Chemistry Reaction Kinetics NUMS 2019
PMDC Verified Question 55 of 80
The unit of rate constant is same as that of rate of reaction in:
A
Third order reaction
B
Zero order reaction
C
First order reaction
D
Second order reaction
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Zero order reaction
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.

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