Understand the use of ΔH to represent enthalpy change for exothermic and endothermic reactions

The symbol ΔH is used to represent the enthalpy change of a reaction.

A negative ΔH means it is an exothermic reaction.

A positive ΔH means it is an endothermic reaction.

Represent exothermic and endothermic reactions on a simple energy level diagram

Understand that the breaking of bonds is endothermic and that the making of bonds is exothermic

Endothermic is the breaking of bonds.

Exothermic is the formation of bonds.

Use average bond energies to calculate the enthalpy change during a simple chemical reaction.

1. Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
2. Add together the bond energies for all the bonds in the products– this is the ‘energy out’.
3. Calculate the energy change: energy in – energy out.