Chemistry Reaction Kinetics PMDC 2020
PMDC Verified Question 46 of 80
The unit of the rate constant is the same as that of the rate of reaction in:
A
Zero order reaction
B
Second order reaction
C
First order reaction
D
Third 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 A: Zero order reaction
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).

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