The iodide ion can be oxidized in an acidic solution by hydrogen peroxide. One proposed
mechanism for the reaction is as follows:
Step 1: H2O2 (aq) + I-(aq) → H2O (l) + OI-(aq) slow
Step 2: H+(aq) + OI-(aq) → HOI(aq) fast
Step 3: HOI (aq) + H+(aq) + I-(aq) → I2 (aq) + H2O (l) fast
A. Write the overall equation for the reaction.
B. What are the rate law and molecularity for each step?
C. What is the overall rate law for the reaction?
Nitric oxide (NO) is one of the by-products of the high-temperature combustion of
gasoline in automobiles. Nitric oxide reacts with oxygen to produce nitrogen dioxide in
the atmosphere, which eventually leads to a degradation of the ozone layer through
reactions of nitrogen dioxide and ozone. The reaction between nitric oxide and oxygen
is second-order for nitric oxide and first-order for oxygen. Below are three proposed
mechanisms for this reaction:
I. 2 NO (g) + O2 (g) → 2 NO2 (g)
II. 2 NO (g) <--> N2O2 (g) fast
N2O2 (g) + O2 (g) → 2 NO2 (g) slow
III. 2 NO (g) <--> N2 (g) + O2 (g) fast
N2 (g) + 2 O2 (g) → 2 NO2 (g) slow
A. Write the rate law and determine the molecularity for each step for the three
proposed mechanisms.
B. Write the overall rate law for each of the proposed mechanisms.
C. The experimentally determined rate law for the reaction is rate = k[NO]^2
[O2]. Which of the three proposed mechanisms is the best choice for the actual mechanism? Why?
A. For the first reaction involving the iodide ion, the overall equation can be written by combining the individual steps:
H2O2(aq) + 2I-(aq) + 2H+(aq) → I2(aq) + 2H2O(l)
B. The rate law for each step can be determined by examining the reactants and their concentrations in the rate-determining step of each mechanism. The rate law is given as:
Rate = k[A]^n[B]^m
For Step 1: H2O2(aq) + I-(aq) → H2O(l) + OI-(aq)
- Rate law: Rate1 = k1[H2O2][I-]
- Molecularity: Unimolecular, as it involves only one molecule in the rate-determining step.
For Step 2: H+(aq) + OI-(aq) → HOI(aq)
- Rate law: Rate2 = k2[H+][OI-]
- Molecularity: Bimolecular, as it involves two molecules in the rate-determining step.
For Step 3: HOI(aq) + H+(aq) + I-(aq) → I2(aq) + H2O(l)
- Rate law: Rate3 = k3[HOI][H+][I-]
- Molecularity: Trimolecular, as it involves three molecules in the rate-determining step.
C. To determine the overall rate law, we need to examine the slowest (rate-determining) step, which is Step 1 in this case. The rate law for the overall reaction will be the rate law of the rate-determining step, so the overall rate law is:
Rate = k1[H2O2][I-]
For the second reaction involving nitric oxide, let's analyze each proposed mechanism:
I. 2 NO(g) + O2(g) → 2 NO2(g)
- Rate law: Rate1 = k1[NO]^2[O2]
- Molecularity: Termolecular, as it involves three molecules in the rate-determining step.
II. 2 NO(g) ⇌ N2O2(g) (fast)
N2O2(g) + O2(g) → 2 NO2(g) (slow)
- Rate law: Rate2 = k2[N2O2][O2]
- Molecularity: Bimolecular for Step 1 (equilibrium), Bimolecular for Step 2 (slow step).
III. 2 NO(g) ⇌ N2(g) + O2(g) (fast)
N2(g) + 2 O2(g) → 2 NO2(g) (slow)
- Rate law: Rate3 = k3[N2][O2]^2
- Molecularity: Bimolecular for Step 1 (equilibrium), Trimolecular for Step 2 (slow step).
B. The overall rate law for each mechanism is the rate law of the rate-determining step.
For Mechanism I, the overall rate law is Rate = k1[NO]^2[O2].
For Mechanism II, the overall rate law is Rate = k2[N2O2][O2].
For Mechanism III, the overall rate law is Rate = k3[N2][O2]^2.
C. The best choice for the actual mechanism is Mechanism I because its rate law (rate = k1[NO]^2[O2]) matches the experimentally determined rate law (rate = k[NO]^2[O2]). Mechanism II and Mechanism III do not match the experimentally determined rate law, so they are not suitable as the actual mechanism.