Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 9.6 Reaction profiles-Study Notes - New Syllabus

9.6 Reaction Profiles for Catalysed and Uncatalysed Reactions

Reaction profiles (energy profile diagrams) show how the energy of a system changes during the course of a reaction.

Reaction Profile

A reaction profile is a graph of energy against reaction progress.

• The y-axis represents energy.
• The x-axis represents the progress of the reaction.
• The curve shows the energy changes as reactants are converted into products.

Uncatalysed Reaction Profile

• The curve rises to a single high peak.
• This peak represents the activation energy.
• A large amount of energy is required to reach the transition state.
• After the peak, the energy decreases to form products.

The height of the peak from the reactants represents the activation energy of the uncatalysed reaction.

Catalysed Reaction Profile

• The catalysed pathway has a lower activation energy.
• The curve usually has two smaller peaks instead of one large peak.
• This is because the reaction proceeds in multiple steps.
• An intermediate is formed between the steps.

Intermediate

An intermediate is a species formed during the reaction which is then used up in a later step.

• It appears as a dip between the two peaks on the reaction profile.
• Its energy level lies between the two activation energy peaks.
• It is not present in the overall equation of the reaction.

Comparison of Catalysed and Uncatalysed Profiles

• The catalysed pathway has a lower maximum activation energy.
• The uncatalysed pathway has a higher single peak.
• Both pathways start and end at the same energy levels.
• Therefore, the enthalpy change of the reaction is unchanged.

Key Features to Include in a Diagram

• Label axes: energy and reaction progress.
• Show reactants and products clearly.
• Draw a higher curve for the uncatalysed pathway.
• Draw a lower, two-step curve for the catalysed pathway.
• Indicate activation energies.
• Show the intermediate between the two peaks.

Therefore, catalysts provide a different reaction route involving intermediates and lower activation energy.