Question

What is the rate law for each of the following elementary reactions? (Rate expressions take the general form: rate=k[H.](Cl₂].)

Cl(g) + ICI(g) + I(g) + Cl₂(g) rate =

Answer 1

The rate law for the elementary reaction Cl(g) + ICI(g) → I(g) + Cl₂(g) is rate = k[Cl][ICI]. A two-step mechanism for the reaction 2 ICI(g) + H₂(g) → 2 HCl(g) + I₂(s) with HI as an intermediate is suggested, which is consistent with the experimental rate law rate = k[ICI][H₂].

Step-by-step explanation:

The rate law for an elementary reaction is directly derived from the reaction's molecularity. For a bimolecular reaction such as Cl(g) + ICI(g) → I(g) + Cl₂(g), the rate law is given by rate = k[Cl][ICI], where k is the rate constant and [Cl] and [ICI] are the concentrations of chlorine and iodine monochloride, respectively.

To find a mechanism consistent with the experimentally determined rate law rate = k[ICI][H₂] for the reaction 2 ICI(g) + H₂(g) → 2 HCl(g) + I₂(s), we can propose a two-step mechanism involving HI as an intermediate:

1. Step 1: ICI(g) + H₂(g) → HI(g) + HCl(g) (fast)
2. Step 2: HI(g) + ICI(g) → HCl(g) + I₂(s) (slow)

The rate-determining step is the slower step 2, which gives the rate law as rate = k[HI][ICI]. However, since HI is an intermediate, its concentration is not directly measurable.

We can use the fast equilibrium established in step 1 to express [HI] in terms of [ICI] and [H₂], resulting in the experimentally observed rate law. This shows that the proposed mechanism is consistent with the experimental data.