Energy Level Diagrams (AQA A-Level Chemistry): Revision Notes

1.6.3 Energy Level Diagrams

The enthalpy change ($ΔH$) represents the energy difference between the reactants and the products. It can be either:

• Endothermic: Energy is absorbed from the surroundings, and $ΔH$ is positive.
• Exothermic: Energy is released to the surroundings, and $ΔH$ is negative.

Features of Energy Level Diagrams

Reactants and Products

• The vertical axis shows the energy levels.
• Reactants are placed on the left side and products on the right side.
• The relative heights of the reactants and products determine if the reaction is exothermic or endothermic.

Activation Energy (Ea)

• Activation energy is the minimum energy required to start a reaction.
• It is represented by a peak between the reactants and the products.
• The height of this peak indicates how much energy is needed to break the initial bonds and begin the reaction.

Exothermic Reaction

• In an exothermic reaction, the products are at a lower energy level than the reactants.
• The difference in height represents the energy released.
• $ΔH$ is negative because energy is released to the surroundings.

Endothermic Reaction

• In an endothermic reaction, the products are at a higher energy level than the reactants.
• The energy absorbed from the surroundings results in $ΔH$ being positive.

Visualising Exothermic and Endothermic Reactions

Exothermic Reaction Diagram

• Initial state: Reactants start at a higher energy level.
• Transition state: A peak (representing the activation energy) is reached, showing the energy required to initiate the reaction.
• Final state: Products are at a lower energy level, indicating that energy has been released.

Endothermic Reaction Diagram

• Initial state: Reactants begin at a lower energy level.
• Transition state: A peak still represents the activation energy.
• Final state: Products are at a higher energy level, demonstrating that energy was absorbed during the reaction.

Activation Energy in Energy Level Diagrams

• Ea (Activation Energy) is critical for understanding how a reaction starts.
• For both exothermic and endothermic reactions, a certain amount of energy (Ea) must be provided to break the bonds of the reactants and initiate the process.
• The peak between the reactants and products on the energy diagram represents the activation energy barrier.

Catalysts and Energy Level Diagrams

A catalyst provides an alternative pathway for the reaction with a lower activation energy. In an energy level diagram:

• The peak for the activation energy is lowered, indicating that less energy is needed to start the reaction.
• The catalyst does not alter the reactants, products, or the overall $ΔH$ of the reaction. It only affects the activation energy (Ea).

Example Energy Level Diagram Interpretation

For a combustion reaction (which is exothermic):

• The reactants (fuel and oxygen) are at a higher energy level than the products (carbon dioxide and water).

• The energy level diagram shows a significant drop in energy as the products are formed, with a large negative $ΔH$ representing the energy released. For a photosynthesis reaction (which is endothermic):

• The reactants (carbon dioxide and water) are at a lower energy level than the products (glucose and oxygen).

• The diagram would show a rise in energy, as energy is absorbed, resulting in a positive $ΔH$.