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
