Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 9.5 Catalysts and activation energy-Study Notes - New Syllabus

9.5 Catalysts and Activation Energy

Catalysts increase the rate of a chemical reaction by providing an alternative reaction pathway.

Definition of a Catalyst

A catalyst is a substance that increases the rate of a reaction without being used up in the process. 

Alternative Reaction Pathway

A catalyst provides an alternative route for the reaction to occur.

• This alternative pathway has a lower activation energy.
• As a result, more particles have sufficient energy to react.
• Therefore, the number of successful collisions increases.

This leads to an increase in the rate of reaction.

Effect on Activation Energy

The catalyst lowers the activation energy required for the reaction.

• This means a greater proportion of particles can overcome the energy barrier.
• The overall energy change of the reaction remains unchanged.

Therefore, the catalyst affects the rate of reaction but does not affect the enthalpy change.

Key Points about Catalysts

• Catalysts are not consumed and remain chemically unchanged at the end of the reaction.
• Only a small amount of catalyst is needed.
• Catalysts speed up both the forward and reverse reactions equally.
• Catalysts do not change the position of equilibrium.

Energy Profile Explanation

• On an energy profile diagram, the catalysed pathway has a lower peak than the uncatalysed pathway.
• This lower peak represents the reduced activation energy.
• The energy difference between reactants and products remains the same.

Therefore, catalysts increase the rate by lowering activation energy without changing the overall energy change of the reaction.