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: ${H_2}(g) + ICl(g)\xrightarrow{{slow}}HCl(g) + HI(g)$
$HI(g) + ICl(g)\xrightarrow{{fast}}HCl(g) + {I_2}(g)$
A.I only
B.II only
C.Both I and II
D.Neither I nor II

Answer 1

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.