Chapter 9, Problem 9.5P

For each of the reactions in parts (a), (b), and (c), suggest a mechanism and apply the PSSH to learn if the mechanism is consistent with the rate law.

1. (a) The gas-phase homogeneous oxidation of nitrogen monoxide (NO) to dioxide (NO₂)

$2\text{NO}+{\text{O}}_2\stackrel{k}{\to }2{\text{NO}}_2$

is known to have a form of third-order kinetics, which suggests that the reaction is elementary as written, at least for low partial pressures of the nitrogen oxides. However, the rate constant k actually decreases with increasing absolute temperature, indicating an apparently negative activation energy. Because the activation energy of any elementary reaction must be positive, some explanation is in order.

Provide an explanation, starting from the fact that an active intermediate species, NO₃, is a participant in some other known reactions that involve oxides of nitrogen. Draw the reaction pathway. Hint: See margin in Section 9.1.2.

1. (b) The rate law for formation of phosgene, COCl₂, from chlorine, Cl₂, and carbon monoxide, CO, has the rate law

$r_{{\text{COCl}}_2}=k{\text{C}}_{\text{CO}}{\text{C}}_{{\text{Cl}}_2}^{3/2}$

Suggest a mechanism for this reaction that is consistent with this rate law and draw the reaction pathway. Hint: Cl formed from the dissociation of Cl₂ is one of the two active intermediates.

1. (c) Suggest an active intermediate(s) and mechanism for the reaction H₂ + Br₂ → 2HBr. Use the PSSH to show whether or not your mechanism is consistent with the rate law

$r_{\text{HBr}}=\frac{k_1{\text{C}}_{{\text{H}}_2}{\text{C}}_{\text{Br}}^{3/2}}{{\text{C}}_{\text{HBr}}+k_2{C}_{{\text{Br}}_2}}$