Question

step 1: no(g) + o₃(g) → no₂(g) + o₂(g)
step 2: no₂(g) + o(g) → no(g) + o₂(g)
a reaction mechanism for the destruction of ozone, o₃(g), is represented above. in the overall reaction, no(g) is best described as
a an inhibitor
b a catalyst
c a reactant
d an intermediate

Explanation:

Step 1: Combine the two reactions

To find the overall reaction, we add Step 1 and Step 2:
$$\text{Step 1: } \ce{NO(g) + O3(g) -> NO2(g) + O2(g)}$$
$$\text{Step 2: } \ce{NO2(g) + O(g) -> NO(g) + O2(g)}$$
Adding them together, we cancel out the species that appear on both sides. $\ce{NO(g)}$ is a reactant in Step 1 and a product in Step 2, and $\ce{NO2(g)}$ is a product in Step 1 and a reactant in Step 2. So the overall reaction is:
$$\ce{O3(g) + O(g) -> 2O2(g)}$$

Step 2: Analyze the role of $\ce{NO(g)}$

A catalyst is a substance that increases the rate of a reaction without being consumed in the overall reaction. In the overall reaction, $\ce{NO(g)}$ is not a reactant (since it's regenerated) and not a product (since it's used and then regenerated). It participates in the reaction mechanism but is present at the start and end of the overall reaction (since it's a reactant in Step 1 and a product in Step 2, so net change is zero). An inhibitor slows the reaction, a reactant is consumed, and an intermediate is produced and then consumed. Since $\ce{NO(g)}$ is regenerated and speeds up the reaction (by providing an alternative mechanism), it acts as a catalyst.

Answer:

B. a catalyst