One application of Hess's Law (which works for ΔH, ΔS, and ΔG) is calculating the overall energy of a reaction using standard energies of formation of products and reactants. What is the standard change in free energy for the reaction: 4 KClO3(s) → 3 KClO4(s) + KCl(s)? Use the following standard gibbs energies of formation: ΔGf(KClO3(s)) = -290.9 kJ/mol; ΔGf(KClO4(s)) = -300.4 kJ/mol; ΔGf(KCl(s)) = -409 kJ/mol

The standard change in free energy for the reaction =  - 437.5 kj/mole

Explanation:

The standard change in free energy for the reaction:

                              4 KClO₃ (s) → 3 KClO₄(s) + KCl(s)

Given that   ΔGf(KClO3(s)) = -290.9 kJ/mol;

                    ΔGf(KClO4(s)) = -300.4 kJ/mol;

                    ΔGf(KCl(s)) = -409 kJ/mol

According to Hess's law

ΔGr (Free energy change of reaction)= ∑(Product free energy - reactant free energy)

               ⇒ ΔGr⁰ = {3 x (-300.4) + (-409)} - {3 x (- 290.9)}

                            = - 901.2 - 409 + 872.7

                            =  - 437.5 kj/mole

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