Science with Tara

An energy level diagram shows the energy change between reactants and products in a chemical reaction. It helps us understand whether a reaction is exothermic or endothermic.

The enthalpy change (ΔH) is the difference in energy between the reactants and products, measured in kilojoules (kJ).

Energy Level Diagram for Exothermic Reactions

In an exothermic reaction, heat is released to the surroundings.

• The total energy of the products is lower than that of the reactants.
• Therefore, ΔH is negative (ΔH < 0).

The difference between the energy levels of the products and the reactants is equal to the amount of energy given out during the reaction.

Diagram Description: Reactants are drawn at a higher energy level. An arrow points downwards to the products, indicating energy is lost to the surroundings. The arrow must touch the energy level of reactants and products.

Energy Level Diagram for Endothermic Reactions

In an endothermic reaction, heat is absorbed from the surroundings.

• The total energy of the products is higher than that of the reactants.
• Therefore, ΔH is positive (ΔH > 0).

The difference between the energy levels of the products and the reactants is equal to the amount of energy taken in during the reaction.

Diagram Description: Reactants are drawn at a lower energy level. An arrow points upwards to the products, indicating energy is gained from the surroundings.

Important Note

• Energy level diagrams show only the overall energy change (ΔH) between reactants and products.
• Energy profile diagrams (also called reaction profile diagrams) show both the activation energy and the enthalpy change. These are usually covered in more advanced topics.

Key Points to Remember

• The vertical arrow always starts from the energy level of the reactants and ends at the energy level of the products.
• Exothermic reactions release energy (feel warm).
• Endothermic reactions absorb energy (feel cold).