Question

The following mechanism has been proposed for the reaction of NO with H₂ to form N₂O and H₂O:

Step 1: NO(g) + NO(g) → N₂O₂(g)
Step 2: N₂O₂(g) + H₂(g) → N₂O(g) + H₂O(g)

Place a check next to each substance that is an intermediate in this reaction mechanism.

A) NO
B) N₂O₂
C) H₂
D) H₂O

If the proposed mechanism is correct, which step is the rate-determining step?
1) NO
2) N₂O₂
3) H₂
4) H₂O

Answer 1

The intermediates in the reaction mechanism are N₂O₂ and H₂O. The rate-determining step is the second step involving N₂O₂ and H₂. This is because the rate law for the overall reaction is consistent with the formation of N₂O₂ and its reaction in the rate-determining step.

Step-by-step explanation:

The substances that are intermediates in the proposed reaction mechanism of NO with H₂ to form N₂O and H₂O are B) N₂O₂ and D) H₂O. An intermediate is a species that is formed in one step of a reaction mechanism and consumed in a subsequent step; it does not exist in the initial or final reaction states. Therefore, NO and H₂ are not intermediates as they are reactants, while H₂O is clearly a product and not an intermediate. However, according to the provided mechanism, N₂O₂ is formed in the first step and consumed in the second step, making it an intermediate.

Regarding the rate-determining step, it is Step 2 (N₂O₂ + H₂ = N₂O + H₂O). The rate-determining step is the slowest step in a reaction mechanism and controls the overall rate of the reaction. The rate law is consistent with the proposed mechanism, where the experimental rate law shows a dependence on the concentration of NO squared and the concentration of H₂, which corresponds to the formation of N₂O₂ (k₁ [NO]²) and its subsequent reaction in the rate-determining step.