Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 9.2 Activation energy-Study Notes - New Syllabus

9.2 Activation Energy and Successful Collisions

For a chemical reaction to occur, colliding particles must have sufficient energy. This minimum energy requirement is explained by the concept of activation energy.

Activation Energy

Activation energy is the minimum energy that colliding particles must possess for a reaction to occur.

This energy is required to break existing bonds in the reactants before new bonds can form to produce products.

If particles collide with energy less than the activation energy, the collision will not result in a reaction.

Successful and Unsuccessful Collisions 

Collisions between particles can be classified based on whether they lead to a reaction.

Successful Collisions

• Occur when particles collide with energy greater than or equal to the activation energy.
• Particles must also have the correct orientation for bonds to break and form.
• These collisions lead to the formation of products.

Therefore, only successful collisions result in a chemical reaction.

Unsuccessful Collisions

• Occur when particles collide with energy less than the activation energy.
• Collisions may also be unsuccessful if particles are not correctly oriented.
• In these cases, no reaction occurs and particles simply separate.

As a result, many collisions in a reaction mixture are unsuccessful, and only a fraction lead to product formation.

Energy Profile Explanation

• During a reaction, energy must be supplied to reach the activation energy barrier.
• Once this barrier is overcome, bonds can break and new bonds can form.
• The activation energy represents the energy difference between reactants and the transition state.

Therefore, reactions only occur when particles collide with sufficient energy to overcome this barrier.