Activation Energy: Minimum Amount Of Energy

When a reaction goes to completion then the reactant present in the reaction is converted into product by passing through the activated state (or the highest energy level in the reaction energy diagram). And to overcome this activated state (which is the highest energy point in the reaction energy diagram), reactant molecules must have enough energy so that they can be easily converted into products and that much amount of energy required by the reactant molecules to overcome this activated state is called as "Activation Energy". Thus, activation energy is the minimum amount of energy needed to start a chemical reaction.

Thus, we can define the activation energy as;

"Activation Energy is the least possible amount of energy required by the reactant molecules to start a chemical reaction". And activation energy is usually expressed in calories or joules per mole.

For Example

Burning of woods using lighter.

Here, the energy given off by the lighter is nothing but the activation energy.

We can also define activation energy in other ways, such as;

According to the Reaction Energy Diagram as shown in above;

"The activation energy of a reaction is the difference between the energy of a transition state (or activated state) and the average energy of the reactant molecules". 

Or

According to the collision theory;

"Activation energy is the minimum amount of energy that colliding molecules need in order to have successful collisions leading to a chemical reaction".

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