Question

For the following reaction, which species is acting as a catalyst? Which is an intermediate? Overall: O3 + O —> 2 O2 Step 1: Cl + O3 —> ClO + O2 Step 2: ClO + O —> Cl + O2

Answer 1

Cl is acting as a catalyst. ClO is an intermediate.

A catalyst is a species that is present at the beginning of a reaction and reappears at the end. It does not appear in the final equation.

An intermediate is not present at the beginning. It forms during the reaction and disappears before the end. It does not appear in the final equation.

Let's apply these concepts to your mechanism.

Step 1: Cl + O₃ → ClO + O₂
Step 2: ClO + O → Cl + O₂
Final: O₃ + O → 2O₂

We see that Cl is present at the beginning of Step 1. It reacts and then forms again in Step 2. It does not appear in the final equation. Cl is a catalyst.

ClO is not present at the beginning. It forms in Step 1 and reacts (disappears) in Step 2. It does not appear in the final equation. ClO is an intermediate.