Question

In an exothermic reaction if \[\Delta {\text{H}}\] is the enthalpy, then activation energy is:
A.More than \[\Delta {\text{H}}\]
B.Less than \[\Delta {\text{H}}\]
C.Equal to \[\Delta {\text{H}}\]
D.None of the above

Answer 1

Hint: For exothermic reactions, the value of enthalpy is negative. The activation energy of a reaction is the difference between the threshold energy of the reactants and the energy of the activated complex. We can calculate the activation energy and the enthalpy change of any reaction from its energy diagram.

Complete step by step answer:
In exothermic reactions, the heat energy is evolved when the reaction occurs. This lowers the enthalpy. Consequently, the enthalpy of products is less than that of the reactants. Thus, exothermic reactions are accompanied by evolution of energy and have negative values of \[\Delta {\text{H}}\]. But even though they release energy, they need a small amount of energy to initiate the reaction.
During a chemical reaction, some bonds are broken and some bonds are created. When the reactant molecules absorb energy, their bonds are loosened and new loose bonds are formed between them. In other words, an intermediate stage is formed. It is called an activated complex or transition state complex. It exists at a higher energy level than the reactants and is unstable. The energy which is necessary to reach this state is the activation energy.Reaction coordinate represents the reaction profile, i.e., the progress of the reaction from reactants to products accompanied by changes in potential energy. In an exothermic reaction, the reactants lie at a higher energy level than the products, i.e., the products are more stable than the reactants.
Since the exothermic reaction is forced to proceed towards more stable species, the overall enthalpy change is negative, i.e., the energy is released in the form of heat. When the activated complex dissociates into products, it is accompanied by release of energy. But, \[\Delta {\text{H}}\] during the reaction at a particular temperature remains constant. Hence, the activation energy must be more than \[\Delta {\text{H}}\].

So, the correct option is A.

Note:
Endothermic reactions are accompanied by absorption of energy and have positive value of enthalpy change. In case of endothermic reactions, the reactants lie at a lower energy level than the products, i.e., the products are less stable than the reactants. Since the endothermic reaction is forced to proceed towards more unstable species, the overall enthalpy change is positive, i.e., the energy is absorbed in the form of heat from the surroundings.