Chemistry Reaction Kinetics SET 2019
PMDC Verified Question 57 of 80
Rates of photo-chemical reactions do not change with the change in concentration of reactants. What is the order of such reactions?
A
Zero order reactions
B
First order reactions
C
Pseudo first order reactions
D
Second order reactions
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 reactions
Concept:

If a reaction's rate is entirely independent of the concentration of its reactants, it is by definition a zero-order reaction.

Formula:

$$ \text{Rate} = k[\text{Reactants}]^{0} = k $$

Solution:

  • Photochemical reactions are driven by the absorption of light photons rather than molecular collisions resulting from concentration.
  • Because increasing the concentration of reactants does not increase the number of photons absorbed (if the light source is constant and the surface is saturated), the rate remains constant.
  • Hence, the reaction is zero order with respect to the reactants.


Why other options are incorrect:

First, pseudo-first, and second-order reactions all exhibit a dependence on the concentration of at least one reactant.

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