Hydrogen iodide is a reactive compound that is sometimes used in the manufacture of methamphetamine. It can be produced by the direct reaction of the elements. Calculate the standard Gibbs free energy of formation of HI(g) at 25°C from its standard molar entropy and standard enthalpy of formation.

PLAN Write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from ΔG° = ΔH° – TΔS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then ΔG° can be identified with ΔGf°.