Question

The reaction at hydrogen and iodine monochloride is represented by the equation is
$H_2(g)+2ICl(g)\to 2HCl(g)+I_2(g)$
This reaction is first order in $H_2(g)$ and also first-order in $ICl(g)$ . Which of these proposed mechanisms can be consistent with the given information about this reaction?
Mechanism I: $H_2(g)+2ICl(g)\to 2HCl(g)+I_2(g)$
Mechanism II: $H_2(g)+ICl(g)\stackrel{slow}{\to }HCl(g)+HI(g)$
$HI(g)+ICl(g)\stackrel{fast}{\to }HCl(g)+I_2(g)$
A.I only
B.II only
C.Both I and II
D.Neither I nor II

Answer 1

Hint:The order of a reaction is the sum of the exponents of the molar concentrations in terms of the reactants in the rate equation of a chemical reaction. A first-order reaction depends on the concentration of only one reactant. Sometimes it is also referred to as a unimolecular reaction. The rate of the reaction depends on the slowest step. Hence the slow step is known as the rate-determining step.

Complete step by step answer:
Now we will discuss it in detail.
-In the question, the above reaction is a first-order reaction concerning both of the reactants $H_2$ and $ICl$ . Hence the slowest step of the mechanism of the reaction should involve both one mole each.
-The rate constant for the above reaction can be written as below. $H(g)+ICl(g)\to HCl(g)+HI(g)$
$rate=k[H_2][ICl]$
-In mechanism-I, it involves both of the reactants. But 2 moles of $ICl$ is there. According to the definition of the first-order reaction, one mole of each reactant should be present. Hence mechanism-I is not correct.
-Mechanism-II seems to be consistent with the given condition of the reaction and it is a unimolecular reaction.
-Thus, we can conclude that Mechanism-II is correct.

Thus, the correct option is (B).


Note:
-It is said that the rate of the first-order reaction varies with the power of concentration of any of the reactants. Hence the rate is directly proportional to the concentration of the reactant.
-Unit of the rate constant for the first order of reaction $K=Mi{n}^{-1}$ or $S^{-1}$ . It has only a time unit and no concentration unit.
-Example of the first-order reaction- Decomposition of $H_2{O}_2$ in aqueous solution, hydrolysis of methyl acetate.
-To recognize the order of the reaction, a graph can be plotted between the logarithm of the concentration of reactant and time.
-Also, it can be noted that the half-life period of the first-order reaction is independent of the initial concentration of the reactant.