A reaction proceeds according to the following mechanism:
Cl2 ⇌ 2 Cl (fast equilibrium)
Cl + H2 ⇌ H2Cl (fast equilibrium)
H2Cl + Cl → 2 HCl (slow)
What is the rate law for this reaction?
To determine the rate law for a reaction, we need to examine the rate-determining step, which is the slowest step in the reaction mechanism. In this case, the slow step is:
H2Cl + Cl → 2 HCl
The rate of the reaction is determined by the concentration of the reactants in this step. However, before we determine the rate law, we need to express the slow step in terms of the elementary steps. From the given mechanism, we can see that the first two elementary steps (Cl2 ⇌ 2 Cl and Cl + H2 ⇌ H2Cl) are in equilibrium, so they do not contribute to the rate law.
Considering the slow step, we can write the rate law using the reactant concentrations as follows:
rate = k[H2Cl][Cl]
Here, k is the rate constant, and [H2Cl] and [Cl] represent the concentrations of H2Cl and Cl, respectively.
Therefore, the rate law for this reaction is rate = k[H2Cl][Cl].