Question

For a reversible reaction where the forward reaction is exothermic, which of the following statements is correct?
A) The backward reaction has higher activation energy than the forward reaction.
B) The backward and forward processes have the same activation energy.
C) The backward reaction has lower activation energy.
D) No activation energy is required at all since energy is liberated in the process.

Answer 1

Hint: For a reversible reaction, the chemical equilibrium involves the two opposing reactions: one favouring g product and the other favouring reactant. The higher the energy released by the reaction, the less than the energy the reaction required to cross the energy barrier. The energy required to cross the energy barrier is the activation energy. If forward reaction is exothermic then the backward reaction is endothermic.

Complete step by step answer:
If equilibrium is subjected to stress, the equilibrium shifts in such a way as to reduce the stress. This is called Le Chatelier’s principle.
According to the principle, the equilibrium can be affected by temperature, concentration, and pressure. In chemical equilibrium, increasing the concentration of the reactants results in the shifting of the equilibrium in favour of the product while an increase in the concentration of the product results in the shifting of the equilibrium favours of reactants.
For a reversible reaction, one is the forward reaction and the other is considered as the backward reaction. The forward reactions favour the formation of products and the backward reaction favours the formation of reactants.
If one of the reactions is exothermic i.e. accompanied with the release of heat then the other must be endothermic i.e. accompanied by the absorption of heat. Such that the reactions follow the law of conservation of energy.
We are given a reversible reaction where the forward reaction is exothermic. Exothermic reactions release higher energy. Thus the forward reaction requires less amount of energy to form the products.
Since we know that the activation energy is the minimum amount of energy possessed by the reactants to undergo the reaction. Exothermic reactions release energy thus they require low activation energy.
The backward reaction will be endothermic which absorbs the energy thus requiring a large amount of energy for the reaction. Therefore the activation energy associated with the backward reaction is higher than the forward reaction.
Hence, the correct statement is the backward reaction has higher activation energy than the forward reaction.

Hence, (A) is the correct option.

Note: The activation energy is related to the temperature .and the rate of the reaction is related to the temperature. Thus one can say that if the reaction absorbs the energy it requires more energy to cross the barrier and thus slowing the rate of reaction.