Question

Consider the following mechanism

step 1: A+B --> AB

step 2: AB+C --> AC+B

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overall: A+C --> AC

1. Which species is an intermediate?

2. Which species is a catalyst?

Answer 1

1. AB is the intermediate.

An intermediate is any species which is believed to be formed during the course of a reaction but they do not appear in the final products. Clearly AB is formed in the first step and consumed in the second step. Hence, AB is an intermediate.

2. B is the catalyst. It reacts with the reactant A and is later formed as a byproduct and released.

A catalyst takes part in the reaction but is regenerated without any chemical change at the end of a reaction. A catalyst increases the rate of reaction by forming the intermediate complex which has much lower energy.