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