Question

Which species is an intermediate? Which species is a catalyst?

Answer 1

An intermediate is a species that is formed during the reaction and disappears before the end.

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

For example let us consider the reaction between O_{​​​​​​3} and O to form O_{​​​​​​2}.

Step 1: Cl + O_{​​​​​​3} ClO + O_{​​​​​​2}

Step 2: ClO + O Cl + O_{​​​​​​2}

Final reaction: O_{​​​​​​3} + O 2O₂

Here Cl is present in the beginning of reaction and reappears in the end and is not involved in the final reaction. Thus Cl is a catyst.

ClO is the intermediate here as it is formed during the reaction and disappeard at the end.