Chemistry Reaction Kinetics MDCAT 2018
PMDC Verified Question 62 of 80
If concentration time graph of a reactant indicates a constant half-life, then the order reaction with respect the reactant is:
A
First order
B
Zero order
C
Second order
D
Half order
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: First order
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.

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