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Chemistry · 2025 · 4 marks
CBSE 2025 · Region 1 · Set 1 · Q29
The rate of a chemical reaction is expressed either in terms of decrease in the concentration of reactants or increase in the concentration of a…
The rate of a chemical reaction is expressed either in terms of decrease in the concentration of reactants or increase in the concentration of a product per unit time. Rate of the reaction depends upon the nature of reactants, concentration of reactants, temperature, presence of catalyst, surface area of the reactants and presence of light. Rate of reaction is directly related to the concentration of reactant. Rate law states that the rate of reaction depends upon the concentration terms on which the rate of reaction actually depends, as observed experimentally. The sum of powers of the concentration of the reactants in the Rate law expression is called order of reaction while the number of reacting species taking part in an elementary reaction which must collide simultaneously in order to bring about a chemical reaction is called molecularity of the reaction. Answer the following questions :
(a)(i)
What is a rate determining step ? $\displaystyle (1+1=2)$
(ii)
Define complex reaction.
(b)
What is the effect of temperature on the rate constant of a reaction ?
OR
(b)
Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions ?
(c)
The conversion of molecule X to Y follows second order kinetics. If concentration of X is increased $\displaystyle 3$ times, how will it affect the rate of formation of Y ?
Marking-scheme solution
(a)(i)
Slowest step.
(ii)
Series of elementary reactions / Reactions involving two or more steps.
(b)
Increases with increase in temperature.
(b)
Molecularity is defined only for elementary reactions whereas order is experimentally
determined hence applicable for both / Because molecularity of each elementary reaction in
complex reaction may be different and hence meaningless for overall complex reaction whereas
order of a complex reaction is experimentally determined by the slowest step in its mechanism
and is therefore applicable for both.
(c)
$\displaystyle 9$ times
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.