Potassium Chlorate(VII) decomposes to produce Potassium Chloride and Oxygen. Using the following data calculate the enthalpy change of this decomposition: Enthalpy of formation(KClO4) = -430 kJ mol-1, Enthalpy of formation(KCl) = -440 kJ mol-1

Firstly the equation of this decomposition should be worked out to be the following
KClO₄(s) --> KCl(s) + 2O₂(g)
The enthalpy of formation has been given for both KClO₄ and KCl, so from this we know that Hess's Law should be used. This states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes, so allows the following enthalpy triangle to be formed.
KClO₄(s) --> KCl(s) + 2O₂(g) ^- K(s) + Cl₂(g) + 2O₂(g) -^This then allows the enthalpy of reaction to be calculated, noting that O₂ doesn't change between the formation and products so it's enthalpy of formation isn't needed.
Enthalpy of reaction = Enthalpy of formation(KCl) - Enthalpy of formation (KClO₄) = -440 kJ mol^-1 + 430 kJ mol^-1= -10 kJ mol^-1