Question

The activation energies for forward and backward reactions ${E_f}$ and ${E_b}$ respectively.
If the reaction is exothermic then:
A.${E_f} > {E_b}$
B.${E_f} < {E_b}$
C.${E_f} = {E_b}$
D.There is no relation between ${E_f}$ and ${E_b}$ directly.

Answer 1

Write down the formula of change in heat which is the difference between activation energy of forward reaction and activation energy of backward reaction. Then for an exothermic reaction analyze the condition for change in heat and accordingly derive the relation between activation energy of forward reaction and activation energy of backward reaction.

Complete answer:
First let us understand what ${E_f}$ and ${E_b}$ are. Here is a term given activation energy. Activation energy can be understood as the energy required by molecules in the reactant side of a chemical reaction to get converted into a product. It is denoted by ${E_a}$ .
Now ${E_f}$ is activation energy for forward reaction. It is the energy required to drive a reaction in a forward direction that is to the product side. ${E_b}$ is activation energy for backward reaction. It is the energy required to drive a reaction in a backward direction that is to the reactant side.
Now the formula of change in heat is, $\Delta H = {E_f} - {E_b}$ . Now for a exothermic reaction since heat is released in the reaction therefore change in heat should be negative so ${E_f} < {E_b}$ and for a endothermic reaction since heat is absorbed in the reaction therefore change in heat should be positive so ${E_f} > {E_b}$ .
Hence we understood for an exothermic reaction, activation energy of forward reaction must be less than activation energy of backward reaction.

So, option B is the correct answer.

Note:
Activation energy of a chemical reaction is affected by a number of factors some of which are surface area, temperature, pressure, reactant concentration, effect of catalyst and nature of reactant. It is measured in joules per mole (J/mol).