Chemistry Reaction Kinetics MDCAT 2017
PMDC Verified Question 65 of 80
Unit of K in first order Reaction is:
A
mol\(^{-1}\) dm\(^3\)
B
moles dm\(^{-3}\)
C
s\(^{-1}\)
D
moles dm\(^{-3}\) s\(^{-1}\)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: s\(^{-1}\)
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.

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