12.6 Reaction mechanisms  (Page 5/13)

In general, can we predict the effect of doubling the concentration of A on the rate of the overall reaction $A+B⟶C$ ? Can we predict the effect if the reaction is known to be an elementary reaction?

Define these terms:

(a) unimolecular reaction

(b) bimolecular reaction

(c) elementary reaction

(d) overall reaction

What is the rate equation for the elementary termolecular reaction $A+2B⟶\text{products?}$ For $3A⟶\text{products?}$

Given the following reactions and the corresponding rate laws, in which of the reactions might the elementary reaction and the overall reaction be the same?

(a) $\begin{array}{l}{\text{Cl}}_2+\text{CO}⟶{\text{Cl}}_2\text{CO}\\ \text{rate}=k{\left[{\text{Cl}}_2\right]}^{\text{3/2}}\left[\text{CO}\right]\end{array}$

(b) $\begin{array}{l}{\text{PCl}}_3+{\text{Cl}}_2⟶{\text{PCl}}_5\\ \text{rate}=k\left[{\text{PCl}}_3\right]\left[{\text{Cl}}_2\right]\end{array}$

(c) $\begin{array}{l}2\text{NO}+{\text{H}}_2⟶{\text{N}}_2+{\text{H}}_2\text{O}\\ \text{rate}=k\left[\text{NO}\right]\left[{\text{H}}_2\right]\end{array}$

(d) $\begin{array}{l}2\text{NO}+{\text{O}}_2⟶2{\text{NO}}_2\\ \text{rate}=k{\text{[NO]}}^2\left[{\text{O}}_2\right]\end{array}$

(e) $\begin{array}{l}\text{NO}+{\text{O}}_3⟶{\text{NO}}_2+{\text{O}}_2\\ \text{rate}=k\left[\text{NO}\right]\left[{\text{O}}_3\right]\end{array}$

Write the rate equation for each of the following elementary reactions:

(a) ${\text{O}}_3\stackrel{\text{sunlight}}{\to }{\text{O}}_2+\text{O}$

(b) ${\text{O}}_3+\text{Cl}⟶{\text{O}}_2+\text{ClO}$

(c) $\text{ClO}+\text{O}⟶\text{Cl}+{\text{O}}_2$

(d) ${\text{O}}_3+\text{NO}⟶{\text{NO}}_2+{\text{O}}_2$

(e) ${\text{NO}}_2+\text{O}⟶\text{NO}+{\text{O}}_2$

Nitrogen(II) oxide, NO, reacts with hydrogen, H ₂ , according to the following equation:
$2\text{NO}+2{\text{H}}_2⟶{\text{N}}_2+2{\text{H}}_2\text{O}$

What would the rate law be if the mechanism for this reaction were:
$\begin{array}{l}2\text{NO}+{\text{H}}_2⟶{\text{N}}_2+{\text{H}}_2{\text{O}}_2\left(\text{slow}\right)\\ {\text{H}}_2{\text{O}}_2+{\text{H}}_2⟶2{\text{H}}_2\text{O}\left(\text{fast}\right)\end{array}$

Experiments were conducted to study the rate of the reaction represented by this equation. This question is taken from the Chemistry Advanced Placement Examination and is used with the permission of the Educational Testing Service.
$\text{2NO}(g)+2{\text{H}}_2(g)⟶{\text{N}}_2(g)+2{\text{H}}_2\text{O}(g)$

Initial concentrations and rates of reaction are given here.

Consider the following questions:

(a) Determine the order for each of the reactants, NO and H ₂ , from the data given and show your reasoning.

(b) Write the overall rate law for the reaction.

(c) Calculate the value of the rate constant, k , for the reaction. Include units.

(d) For experiment 2, calculate the concentration of NO remaining when exactly one-half of the original amount of H ₂ had been consumed.

(e) The following sequence of elementary steps is a proposed mechanism for the reaction.

Step 1: $\text{NO}+\text{NO}\rightleftharpoons {\text{N}}_2{\text{O}}_2$

Step 2: ${\text{N}}_2{\text{O}}_2+{\text{H}}_2\rightleftharpoons {\text{H}}_2\text{O}+{\text{N}}_2\text{O}$

Step 3: ${\text{N}}_2\text{O}+{\text{H}}_2\rightleftharpoons {\text{N}}_2+{\text{H}}_2\text{O}$

Based on the data presented, which of these is the rate determining step? Show that the mechanism is consistent with the observed rate law for the reaction and the overall stoichiometry of the reaction.

The reaction of CO with Cl ₂ gives phosgene (COCl ₂ ), a nerve gas that was used in World War I. Use the mechanism shown here to complete the following exercises:
${\text{Cl}}_2(g)\rightleftharpoons 2\text{Cl}(g)$ (fast, k ₁ represents the forward rate constant, k ₋₁ the reverse rate constant)
$\text{CO}(g)+\text{Cl}(g)⟶\text{COCl}(g)$ (slow, k ₂ the rate constant)
$\text{COCl}(g)+\text{Cl}(g)⟶{\text{COCl}}_2(g)$ (fast, k ₃ the rate constant)

(a) Write the overall reaction.

(b) Identify all intermediates.

(c) Write the rate law for each elementary reaction.

(d) Write the overall rate law expression.