Chemical Kinetics
Kinetic studies not only help us to determine the speed or rate of a chemical reaction but also describe the conditions under which the reaction rates can be altered. Some reactions such as ionic reactions are very fast while some others like rusting of iron are very slow. Also, there are reactions like inversion of cane sugar and hydrolysis of starch, which proceed at a moderate speed. The rate of a reaction can be defined as the change in concentration of a reactant or product per unit time. The unit used to express the rate of a reaction is concentration time−1. The rate of a reaction can be determined in terms of the average rate of reaction or can be determined at a particular instant of time called the instantaneous rate of reaction. The rate of a chemical reaction at a given temperature depends on the concentration of one or more reactants. The representation of the rate of reaction in terms of concentration of the reactants is known as rate law. Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically, but must be determined experimentally.
For a reaction, xA + yB → Products, the rate law is given as Rate = k[A]a[B]b.
If the initial concentration of A is doubled, keeping the concentration of B same, the rate of the reaction is doubled. Similarly, if the concentration of B is doubled, keeping the concentration of A same, the rate of the reaction becomes four times. The value of a and b, respectively, in the rate law is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → one, two
In a rate law, the power of each concentration term is the order with respect to that reactant. It is found by experiment, by changing the concentration of one reactant while keeping the others fixed. It is not taken from the coefficients x and y of the balanced equation.
\( \displaystyle \text{Rate}=k[A]^a[B]^b \)
If [A] is multiplied by 2 with [B] fixed, the rate is multiplied by 2a; if [B] is multiplied by 2 with [A] fixed, the rate is multiplied by 2b.
Doubling A doubles the rate:
\( \displaystyle 2^a=2\Rightarrow a=1 \)
Doubling B makes the rate four times:
\( \displaystyle 2^b=4=2^2\Rightarrow b=2 \)
So a and b are one and two, and the rate law is Rate = k[A][B]2; the order of the reaction is 1 + 2 = 3.