Question

The gases hydrogen and chloride do not react with each other even if they are kept together far a long time, but when they are kept in sunlight they redily react. what is the reason

Answer 1

All reactions require a certain amount of energy to get started, which is known as the activation energy. When hydrogen and chlorine gas are kept at room temperature, they do not have enough energy to readily reach with each other and therefore will not react even after a long time.
But when kept in sunlight, they obtain the required activation energy and the reaction can proceed to form hydrogen chloride.