Activation energy is the least potential energy required to start a chemical reaction. The molecules need some kinetic energy or velocity to collide with other molecules to start a reaction. No reaction will occur if the collision doesn’t happen or molecules don’t have enough kinetic energy. An energy needed to start the reaction is defined as Activation energy.
The activation energy (Ea) of a reaction is measured in joules(J), kilojoules per mole(kJ/mol), or kilocalories per mole (kcal/mol)