Q. What is Activation Energy?

Ans.

Colliding molecules which overcome the activation energy form an activated complex. In the activated complex, original bonds are partially broken, and new bonds are partially formed. The activated complex is unstable and may change to the products or back to the reactants.

For a reaction with low activation energy (see Ea' in above graph), a higher proportion of molecules will have the necessary energy. Hence the reaction rate will be faster. Conversely, for a reaction with high activation energy, reaction rate will be slower.

For example, let us consider the reaction NO₂ + CO -> NO + CO₂

There are two activation energies for this reaction, as shown in the graph below. E_af is the activation energy for the forward reaction. E_ar is the activation energy for the reverse reaction. In this reaction, the difference between E_af and E_ar is 226 kJ/mol and is called the heat of reaction.

The heat of reaction can also be defined as the difference between the heat content of the reactants and the heat content of the products.

The activation energy for a reaction is usually positive since there is an energy barrier to overcome.