The rate of reaction is concerned with decrease in concentration of reactants or increase in the concentration of products per unit time. It can be expressed as instantaneous rate at a particular instant of time and average rate over a large interval of time. Mathematical representation of rate of reaction is given by rate law. Rate constant and order of a reaction can be determined from rate law or its integrated rate equation.
(i) What is average rate of reaction?
(ii) Write two factors that affect the rate of reaction.
(iii) (1) What happens to rate of reaction for zero order reaction?
(2) What is the unit of $k$ for zero order reaction?
OR
(iii) (1) For a reaction $P+2 Q \longrightarrow$ Products Rate $=k[P]^{1 / 2}[Q]^1$. What is the order of the reaction?
(2) Define pseudo first order reaction with an example.
Updated On 18th June 2025
Expert Explanation :
(i) Average rate of reaction : Ratio of change in concentration and time required for the change is average rate of reaction.
$
r_{a v}=\frac{\Delta x}{\Delta t}=\frac{\text { Change in concentration }}{\text { Time required for the change }}
$
(ii) Two factors that affect rate of reaction are:
(a) Nature of reactants
(b) Temperature
(iii) (1) The rate of reaction of a zero order reaction is proportional to zero power of the concentration of reactants. Rate of reaction for zero order reaction is given as :
$
k=\frac{[R]_0-[R]}{t}
$
(2) Units of $k$ for zero order reaction is $\mathrm{mol} \mathrm{L}^{-1} \mathrm{~s}^{-1}$.
OR
(iii) (1) $P+2 Q \longrightarrow$ Products
$
\text { Rate }=k[P]^{1 / 2}[Q]^1
$
Order of reaction $=\frac{1}{2}+1=3$
(2) The reactions which are of second order but behave like first order reactions are called pseudo first order reactions e.g., during hydrolysis of ethyl acetate (ester) with water, the concentration of water does not alter much during the reaction. So, in the rate equation, the rate of change in water concentration (the reactant which is in excess) can be taken as constant hence the reaction behaves as first order reaction.
$
\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5+\mathrm{H}_2 \mathrm{O} \xrightarrow{\mathrm{H}^{+}} \mathrm{CH}_3 \mathrm{COOH}+\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}
$
Rate $=k^{\prime}\left[\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5\right]\left[\mathrm{H}_2 \mathrm{O}\right]$ the term $\left[\mathrm{H}_2 \mathrm{O}\right]$ can be taken as constant. Hence, the rate equation becomes Rate $=k\left[\mathrm{CH}_3 \mathrm{COOC}_2 \mathrm{H}_5\right]$ where $k=k^{\prime}\left[\mathrm{H}_2 \mathrm{O}\right]$.
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