Activation energy

Activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius to describe the minimum energy which must be available to a chemical system with potential reactants to result in a chemical reaction. Activation energy may also be defined as the maximum energy required to start a chemical reaction.

Activation Energy Formula

The activation energy is usually represented by the symbol Ea in mathematical expressions for such quantities as the reaction rate constant,

k = Aexp(−Ea/RT),


k = Rate Constant

A = Arrhenius Constant

Ea = Activation Energy

R = Gas constant = 8.34J/K/mol

and the diffusion coefficient,

D =Doexp(−Ea/RT).

Activation energies are determined from experimental rate constants or diffusion coefficients that are measured at different temperatures.

SI Unit for Activation Energy

It is denoted by Ea. It is usually measured in joules (J) and or kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol).

Factors Affecting Activation Energy

The following two factors Affecting Activation Energy.

1. Nature of Reactants

In the case of ionic reactant, the value of (Ea) will be low because there is an attraction between reacting species. While in the case of covalent reactant the value of Ea will be high because energy is required to break the older bonds.

2. Effect of Catalyst

Positive catalyst provides such an alternate path in which the value of Ea will be low, while the negative catalyst provides such an alternate path in which the value of Ea will be high.


To start a car engine, the activation energy is needed means when we are turning the key causes a spark which activates the burning of gasoline in the engine.

The combustion of gas can not occur without the spark of energy to begin the reaction.