction profile, the coefficients become the exponents in the theoretical rate law.
- Thus, the theoretically represented rate equation is: \( \text{Rate} = k[\text{A}]^a[\text{B}]^b \).
Why other options are incorrect:Option A assumes all coefficients are 1. Options C and D ignore one of the reactants entirely.
If a reaction rate does not change with concentration then it is? [UHS 2024]
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Correct Key: Option D
Diagnostic Explanation
Concept:The mathematical dependence of the reaction speed on reactant concentration defines its kinetic order.
Formula:$$ \text{Rate} = k[\text{Reactant}]^{0} = k(1) = k $$
Solution:- When a reaction rate is entirely independent of the reactant concentration, it means changing the concentration has zero effect on the speed.
- Mathematically, this corresponds to the concentration term being raised to the power of zero.
- Because anything raised to the power of zero is 1, the rate stays locked at the constant value \( k \).
- This is the absolute definition of a zero order reaction.
Why other options are incorrect:In 1st, 2nd, and 3rd order reactions, the rate directly scales (linearly, quadratically, or cubically) with changes in concentration.
Which of the following is CORRECT Arrhenius equation? [UHS 2024]
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Correct Key: Option B
Diagnostic Explanation
Concept:The Arrhenius equation mathematically models how the rate constant of a chemical reaction varies with absolute temperature and activation energy.
Formula:$$ k = A e^{-E_a / RT} $$
Solution:- \( k \) is the rate constant.
- \( A \) is the Arrhenius pre-exponential factor (frequency of collisions).
- \( E_a \) is the activation energy.
- \( e \) is the mathematical base of natural logarithms.
- \( R \) is the universal gas constant.
- \( T \) is the absolute temperature in Kelvin.
- Therefore, the correct constant in the denominator of the exponent is \( R \).
Why other options are incorrect:The variables \( Q, S, \) and \( U \) do not represent the universal gas constant in standard thermodynamic or kinetic formulas.
Which of the following is the unit of rate of reaction? [SZABMU 2024]
A
\( \text{mol}^{-1}\text{dm}^3\text{s}^{-1} \)
B
\( \text{mol}\cdot\text{dm}^3\text{s}^{-1} \)
C
\( \text{mol}\cdot\text{dm}^{-3}\text{s} \)
D
\( \text{mol}\cdot\text{dm}^{-3}\text{s}^{-1} \)
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The rate of a reaction is defined as the change in concentration over a specific period of time.
Formula:$$ \text{Rate} = \frac{\text{Concentration}}{\text{Time}} $$
Solution:- The standard SI-derived unit for chemical concentration is moles per cubic decimeter (\( \text{mol dm}^{-3} \)).
- The standard unit for time is seconds (\( \text{s} \)).
- Dividing the two yields: \( \frac{\text{mol dm}^{-3}}{\text{s}} \).
- Using exponent rules to bring seconds to the numerator gives: \( \text{mol dm}^{-3} \text{s}^{-1} \), which is structurally represented in option D.
Why other options are incorrect:Option B represents \( \text{mol dm}^{3} \text{s}^{-1} \) (wrong sign on dm). Option A inverted the mole unit. Option C puts time in the numerator (\( \text{s} \) instead of \( \text{s}^{-1} \)).
Consider a reaction of A into B, if K value is \( 3 \times 10^{-12} \) at 200°C then what will be the value of K at 250°C? [SZABMU 2024]
A
\( 9 \times 10^{-3} \text{ s}^{-1} \)
B
\( 12 \times 10^{-3} \text{ s}^{-1} \)
C
\( 6 \times 10^{-12} \text{ s}^{-1} \)
D
\( 15 \times 10^{-12} \text{ s}^{-1} \)
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:Standard kinetic theory states that a rate constant typically doubles for every 10°C rise. However, examiners sometimes mistakenly apply a simplistic linear multiplier instead of an exponential one.
Solution:- Scientific Reality: A 50°C increase equals five 10-degree intervals. The rate should increase by \( 2^5 = 32 \) times, yielding \( 96 \times 10^{-12} \). This is not an option.
- Exam Logic (Flawed but Required): The paper setter incorrectly assumed a linear proportionality.
- Temperature difference: \( 250^\circ\text{C} - 200^\circ\text{C} = 50^\circ\text{C} \).
- Number of 10-degree intervals: \( 50 / 10 = 5 \).
- The setter simply multiplied the base rate constant by the number of intervals: \( 5 \times (3 \times 10^{-12}) = 15 \times 10^{-12} \).
- Therefore, following the examiner's flawed mathematical intent, the answer is D.
Why other options are incorrect:While chemically inaccurate, options A, B, and C do not even align with the linear miscalculation model utilized by the examiner to generate the official key.
If half-life of a chemical reaction is 30 minutes, how much time is required for its 87.5% completion? [SZABMU 2024]
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Correct Key: Option C
Diagnostic Explanation
Concept:For a first-order reaction, the percentage of reactant remaining can be calculated by halving the concentration successively over consecutive half-lives.
Solution:- If 87.5% of the reaction is completed, the amount of reactant remaining is: \( 100\% - 87.5\% = 12.5\% \).
- Let's trace the decay through half-lives (\( t_{1/2} = 30 \text{ min} \)):
- After 1st half-life: 100% falls to 50% (30 mins elapsed).
- After 2nd half-life: 50% falls to 25% (60 mins elapsed).
- After 3rd half-life: 25% falls to 12.5% (90 mins elapsed).
- It takes exactly 3 half-lives to reach 12.5% remaining (87.5% complete).
- Total time = \( 3 \times 30 \text{ minutes} = 90 \text{ minutes} \).
Why other options are incorrect:60 minutes equals two half-lives (75% completion). 120 minutes equals four half-lives (93.75% completion).
The study of rates of chemical reactions and the factors that affect the rates of chemical reactions is known as: [SZABMU 2024]
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Correct Key: Option D
Diagnostic Explanation
Concept:Physical chemistry is partitioned into domains. The domain tracking the variable of
time and reaction speed is chemical kinetics.
Solution:- Chemical kinetics explicitly deals with measuring the rate (velocity) of chemical reactions.
- It also investigates the mechanisms by which reactions occur and how external variables (temperature, pressure, concentration, and catalysts) influence that rate.
Why other options are incorrect:Thermodynamics studies energy changes (enthalpy, entropy) but not the factor of time. Stoichiometry deals with mass conservation and balancing. Electrochemistry deals with electricity and redox.
Decomposition of ammonia on heated tungsten is an example of: [SZABMU-RC 2024]
A
Pseudo first order reaction
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:Reactions catalyzed by solid surfaces often exhibit unique kinetics at high concentrations when the surface becomes completely saturated with reactant molecules.
Formula:$$ 2\text{NH}_{3(g)} \xrightarrow{\text{W (Tungsten)}} \text{N}_{2(g)} + 3\text{H}_{2(g)} $$
Solution:- When ammonia decomposes on a solid tungsten catalyst, it must first adsorb onto the tungsten surface.
- At moderate to high pressures, the entire active surface of the tungsten is completely covered by ammonia molecules.
- Adding more ammonia gas cannot increase the rate because there are no available binding sites on the catalyst.
- Because the rate becomes completely independent of the gaseous ammonia concentration, it acts as a classic zero-order reaction.
Why other options are incorrect:At extremely low pressures (where the surface is mostly empty), it acts as first order, but standard exam context implies the saturated zero-order condition, which is a textbook hallmark.
For a first order reaction A \(\longrightarrow\) B, the rate constant is \( 0.0458 \text{ s}^{-1} \). Calculate rate of the reaction if the concentration of reactant is \( 0.35 \text{ mol dm}^{-3} \): [ETEA 2024]
A
\( 0.012 \text{ mol dm}^{-3} \text{ s}^{-1} \)
B
\( 0.014 \text{ mol dm}^{-3} \text{ s}^{-1} \)
C
\( 0.016 \text{ mol dm}^{-3} \text{ s}^{-1} \)
D
\( 0.018 \text{ mol dm}^{-3} \text{ s}^{-1} \)
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The rate of a first-order reaction is calculated by multiplying the rate constant by the reactant's instantaneous concentration.
Formula:$$ \text{Rate} = k[\text{A}]^1 $$
Solution:- We are given the rate constant, \( k = 0.0458 \text{ s}^{-1} \).
- We are given the reactant concentration, \( [\text{A}] = 0.35 \text{ mol dm}^{-3} \).
- Substitute the values into the rate law:
- $$ \text{Rate} = (0.0458) \times (0.35) $$
- $$ \text{Rate} = 0.01603 \text{ mol dm}^{-3} \text{ s}^{-1} $$
- Rounding to proper significant figures gives \( 0.016 \).
Why other options are incorrect:The other options are mathematically incorrect products of the basic multiplication.
A reaction is first order with respect to A and second order with respect to B, the rate equation is: [ETEA 2024]
A
Rate = \( k[\text{A}] \)
B
Rate = \( k[\text{A}][\text{B}] \)
C
Rate = \( k[\text{A}]^2[\text{B}] \)
D
Rate = \( k[\text{A}][\text{B}]^2 \)
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The experimental order of a reaction with respect to a specific reactant becomes the mathematical exponent for that reactant's concentration in the rate law.
Solution:- The problem explicitly states the reaction is "first order with respect to A". This means the concentration of A, \( [\text{A}] \), must have an exponent of 1.
- It states it is "second order with respect to B". This means the concentration of B, \( [\text{B}] \), must have an exponent of 2.
- Combining these into the standard rate equation format yields: Rate = \( k[\text{A}]^1[\text{B}]^2 \).
Why other options are incorrect:Option B represents a reaction that is first order for both A and B. Option C represents a reaction that is second order for A and first order for B (the inverse of the question).
The unit of rate constant for the first order reaction is: [DUHS 2024]
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The unit for the rate constant (\( k \)) adjusts based on the reaction order so that multiplying it by the concentration terms always yields the fixed units of reaction rate (Concentration/time).
Formula:$$ \text{Rate} = k[\text{Concentration}]^{1} $$
Solution:- Note on Source Key: The book's official answer key incorrectly lists D as the answer, but the book's own Explanatory Notes correctly calculate it as option C. We apply the scientifically accurate answer here.
- For a first-order reaction: \( \text{Rate} (\text{M s}^{-1}) = k \times [\text{A}] (\text{M}) \).
- Isolating \( k \): \( k = \frac{\text{M s}^{-1}}{\text{M}} \).
- The Molarity (M) units cancel out entirely, leaving only \( \text{s}^{-1} \).
Why other options are incorrect:Option D (\( \text{M}^{-1}\text{s}^{-1} \)) is the unit for a second-order reaction. Option A represents zero-order. Option B implies a negative first-order reaction.
The formation of hydrogen gas can be increased by reacting which Zn sample with 1M HCl solution? [NUMS 2024]
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Correct Key: Option D
Diagnostic Explanation
Concept:In heterogeneous reactions involving a solid reacting with a liquid, the rate of reaction is heavily dependent on the total exposed surface area of the solid.
Formula:$$ \text{Rate} \propto \text{Surface Area of Solid Reactant} $$
Solution:- All options contain exactly 1 gram of Zinc, so the total mass is constant.
- A solid rod, pellet, or ribbon has a relatively small surface-to-volume ratio, meaning fewer Zinc atoms are exposed to the HCl molecules at any given moment.
- When Zinc is ground into a fine powder, it maximizes the surface area.
- This massive increase in exposed atoms drastically increases the collision frequency between Zn and HCl, massively increasing the rate of hydrogen gas formation.
Why other options are incorrect:Rods, pallets, and ribbons all have significantly less exposed surface area compared to finely divided powder, leading to slower reactions.
The rate of reaction between two specific time intervals is called? [UHS 2023]
A
Average rate of reaction
B
Instantaneous rate of reaction
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:Rates of reaction can be measured over a finite duration or at an exact, specific moment in time.
Formula:$$ \text{Average Rate} = \frac{\Delta [\text{Concentration}]}{\Delta t} = \frac{C_2 - C_1}{t_2 - t_1} $$
Solution:- When the rate is calculated over a macroscopic, finite period of time (between two specific, separated time intervals \( t_1 \) and \( t_2 \)), it is called the average rate of reaction.
- It provides the overall speed of the reaction during that specific window, even if the rate fluctuated during it.
Why other options are incorrect:Instantaneous rate is measured at a single infinitesimal moment (using the derivative \( dC/dt \)). Initial rate is the instantaneous rate specifically at \( t = 0 \).
How will be the rate of reaction, if the slope of the curve is greater near the start of the reaction? [UHS 2023]
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Correct Key: Option B
Diagnostic Explanation
Concept:In a concentration-time graph, the instantaneous rate of the reaction is equal to the mathematical slope of the tangent to the curve at any given point.
Formula:$$ \text{Rate} = |\text{Slope}| = \left| \frac{dC}{dt} \right| $$
Solution:- The steeper (greater) the slope of the curve, the faster the concentration is changing per unit of time.
- Near the start of a reaction, reactant concentrations are at their highest, leading to the maximum number of collisions.
- Therefore, a greater slope directly translates to a greater (faster) rate of reaction.
Why other options are incorrect:A lesser slope means a slower reaction. Constant slope implies a zero-order reaction. Equilibrium occurs when the slope becomes exactly zero (flat horizontal line).
The rate constant "k" in the rate equation is the rate of reaction when the concentration of each of the reactant is: [SZABMU 2023]
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Correct Key: Option B
Diagnostic Explanation
Concept:The specific rate constant is defined mathematically by simplifying the rate law equation.
Formula:$$ \text{Rate} = k[\text{A}]^{a}[\text{B}]^{b} $$
Solution:- If we set the concentration of all participating reactants to unity (exact value of 1 M or \( 1 \text{ mol dm}^{-3} \)), the equation simplifies.
- $$ \text{Rate} = k(1)^{a}(1)^{b} = k(1) = k $$
- Thus, \( k \) is exactly equal to the rate of reaction exclusively when all reactant concentrations are exactly 1 M.
Why other options are incorrect:If concentrations were 2 M, 3 M, or 4 M, the rate would be a multiple of the rate constant depending on the reaction orders, not strictly equal to it.
The rate of reaction: [SZABMU 2023]
A
Remains the same as the reaction proceeds
B
Increases as the reaction proceeds
C
Decreases as the reaction proceeds
D
Remains the same initially and then increases
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:According to the Law of Mass Action, the speed of a chemical reaction is proportional to the concentration of the reacting substances.
Solution:- As a chemical reaction progresses over time, the reactant molecules collide and are converted into products.
- Consequently, the concentration of the reactants continuously decreases.
- Because there are fewer reactant molecules available to collide, the frequency of effective collisions drops, causing the rate of reaction to decrease continuously as it proceeds toward completion or equilibrium.
Why other options are incorrect:The rate only remains the same in a strict zero-order reaction (rare). It does not increase because reactants are being depleted, not generated.
Reaction that follows third order kinetics is: [ETEA 2023]
A
Decomposition of nitrogen dioxide
B
Decomposition of hydrogen iodide
C
Gas phase oxidation of nitric oxide
D
Formation of hydrogen iodide
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The order of a chemical reaction is experimentally determined. Some classic gas-phase reactions are rigorously established in chemical literature as following third-order kinetics.
Formula:$$ 2\text{NO}_{(g)} + \text{O}_{2(g)} \longrightarrow 2\text{NO}_{2(g)} $$
Solution:- The gas-phase oxidation of nitric oxide (NO) by oxygen is a well-known example of a third-order reaction.
- Experimental data yields the rate law: \( \text{Rate} = k[\text{NO}]^2[\text{O}_2]^1 \).
- The sum of the exponents (2 + 1) equals 3, confirming it follows third-order kinetics overall.
Why other options are incorrect:Decomposition of \( \text{HI} \) and formation of \( \text{HI} \) are standard second-order reactions. Decomposition of \( \text{N}_2\text{O}_5 \) is first-order.
As compared to exothermic reaction, formation of activated complex in endothermic reaction requires: [ETEA 2023]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:Activation energy (\( E_a \)) is the energy difference between the reactants and the transition state (activated complex).
Solution:- In an endothermic reaction, the products have a higher potential energy than the reactants, and a large amount of energy must be absorbed to break the strong initial bonds.
- Because the reactants sit at a much lower energy well relative to the transition state, the "energy hill" they must climb is significantly steeper.
- Therefore, endothermic reactions typically require a greater activation energy (\( E_a \)) compared to exothermic reactions, where the reactants are inherently closer to the transition state energy.
Why other options are incorrect:If it required smaller or equal \( E_a \), it would likely not be heavily endothermic. All standard reactions require
some \( E_a \).
What is the order of reaction if the unit of K is sec\(^{-1}\)? [DUHS 2023]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The units of the rate constant (\( k \)) depend strictly on the overall reaction order to balance the rate equation.
Formula:$$ \text{Unit of } k = (\text{mol dm}^{-3})^{1-n} \text{s}^{-1} $$
Solution:- For a first-order reaction (\( n = 1 \)):
- Substitute \( n=1 \) into the formula: \( (\text{mol dm}^{-3})^{1-1} \text{s}^{-1} \).
- This simplifies to \( (\text{mol dm}^{-3})^{0} \text{s}^{-1} \).
- Since any value to the power of 0 is 1, the concentration units completely cancel out.
- The final unit for \( k \) is simply \( \text{s}^{-1} \) (or sec\(^{-1}\)).
Why other options are incorrect:Zero order yields \( \text{mol dm}^{-3}\text{s}^{-1} \). Second order yields \( \text{mol}^{-1}\text{dm}^3\text{s}^{-1} \). Third order yields \( \text{mol}^{-2}\text{dm}^6\text{s}^{-1} \).
Which of the following best describes the ionic reactions of inorganic compounds? [DUHS 2023]
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Correct Key: Option B
Diagnostic Explanation
Concept:The speed of a chemical reaction is heavily dictated by the types of bonds that must be broken and formed.
Solution:- Inorganic ionic compounds completely dissociate into free-moving cations and anions when dissolved in aqueous solutions.
- Because the ions are already formed, there are no strong covalent bonds that need to be broken before a reaction can occur.
- When oppositely charged ions encounter each other, strong electrostatic forces pull them together instantly.
- Consequently, inorganic ionic reactions (like precipitation or neutralization) are typically very fast, often occurring instantaneously.
Why other options are incorrect:Reactions involving complex covalent organic molecules are usually slow or moderate because they require significant bond-breaking energy. Ionic reactions do not.
In which method the rate of reaction involving ions can be studied? [BUMHS 2023]
A
Electrical conductivity method
B
Optical rotation method
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:Kinetic studies require monitoring a physical property that changes predictably as reactants turn into products.
Solution:- Ions in a solution carry electrical current.
- If a chemical reaction produces new ions, consumes existing ions, or changes the mobility of the ions present, the overall electrical conductivity of the solution will change proportionally over time.
- By continuously measuring this change using a conductivity meter, the rate of the reaction can be accurately tracked.
Why other options are incorrect:Optical rotation is for chiral molecules (like sugars). Refractometry is for refractive index changes (typically pure liquids/organics). Spectrometry is for light absorbance (colored species/UV active).
Which order of reaction obeys the expression \( t_{1/2} = \frac{1}{ka} \)? [BUMHS 2023]
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Correct Key: Option C
Diagnostic Explanation
Concept:The mathematical relationship between half-life (\( t_{1/2} \)) and the initial concentration (\( a \)) is unique to each reaction order.
Formula:$$ t_{1/2} \propto \frac{1}{a^{n-1}} $$
Solution:- We are given the expression: \( t_{1/2} = \frac{1}{ka} \).
- This shows that the half-life is inversely proportional to the first power of the initial concentration (\( a^1 \)).
- Using the general proportionality formula, if \( a^{n-1} = a^1 \), then \( n - 1 = 1 \).
- Solving for \( n \) yields \( n = 2 \).
- Therefore, this formula strictly governs a second-order reaction.
Why other options are incorrect:Zero order: \( t_{1/2} = a / 2k \). First order: \( t_{1/2} = 0.693 / k \) (independent of \( a \)). Third order: \( t_{1/2} \propto 1/a^2 \).
When temperature of reacting gases is raised by 10 K, reaction rate will increase to: [NUMS 2023]
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Correct Key: Option C
Diagnostic Explanation
Concept:The temperature coefficient of a reaction dictates how the rate changes with a standard increase in temperature.
Formula:$$ \frac{k_{(T + 10)}}{k_T} \approx 2 \text{ to } 3 $$
Solution:- According to collision theory and the Maxwell-Boltzmann distribution, raising the temperature by 10 K (or 10°C) slightly increases collision frequency but drastically increases the fraction of molecules possessing energy greater than the activation energy.
- For most standard homogeneous reactions, this specific 10 K increase causes the reaction rate to approximately double (increase by a factor of 2).
Why other options are incorrect:While it can sometimes triple in specific high-barrier reactions, 'double' is the standard, universally accepted rule of thumb in foundational kinetics.
The minimum amount of energy required by the colliding particles for effective collision is called: [NUMS 2023]
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Correct Key: Option B
Diagnostic Explanation
Concept:According to collision theory, molecules must collide with sufficient force to disrupt their current electron clouds and break existing bonds.
Solution:- The specific minimum threshold of kinetic energy that colliding molecules must possess to overcome their mutual electron-cloud repulsion and successfully form an activated complex is termed the Activation Energy (\( E_a \)).
- Collisions possessing energy equal to or greater than \( E_a \) are termed 'effective' or 'fruitful' collisions.
Why other options are incorrect:Lattice energy relates to the formation of ionic solids. Bond energy is the energy holding a specific bond together. Hydration energy is released when ions dissolve in water.
When nitric oxide reacts with ozone, the order of reaction will be: [NUMS 2023]
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Correct Key: Option A
Diagnostic Explanation
Concept:The reaction between nitric oxide and ozone in the atmosphere is a fundamental gas-phase reaction whose kinetics are determined experimentally.
Formula:$$ \text{NO}_{(g)} + \text{O}_{3(g)} \longrightarrow \text{NO}_{2(g)} + \text{O}_{2(g)} $$
$$ \text{Rate} = k[\text{NO}][\text{O}_3] $$
Solution:- Experimental observations show that the rate of this reaction is directly proportional to the first power of the concentration of NO and the first power of the concentration of O\(_3\).
- Adding the exponents from the rate law: \( 1 + 1 = 2 \).
- Therefore, the reaction is strictly 2nd order overall.
Why other options are incorrect:It is an elementary bimolecular reaction; hence it does not exhibit 1st, 3rd, or zero-order kinetics.
What is the overall order of this rate equation? Rate = \( k[\text{H}_{2}][\text{NO}_{2}]^{2} \) [UHS 2022]
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:The overall order of a chemical reaction is defined as the mathematical sum of the individual powers (exponents) to which the concentration terms are raised in the rate law.
Formula:$$ \text{Overall Order} = m + n $$
Solution:- Examine the rate law: \( \text{Rate} = k[\text{H}_{2}]^{1}[\text{NO}_{2}]^{2} \).
- The exponent for \( [\text{H}_{2}] \) is 1.
- The exponent for \( [\text{NO}_{2}] \) is 2.
- Summing the exponents gives: \( 1 + 2 = 3 \).
- Therefore, it is a third-order reaction overall.
Why other options are incorrect:Option A is just the order w.r.t hydrogen. Option C is just the order w.r.t nitrogen dioxide. Option D miscalculates basic addition.
The catalysis in which the catalyst and the reactants are in the same phase is known: [UHS 2022]
A
Heterogeneous catalysis
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Correct Key: Option B
Diagnostic Explanation
Concept:Catalytic systems are classified based on the physical state (solid, liquid, gas) of the catalyst compared to the physical state of the reacting mixture.
Solution:- The prefix "homo-" means "same".
- When the catalyst dissolves completely into the reactant mixture (e.g., both are liquids, or both are gases) so that only a single phase exists, it is termed homogeneous catalysis.
Why other options are incorrect:Heterogeneous catalysis (hetero = different) occurs when the catalyst is in a different phase (e.g., solid metal in a gas mixture). Slow/fast describe reaction speeds, not the classification of the catalytic phase.
What is incorrect about activated complex? [SZABMU 2022]
A
It is a high energy specie
C
It is an unstable specie
D
Potential energy is maximum at activated complex state
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:The activated complex (or transition state) is the transient, intermediate structure formed at the absolute peak of the activation energy barrier during a collision.
Solution:- Because the activated complex sits at the maximum point of potential energy on the reaction coordinate diagram, it is incredibly unstable.
- It exists only for a fraction of a picosecond before breaking apart to form either products or reverting back to reactants.
- Therefore, calling it a "stable specie" is entirely incorrect.
Why other options are incorrect:Options A, C, and D accurately describe the activated complex: it has maximum potential energy, making it a highly unstable, high-energy species.
The unit of rate constant is the same as that of the rate of reaction in which of the following order of reaction: [SZABMU 2022]
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Correct Key: Option A
Diagnostic Explanation
Concept:The units of the rate constant (\( k \)) are determined by the reaction order to ensure dimensional consistency with the reaction rate.
Formula:$$ \text{Rate} = k[\text{Concentration}]^{n} $$
Solution:- For a zero-order reaction, the power \( n = 0 \).
- $$ \text{Rate} = k[\text{A}]^{0} = k(1) = k $$
- Because \( \text{Rate} = k \), \( k \) must inherit the exact units of rate: \( \text{mol dm}^{-3} \text{s}^{-1} \).
Why other options are incorrect:In first order, \( k \) is in \( \text{s}^{-1} \). In second order, \( k \) is in \( \text{mol}^{-1}\text{dm}^{3}\text{s}^{-1} \). They do not match the rate units.
If a reaction is first order with respect to a reactant then the rate will be if concentration of reactant is doubled: [SZABMU 2022]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:In a first-order reaction, the rate is directly and linearly proportional to the concentration of the reactant.
Formula:$$ \text{Rate}_1 = k[\text{A}]^{1} $$
$$ \text{Rate}_2 = k[2\text{A}]^{1} $$
Solution:- Substitute the doubled concentration into the rate law: \( \text{Rate}_2 = 2 \times k[\text{A}] \).
- Since \( k[\text{A}] \) is the original rate, \( \text{Rate}_2 = 2 \times \text{Rate}_1 \).
- Therefore, the rate simply doubles.
Why other options are incorrect:Quadrupling happens in a second-order reaction. Halving would occur if concentration was halved, not doubled.
The unit of rate constant K is \( \text{dm}^{3}\text{mole}^{-1}\text{s}^{-1} \) for a chemical reaction, the order of reaction is: [ETEA 2022]
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The generic formula to find the units of a rate constant (\( k \)) for an \( n \)-th order reaction is derived from equating rate units with concentration units.
Formula:$$ \text{Unit of } k = (\text{mol dm}^{-3})^{1-n} \text{s}^{-1} $$
Solution:- We are given the unit: \( \text{dm}^{3}\text{mol}^{-1}\text{s}^{-1} \).
- This can be rewritten as \( (\text{mol dm}^{-3})^{-1} \text{s}^{-1} \).
- Comparing this to the generic formula, we set the exponents equal: \( 1 - n = -1 \).
- Solving for \( n \): \( n = 2 \).
- Therefore, it is a second-order reaction.
Why other options are incorrect:First order gives \( \text{s}^{-1} \). Zero order gives \( \text{mol dm}^{-3}\text{s}^{-1} \). Third order gives \( \text{dm}^{6}\text{mol}^{-2}\text{s}^{-1} \).
One mole of a reactant reacts with a rate of \( 0.6 \text{ mol dm}^{-3} \text{s}^{-1} \). What is the rate constant of this reaction if reaction is first order? [ETEA 2022]
B
\( 0.3 \text{ s}^{-1} \)
C
\( 0.6 \text{ s}^{-1} \)
D
\( 0.9 \text{ s}^{-1} \)
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The rate constant for a first-order reaction is found by dividing the given rate by the given concentration.
Formula:$$ \text{Rate} = k[\text{A}]^{1} $$
Solution:- We are given Rate = \( 0.6 \text{ mol dm}^{-3} \text{s}^{-1} \).
- We are given the concentration of the reactant is 1 mole (implied 1 \( \text{mol dm}^{-3} \) in a standard 1 \( \text{dm}^{3} \) vessel).
- Substitute the values: \( 0.6 = k(1) \).
- Therefore, \( k = 0.6 \text{ s}^{-1} \).
Why other options are incorrect:Because the concentration is exactly 1, the numerical value of the rate constant exactly matches the numerical value of the rate. Dividing or multiplying by other factors is mathematically unjustified here.
The thermal decomposition of nitrogen pentaoxide in gaseous state follows. Which one of the following order of reaction? \( \text{N}_{2}\text{O}_{5(g)} \xrightarrow{\text{heat}} \text{N}_{2}\text{O}_{4(g)} + \frac{1}{2}\text{O}_{2(g)} \) [ETEA 2022]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The thermal decomposition of dinitrogen pentoxide (\( \text{N}_{2}\text{O}_{5} \)) is a classic textbook example of a reaction whose kinetics are experimentally determined.
Formula:$$ \text{Rate} = k[\text{N}_{2}\text{O}_{5}]^{1} $$
Solution:- Despite the stoichiometry, experiments show that the half-life of this specific decomposition is completely independent of the initial concentration of \( \text{N}_{2}\text{O}_{5} \).
- A concentration-independent half-life is the definitive hallmark of a strict first-order reaction.
Why other options are incorrect:One might incorrectly assume an order based on intermediate mechanisms, but experimental data universally confirms it follows first-order kinetics.
Rate of reaction increases by the addition of catalyst because: [DUHS 2022]
A
Catalyst reacts with reactant
B
Catalyst reacts with product
C
Energy of activation decreases
D
Energy of activation increases
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:A catalyst provides a new reaction mechanism that requires less energy to reach the transition state.
Formula:$$ k = A e^{-E_a / RT} $$
Solution:- By lowering the activation energy (\( E_a \)), a much larger percentage of colliding reactant molecules possess enough energy to successfully cross the barrier.
- According to the Arrhenius equation, a smaller \( E_a \) in the negative exponent dramatically increases the rate constant \( k \).
Why other options are incorrect:Increasing activation energy slows down a reaction. While catalysts do form temporary intermediate bonds with reactants (Option A), the fundamental
thermodynamic reason the rate increases is specifically the decrease in the activation energy barrier.
Reaction, \( \text{H}_{2} + \text{Cl}_{2} \longrightarrow 2\text{HCl} \) is a: [DUHS 2022]
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The reaction between hydrogen and chlorine gas in the presence of sunlight is a classic photochemical reaction.
Formula:$$ \text{Rate} = k[\text{H}_2]^0[\text{Cl}_2]^0 = k $$
Solution:- This specific reaction is initiated and entirely driven by the absorption of photons from light, not by thermal molecular collisions based on concentration.
- Because changing the concentration of \( \text{H}_2 \) or \( \text{Cl}_2 \) does not change the amount of light absorbed, the reaction rate remains constant.
- Thus, it is independent of reactant concentration, making it a zero-order reaction.
Why other options are incorrect:Looking at the stoichiometry, one might incorrectly guess second order (1+1). However, photochemical kinetics overrule stoichiometry.
Rate of any reaction depends upon: [NUMS 2022]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:While many factors affect chemical reactions, temperature is the universal factor that alters the fundamental rate constant (\( k \)) of
all reactions via the Arrhenius equation.
Formula:$$ k = A e^{-E_a / RT} $$
Solution:- The question asks what the rate of any reaction depends upon.
- Zero-order reactions do not depend on concentration or pressure.
- However, all reactions, regardless of their order, have a rate constant that is exponentially dependent on the absolute Temperature (\( T \)).
- Therefore, temperature is the only universally applicable dependency among the choices.
Why other options are incorrect:Concentration and pressure do not affect zero-order reactions. Rate constant is a parameter defining the rate, not an external variable that the rate "depends upon" in this context.
What is "NOT TRUE" about rate of a reaction? [NUMS 2022]
A
Surface area of reactants affects rate of reaction
B
Catalyst affects rate of a reaction
C
Concentration of reactants does not affect the rate of a reaction
D
Concentration affects rate of reaction
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The Law of Mass Action and general chemical kinetics state that the rate of a reaction is directly influenced by the concentration of its reactants (except in zero-order reactions).
Solution:- The statement "Concentration of reactants does not affect the rate of a reaction" is a broadly false generalization.
- For the vast majority of chemical processes (first, second, third order, etc.), increasing the concentration of reactants increases the frequency of collisions, thereby increasing the rate.
- Therefore, this statement is the "NOT TRUE" option.
Why other options are incorrect:Surface area, catalysts, and concentration all genuinely affect the rate of chemical reactions, making those statements true.
Catalyst can: [PMC 2021]
A
Increase rate of reaction
B
Decrease rate of reaction
C
Increase conc of product
D
Increase conc of reactant
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:A positive catalyst is a substance introduced to a reaction system to accelerate it.
Solution:- The primary function of a catalyst is to increase the rate at which equilibrium is achieved by providing a lower energy pathway (decreasing \( E_a \)).
- It achieves this without being permanently consumed in the process.
Why other options are incorrect:While a negative catalyst (inhibitor) decreases the rate, standard usage of 'catalyst' without a prefix implies a positive catalyst. A catalyst cannot shift equilibrium, so it cannot change the final concentrations (yield) of reactants or products.
Energy required to start a reaction is called: [PMC 2021]
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:Molecules are generally stable. To initiate a chemical transformation, existing bonds must first be weakened or broken, which requires an initial energy input.
Solution:- This minimum amount of external energy required to initiate a chemical reaction is universally termed the Activation Energy (\( E_a \)).
- It represents the energy hill that reactants must climb to reach the unstable transition state (activated complex).
Why other options are incorrect:Ionization energy is for removing an electron from an atom. Bond energy is the exact energy within a specific bond. Lattice energy holds ionic crystals together.
Rate = \( k[\text{NO}_{2}] \), the order of this reaction is: [PMC 2021]
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The overall order of a chemical reaction is calculated directly from its experimental rate law equation.
Solution:- The rate law is given as: Rate = \( k[\text{NO}_{2}] \).
- When no exponent is explicitly written next to a concentration bracket, it is mathematically understood to be 1.
- The sum of all exponents in this rate law is exactly 1.
- Therefore, this is a first-order reaction.
Why other options are incorrect:The exponent is clearly 1, making zero, two, or three mathematically impossible given the explicit formula.
When rate of reaction is measured by the amount of radiation absorbed, it is called: [PMC 2021]
A
Optical rotation method
C
Electrical conductivity method
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:Different physical properties are used to track the progress of chemical reactions based on the nature of the reactants and products.
Solution:- Spectrometry (or spectrophotometry) is the specific analytical method based on measuring the interaction between matter and electromagnetic radiation.
- If a reaction mixture absorbs specific wavelengths of light (UV, visible, IR), a spectrometer measures the exact amount of radiation absorbed over time to calculate the rate.
Why other options are incorrect:Optical rotation measures changes in polarized light angle. Refractometry measures the bending of light. Electrical conductivity measures ion movement.
The unit of the rate constant is the same as that of the rate of reaction in: [NMDCAT 2020]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The rate constant (\( k \)) acts as the proportionality constant in a rate law. Its units must balance the equation.
Formula:$$ \text{Rate} = k[\text{A}]^{n} $$
Solution:- Rate always has the unit \( \text{mol dm}^{-3}\text{s}^{-1} \).
- For a zero-order reaction (\( n = 0 \)): \( \text{Rate} = k[\text{A}]^{0} = k(1) = k \).
- Because \( \text{Rate} = k \), \( k \) must have the exact same units as Rate: \( \text{mol dm}^{-3}\text{s}^{-1} \).
Why other options are incorrect:Higher-order reactions contain concentration terms that do not equal 1, which alter the units of \( k \) (e.g., \( \text{s}^{-1} \) for first order).
The study of rates of chemical reactions and the factors that affect the rates of chemical reactions is known as: [NMDCAT 2020]
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:Chemistry is divided into branches based on what physical properties of matter are being studied.
Solution:- Chemical kinetics is the specific branch of physical chemistry concerned entirely with measuring the speed (rate) of chemical reactions and understanding the mechanisms and factors (like temperature, concentration, and catalysts) that alter this speed.
Why other options are incorrect:Thermodynamics studies energy changes and reaction spontaneity, not speed. Stoichiometry studies mass relationships. Electrochemistry studies electron transfer.
For which reaction, \( \text{A}_{(g)} \longrightarrow \text{product} \) [NMDCAT 2020]
When the concentration of \( \text{A}_{(g)} \) doubles, the rate of reaction increased four folds, which means it is:
A
Negative order of reaction
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The mathematical relationship between concentration change and rate change determines the reaction order.
Formula:$$ \text{Rate}_1 = k[\text{A}]^n $$
$$ \text{Rate}_2 = k[2\text{A}]^n $$
Solution:- We are given that when concentration doubles (factor of 2), the rate increases by a factor of 4.
- Set up the ratio: \( \frac{\text{Rate}_2}{\text{Rate}_1} = \frac{k[2\text{A}]^n}{k[\text{A}]^n} \)
- $$ 4 = (2)^n $$
- Since \( 2^2 = 4 \), the exponent \( n \) must be exactly 2.
- Therefore, this is a second-order reaction.
Why other options are incorrect:If it were 1st order, doubling concentration would only double the rate. If zero order, the rate wouldn't change at all.
For which of the following order of the reaction, rate of reaction is inversely proportional to the concentration reaction? [NMDCAT 2020]
C
Negative order of reaction
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:An inverse relationship in a rate law implies that as the concentration of a reactant increases, the rate of the overall reaction actually decreases.
Formula:$$ \text{Rate} \propto [\text{A}]^{-n} \implies \text{Rate} \propto \frac{1}{[\text{A}]^n} $$
Solution:- Mathematically, a variable in the denominator corresponds to a negative exponent.
- If the rate is inversely proportional to concentration, the exponent in the rate law must be negative.
- This is referred to as a negative order of reaction (often seen when a product or reactant acts as an inhibitor).
Why other options are incorrect:Positive orders (1st, 2nd) mean rate increases with concentration. Zero order means rate is constant regardless of concentration.
What is the measure of activation energy in an endothermic reaction? [MDCAT 2019]
A
The energy of activation of backward-backward reaction is same
B
The energy of activation of backward reaction is less than that of forward reaction
C
The energy of activation of backward reaction is more than that of forward reaction
D
The energy of activation of forward reaction is less than that of backward reaction
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:In an endothermic reaction, the products have a higher potential energy than the reactants. Therefore, the energy barrier from the reactant side is larger than from the product side.
Formula:$$ \Delta H = E_{a(\text{forward})} - E_{a(\text{backward})} > 0 $$
Solution:- Because the reaction absorbs heat, the enthalpy change (\( \Delta H \)) is positive.
- This means the activation energy for the forward reaction (\( E_{a(\text{forward})} \)) must be strictly greater than the activation energy for the reverse/backward reaction (\( E_{a(\text{backward})} \)).
- Consequently, the backward activation energy is less than the forward activation energy.
Why other options are incorrect:If forward activation energy is less, the reaction is exothermic. They are only the same if \( \Delta H = 0 \) (a thermoneutral reaction).
If energy of activated complex is close to energy of reactants, it means that the reaction is: [NUMS 2019]
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The rate of a chemical reaction is inversely dependent on its activation energy. A smaller energy gap means a faster reaction.
Formula:$$ E_{a} = E_{\text{activated complex}} - E_{\text{reactants}} $$
Solution:- If the energy of the activated complex is very close to the energy of the reactants, the activation energy (\( E_a \)) is very low.
- A low activation energy means a larger fraction of reactant molecules will possess enough kinetic energy to cross the barrier at any given temperature.
- Therefore, the reaction proceeds extremely fast.
Why other options are incorrect:Slow or moderate reactions imply a significantly higher activation energy barrier that fewer molecules can successfully overcome.
If the energy of activation of a chemical reaction is very low, the rate of that chemical reaction is observed to be very high because? [MDCAT 2019]
A
Concentration of the reactants becomes irrelevant
B
Reaction proceeds without any transition state
C
Molecules of the reactants move slowly
D
Number of efficient or fruitful collisions increase
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The Arrhenius equation and collision theory dictate that rate depends on the frequency of effective (fruitful) collisions that overcome the activation energy barrier.
Formula:$$ k = A e^{-E_a / RT} $$
Solution:- A very low activation energy (\( E_a \)) means the threshold energy required for a reaction is minimal.
- Consequently, a much larger proportion of the colliding molecules will have enough kinetic energy to exceed this low barrier.
- This drastically increases the number of fruitful collisions, making the reaction rate very high.
Why other options are incorrect:Concentration always matters. A transition state always exists regardless of how low the energy is. Slow molecules would actually decrease the collision frequency.
The influence of temperature on reaction rate is predicted by: [NUMS 2019]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The mathematical relationship between the rate constant of a reaction and absolute temperature was formulated by Svante Arrhenius.
Formula:$$ k = A e^{-E_a / RT} $$
Solution:- The Arrhenius equation explicitly links the rate constant (\( k \)) to Temperature (\( T \)).
- It predicts that as temperature increases, the exponential term becomes larger, exponentially increasing the rate of the reaction.
Why other options are incorrect:Van der Waals equation (Wander Waal's) relates to real gases. Free energy change (\( \Delta G \)) predicts spontaneity, not kinetic rate. 'Kinetic equation' is a generic term without specific mathematical bearing on temperature dependence.
The unit of rate constant is same as that of rate of reaction in: [NUMS 2019]
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:When the concentration term in the rate law disappears, the rate constant (\( k \)) mathematically equals the rate of the reaction.
Formula:$$ \text{Rate} = k[\text{A}]^{n} $$
Solution:- For a zero-order reaction, the order \( n = 0 \).
- Substitute 0 into the rate law: \( \text{Rate} = k[\text{A}]^{0} \).
- Since anything to the power of 0 is 1, \( \text{Rate} = k \).
- Therefore, the unit of \( k \) is identical to the unit of Rate, which is \( \text{mol dm}^{-3} \text{s}^{-1} \).
Why other options are incorrect:In first, second, and third-order reactions, the concentration terms do not reduce to 1, thus \( k \) must have different units to balance the dimensional equation.
The decomposition of phosphorus pentachloride in the presence of moisture takes place by the following mechanism,
\( \text{PCl}_{5(s)} + \text{H}_{2}\text{O}_{(l)} \longrightarrow \text{POCl}_{3(l)} + 2\text{HCl}_{(aq)} \) (Slow Step)
\( \text{POCl}_{3(l)} + 3\text{H}_{2}\text{O}_{(l)} \longrightarrow \text{H}_{3}\text{PO}_{4(aq)} + 3\text{HCl}_{(aq)} \) (Fast Step)
Overall: \( \text{PCl}_{5(s)} + 4\text{H}_{2}\text{O}_{(l)} \longrightarrow \text{H}_{3}\text{PO}_{4(aq)} + 5\text{HCl}_{(aq)} \)
The rate equation for this reaction will be: [MDCAT 2019]
A
Rate = \( k[\text{PCl}_{5}][\text{H}_{2}\text{O}]^{4} \)
B
Rate = \( k[\text{POCl}_{3}][\text{H}_{2}\text{O}]^{3} \)
C
Rate = \( k[\text{PCl}_{5}][\text{H}_{2}\text{O}] \)
D
Rate = \( [\text{PCl}_{5}][\text{H}_{2}\text{O}] \)
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:In a multi-step reaction mechanism, the overall rate of the reaction is strictly governed by the slowest step, known as the Rate-Determining Step (RDS).
Solution:- Identify the slow step from the mechanism: \( \text{PCl}_{5} + \text{H}_{2}\text{O} \longrightarrow \text{POCl}_{3} + 2\text{HCl} \).
- The rate law is derived directly from the molecularity of this slow step.
- The reactants in the slow step are one molecule of \( \text{PCl}_{5} \) and one molecule of \( \text{H}_{2}\text{O} \).
- Therefore, the rate equation is: \( \text{Rate} = k[\text{PCl}_{5}][\text{H}_{2}\text{O}] \).
Why other options are incorrect:Option A incorrectly uses the stoichiometry of the overall reaction. Option B uses the fast step. Option D is missing the essential rate constant (\( k \)).
For exothermic reversible reaction activation energy for forward direction depends upon: [ETEA 2019]
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:Activation energy (\( E_a \)) is primarily determined by the inherent chemical bonds that must be broken in the reactants.
Solution:- The nature of the reactants (the strength and type of their chemical bonds) is the primary determinant of the activation energy barrier.
- Note on exact exam key: While standard physical chemistry states that \( E_a \) is fundamentally independent of temperature, the specific local examining body (ETEA) marked 'Both A and B' (Temperature and Nature) as the correct answer in their official key, likely conflating effective kinetic energy distribution with the barrier itself. We retain the historical key for accuracy.
Why other options are incorrect:Strictly speaking, only the nature of the reactant and presence of a catalyst define \( E_a \). However, one must follow the historical exam key provided.
What is the measure of activation energy is an endothermic reaction? [MDCAT 2019]
A
The energy of activation of backward reaction is less than that of forward reaction
B
The energy of activation of forward-backward reaction is same
C
The energy of activation of backward reaction is more than that of forward reaction
D
The energy of activation of forward reaction is less than that of backward reaction
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:In an endothermic profile, heat is absorbed, leaving products at a higher potential energy state than the reactants.
Formula:$$ \Delta H = E_{a(\text{forward})} - E_{a(\text{backward})} > 0 $$
Solution:- Since \( \Delta H \) is positive, \( E_{a(\text{forward})} \) must be mathematically greater than \( E_{a(\text{backward})} \).
- This means the backward reaction has a smaller energy hill to climb to reach the transition state compared to the forward reaction.
Why other options are incorrect:If backward is more, the reaction is exothermic. If they are equal, there is no net energy change.
Role of a catalyst in a chemical reaction is to: [MDCAT 2018]
A
Increase rate of a reaction
B
Decrease yield of a reaction
C
Decrease rate of a reaction
D
Increase yield of product
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:A positive catalyst alters the kinetics of a reaction without changing its thermodynamics.
Solution:- The core function of a catalyst is to increase the rate (speed) at which a chemical reaction approaches equilibrium.
- It does this by lowering the activation energy barrier.
- Importantly, a catalyst does NOT alter the equilibrium position, so it cannot increase or decrease the final yield of the product.
Why other options are incorrect:Catalysts speed up equilibrium attainment but cannot shift thermodynamics to change the final yield. Decreasing the rate is the role of an inhibitor (negative catalyst).
If concentration time graph of a reactant indicates a constant half-life, then the order reaction with respect the reactant is: [MDCAT 2018]
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:The mathematical relationship between half-life and initial concentration is unique to each reaction order.
Formula:$$ t_{1/2} \propto \frac{1}{[\text{A}_0]^{n-1}} $$
Solution:- For a zero-order reaction (\( n = 0 \)), \( t_{1/2} \propto [\text{A}_0] \).
- For a second-order reaction (\( n = 2 \)), \( t_{1/2} \propto 1/[\text{A}_0] \).
- For a first-order reaction (\( n = 1 \)), \( t_{1/2} \propto [\text{A}_0]^{0} \), which equals 1.
- This proves that ONLY for a first-order reaction is the half-life completely independent of the concentration, making it a constant value over time.
Why other options are incorrect:Zero and second-order half-lives change dynamically as the concentration of the reactant changes during the reaction.
Choose the type of catalysis in the following reaction:
\( 2\text{SO}_{2(g)} + \text{O}_{2(g)} \xrightarrow{\text{NO}_{(g)}} 2\text{SO}_{3(g)} \) [MDCAT 2017]
D
Heterogeneous catalysis
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:Catalysis is classified by the physical state of the catalyst relative to the reactants. If they are in the same phase, it is homogeneous catalysis.
Formula:$$ \text{Reactants (gas)} + \text{Catalyst (gas)} \longrightarrow \text{Products} $$
Solution:- The reactants \( \text{SO}_{2} \) and \( \text{O}_{2} \) are in the gaseous phase.
- The catalyst, Nitric Oxide (\( \text{NO} \)), is also in the gaseous phase.
- Since both the catalyst and the reacting substances exist in the same phase, this is a classic example of homogeneous catalysis (Lead Chamber Process).
Why other options are incorrect:Heterogeneous catalysis requires the catalyst to be in a different phase (e.g., solid catalyst with gas reactants). Biological catalysis involves enzymes. 'Gas catalysis' is not a standard formal classification.
Which one of the following graphs is representation for more rapid catalyzed reaction? [MDCAT 2017]
A
Graph showing a lowered activation energy path (solid line) compared to original higher path (dotted line)
B
Graph showing equilibrium shifted entirely to the right with no activation barrier
C
Graph showing a single high activation energy peak (uncatalyzed pathway)
D
Graph showing multiple consecutive high-energy intermediate peaks without lowering overall energy
View Answer & Propolis Autopsy
Correct Key: Option A
Diagnostic Explanation
Concept:A catalyzed reaction provides an alternative pathway that requires less activation energy compared to the uncatalyzed reaction.
Solution:- In potential energy diagrams, the vertical axis represents energy.
- The peak of the curve represents the activation energy barrier.
- A rapid catalyzed reaction is visually represented by a curve with a significantly lower peak (solid line) than the uncatalyzed pathway (dotted line), indicating a lowered activation energy (\( E_a \)).
Why other options are incorrect:A single high peak represents a slow, uncatalyzed reaction. Multiple high peaks or completely flat barriers do not accurately represent standard positive catalytic lowering of \( E_a \).
Unit of K in first order Reaction is: [MDCAT 2017]
D
moles dm\(^{-3}\) s\(^{-1}\)
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The units of the rate constant (\( k \)) depend on the overall order of the reaction to ensure both sides of the rate law equation have the units of rate (concentration/time).
Formula:$$ \text{Rate} = k[\text{A}]^{1} $$
Solution:- The unit for Rate is \( \text{mol dm}^{-3} \text{s}^{-1} \).
- The unit for concentration [A] is \( \text{mol dm}^{-3} \).
- Isolating \( k \): $$ k = \frac{\text{Rate}}{[\text{A}]} = \frac{\text{mol dm}^{-3} \text{s}^{-1}}{\text{mol dm}^{-3}} $$
- The concentration units cancel out, leaving exactly \( \text{s}^{-1} \).
Why other options are incorrect:Option D is the unit for a zero-order reaction. Option A is the unit for a second-order reaction.
Rate of first order reaction depends on: [MDCAT 2017]
A
Independence of the initial concentration
B
Concentration of one reactant
C
Concentration of two reactants
D
Concentration of three reactants
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:The order of a reaction dictates how the reaction rate scales with the concentration of its reactants.
Formula:$$ \text{Rate} = k[\text{A}]^{1} $$
Solution:- By definition, a first-order reaction has a rate law where the sum of the exponents of concentration terms is equal to 1.
- This means the rate depends strictly on the first power of the concentration of a single reacting species.
Why other options are incorrect:Dependence on two or three reactants implies second or third-order kinetics, respectively. The rate (not half-life) is highly dependent on the initial concentration.
\( 2\text{A} + \text{B} \longrightarrow \text{Product} \). If the reactant 'B' is in excess, the order of reaction with respect to 'A' in given rate law \( \text{Rate} = k[\text{A}]^{2}[\text{B}] \) is: [MDCAT 2016]
B
Pseudo 1st order reaction
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:The order of a reaction with respect to a specific reactant is defined strictly by its exponent in the experimental rate law.
Formula:$$ \text{Rate} = k[\text{A}]^{2}[\text{B}] $$
Solution:- The question asks for the order specifically with respect to 'A'.
- Looking at the given rate law, the concentration of A is raised to the power of 2.
- Therefore, regardless of what happens to B, the reaction is strictly 2nd order with respect to A.
- (Note: Because B is in excess, its concentration remains effectively constant, making the overall order pseudo-2nd order, but the order specifically w.r.t A remains 2).
Why other options are incorrect:The exponent for A is 2, not 1 or 3. Pseudo-first order would occur if A's exponent was 1 and B was in excess.
The rate constant 'k' is 0.693 min\(^{-1}\). The half-life for the 1st order reaction will be: [MDCAT 2016]
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:For a first-order reaction, the half-life (\( t_{1/2} \)) is a constant value independent of initial concentration, mathematically derived from the natural logarithm of 2.
Formula:$$ t_{1/2} = \frac{0.693}{k} $$
Solution:- We are given \( k = 0.693 \text{ min}^{-1} \).
- Substitute this into the half-life formula:
- $$ t_{1/2} = \frac{0.693}{0.693} $$
- $$ t_{1/2} = 1 \text{ min} $$
Why other options are incorrect:Failing to divide 0.693 by the rate constant or incorrectly multiplying them leads to the wrong numerical answers.
Choose the one which is not the assumption of collision theory of reaction rate: [ETEA 2016]
A
For chemical reaction to occur molecule / particles must colloids
B
Every collision is not productive
C
For reacting particles must possess a certain minimum amount of energy
D
For hydrogen molecule formation from atoms require specific orientation
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:Collision theory dictates that for a reaction to occur, particles must collide with sufficient energy (activation energy) and proper spatial orientation.
Solution:- While orientation is crucial for complex molecules, the formation of a hydrogen molecule (\( \text{H}_{2} \)) from two hydrogen atoms is a unique exception.
- Hydrogen atoms possess a perfectly spherical (1s) electron cloud.
- Because they are completely spherically symmetrical, they look identical from every angle. Therefore, their collision does NOT require any specific orientation to be effective.
Why other options are incorrect:Options A, B, and C are true core postulates of standard collision theory (particles must collide, not all collisions work, and they need minimum energy).
The half-life of N\(_{2}\)O\(_{5}\) at 45°C is 24 minutes. How long will it take for sample of N\(_{2}\)O\(_{5}\) to decay to 25% of its original concentration? [MDCAT 2015]
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:The half-life (\( t_{1/2} \)) is the time required for a reactant concentration to drop to half (50%) of its initial value. The decay of N\(_{2}\)O\(_{5}\) is a first-order process, meaning its half-life remains constant regardless of concentration.
Solution:- Initial concentration is 100%.
- After one half-life (24 mins), the concentration decays to 50%: \( 100\% \rightarrow 50\% \).
- After a second half-life (another 24 mins), the remaining 50% decays to 25%: \( 50\% \rightarrow 25\% \).
- Total time = \( 24 \text{ mins} + 24 \text{ mins} = 48 \text{ minutes} \).
Why other options are incorrect:24 minutes is only one half-life (decay to 50%). 72 minutes would be three half-lives (decay to 12.5%).
When the change in concentration is \( 6 \times 10^{-4} \text{ mol dm}^{-3} \) and time for that change is 10 seconds, the rate of reaction will be? [MDCAT 2015]
A
\( 6 \times 10^{-3} \text{ mol dm}^{-3} \text{sec}^{-1} \)
B
\( 6 \times 10^{-4} \text{ mol dm}^{-3} \text{sec}^{-1} \)
C
\( 6 \times 10^{-2} \text{ mol dm}^{-3} \text{sec}^{-1} \)
D
\( 6 \times 10^{-5} \text{ mol dm}^{-3} \text{sec}^{-1} \)
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit of time.
Formula:$$ \text{Rate} = \frac{\Delta C}{\Delta t} $$
Solution:- Given change in concentration, \( \Delta C = 6 \times 10^{-4} \text{ mol dm}^{-3} \).
- Given time interval, \( \Delta t = 10 \text{ seconds} \).
- Substitute the values: $$ \text{Rate} = \frac{6 \times 10^{-4}}{10} $$
- $$ \text{Rate} = 6 \times 10^{-5} \text{ mol dm}^{-3} \text{sec}^{-1} $$
Why other options are incorrect:Option A results from incorrectly multiplying by 10 instead of dividing. The others are simple arithmetic errors.
For the reaction, \( 2\text{NO} + \text{O}_{2} \rightleftharpoons 2\text{NO}_{2} \), the rate equation for the forward reaction is: [MDCAT 2014]
A
Rate = \( k[\text{NO}][\text{O}_{2}] \)
B
Rate = \( k[\text{NO}_{2}]^{2} \)
C
Rate = \( k[\text{NO}_{2}] \)
D
Rate = \( k[\text{NO}]^{2}[\text{O}_{2}] \)
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The rate law is determined experimentally. For the gas-phase oxidation of nitric oxide, it follows third-order kinetics overall.
Formula:$$ \text{Rate} = k[\text{NO}]^{2}[\text{O}_{2}] $$
Solution:- Experimental data shows the reaction is second order with respect to \( \text{NO} \).
- It is first order with respect to \( \text{O}_{2} \).
- Combining these gives the overall rate law: \( \text{Rate} = k[\text{NO}]^{2}[\text{O}_{2}] \).
Why other options are incorrect:The other expressions do not match the experimentally determined orders of the reactants. Option B represents the reverse reaction rate, not the forward.
If the reactants or product of a chemical reaction can absorb ultraviolet, visible or infra-red radiation then the rate of a chemical reaction can best be measured by: [MDCAT 2014]
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:Spectrometry (or spectrophotometry) is an analytical technique used to measure how much light a chemical substance absorbs by measuring the intensity of light as a beam passes through the sample solution.
Solution:- According to the Beer-Lambert Law, the absorbance of light is directly proportional to the concentration of the absorbing species.
- If a reactant or product uniquely absorbs UV, Visible, or IR light, measuring the continuous change in absorbance provides a highly accurate, non-destructive, and continuous measure of concentration changes over time.
Why other options are incorrect:Chemical methods (like titration) are slow and destructive. Graphical and differential methods are mathematical ways to analyze data, not physical instruments to measure absorbance.
In zero order reaction, the rate is independent of: [MDCAT 2013]
A
Concentration of the product
B
Temperature of the reaction
C
Surface area of the product
D
Concentration of the reactant
View Answer & Propolis Autopsy
Correct Key: Option D
Diagnostic Explanation
Concept:The defining characteristic of a zero-order reaction is that its rate does not change as the reactant concentration changes.
Formula:$$ \text{Rate} = k[\text{A}]^{0} = k $$
Solution:- In a rate law expression for a zero-order reaction, the sum of the exponents of the concentration terms is zero.
- Any value raised to the power of zero is 1. Therefore, the rate equals the rate constant (\( k \)).
- This means the rate is completely independent of the concentration of the reactant(s).
Why other options are incorrect:Rate is always independent of product concentration in standard initial rate laws. However, the specific hallmark defining 'zero order' is its independence from the
reactant concentration. Temperature always affects the rate constant \( k \).
In some reactions a product formed acts as a catalyst. This phenomenon is called: [MDCAT 2012]
C
Heterogeneous catalysis
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:When one of the products of a chemical reaction acts as a catalyst for that same reaction, the process is known as autocatalysis.
Formula:$$ \text{A} + \text{B} \longrightarrow \text{C} + \text{D (catalyst)} $$
Solution:- As the reaction proceeds, the concentration of the product (catalyst) increases.
- This leads to a gradual, spontaneous increase in the rate of the reaction over time.
Why other options are incorrect:Negative catalysis decreases the rate, heterogeneous catalysis involves different phases, and activation of a catalyst refers to promoters, not the product acting as a catalyst.
The reaction rate in forward direction decreases with the passage of time because: [MDCAT 2012]
A
Concentration of product decreases
B
Temperature of the system changes
C
Concentration of reactants decreases
D
The order of reaction changes
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:According to the Law of Mass Action, the rate of a chemical reaction is directly proportional to the active masses (concentrations) of the reacting substances.
Formula:$$ \text{Rate} \propto [\text{Reactants}]^{n} $$
Solution:- As the reaction proceeds in the forward direction, reactants are continuously consumed to form products.
- Consequently, the concentration of reactants decreases steadily.
- Since the rate is directly dependent on reactant concentration, it also decreases with the passage of time.
Why other options are incorrect:The concentration of products increases (not decreases) over time. The order of the reaction is a constant experimental value and does not change over time. Temperature is typically kept constant unless specified otherwise.
According to the collision theory of bimolecular reactions in gas phase, minimum amount of energy required for an effective collision is known as: [MDCAT 2011]
D
Has no effect on the reaction
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:Not all collisions between reactant molecules lead to product formation. Only those possessing a specific minimum kinetic energy are effective.
Formula:$$ E \geq E_a \text{ (for effective collision)} $$
Solution:- This required threshold energy is formally defined as the Energy of Activation (\( E_a \)).
- It is the minimum extra amount of energy (above the average kinetic energy) that reactants must possess to convert into products.
Why other options are incorrect:Heat of reaction (\( \Delta H \)) is the energy difference between products and reactants. Rate of reaction is a speed, not an energy measure.
It is experimentally found that catalyst is used to: [MDCAT 2011]
A
Increase the activation energy
B
Lower the activation energy
C
Decrease the temperature of other reactants
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:A catalyst speeds up a chemical reaction without being consumed by providing an alternative reaction pathway.
Formula:$$ E_{a(\text{catalyzed})} < E_{a(\text{uncatalyzed})} $$
Solution:- The primary function of a positive catalyst is to provide an alternative mechanism or pathway for the reaction.
- This new pathway has a lower activation energy (\( E_a \)).
- Because the barrier is lower, more molecules can react at a given temperature, accelerating the reaction.
Why other options are incorrect:Increasing activation energy would slow the reaction (negative catalyst). Catalysts do not inherently alter the pH or change the bulk temperature of the reactants.
Glucose is converted into ethanol by the enzyme present in the yeast: [MDCAT 2010]
View Answer & Propolis Autopsy
Correct Key: Option C
Diagnostic Explanation
Concept:Fermentation of simple sugars into alcohol is a biochemical process catalyzed by specific biological enzymes.
Formula:$$ \text{C}_{6}\text{H}_{12}\text{O}_{6} \xrightarrow{\text{Zymase}} 2\text{C}_{2}\text{H}_{5}\text{OH} + 2\text{CO}_{2} $$
Solution:- Yeast secretes a complex of enzymes known as zymase.
- Zymase acts directly on glucose (and fructose), catalyzing its breakdown into ethanol and carbon dioxide.
Why other options are incorrect:Invertase (sucrase) hydrolyzes sucrose into glucose and fructose. Urease breaks down urea into ammonia and carbon dioxide.
The rate of reaction involving ions can be studied by which method? [MDCAT 2010]
B
Electrical conductivity method
C
Optical rotation method
View Answer & Propolis Autopsy
Correct Key: Option B
Diagnostic Explanation
Concept:The rate of a reaction is measured by tracking a physical property that changes linearly as the reaction proceeds. For ions, electrical mobility is the key property.
Solution:- Ions in a solution are responsible for carrying electrical charge.
- If a reaction involves a net change in the number of ions or a change in the type of ions (and therefore their individual mobilities), the overall electrical conductivity of the solution will change as the reaction proceeds.
- This change in conductivity can be continuously measured using a conductivity meter to determine the reaction rate.
Why other options are incorrect:Refractometry is for changes in refractive index (usually organic liquids). Optical rotation is for optically active chiral substances. Dilatometry measures volume changes.
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