Calculate the free energy change for the complete combustion of one mole of methane, CH4(g),

We begin by writing the equation that represents this reaction. Recall that "complete combustion," or burning, is a reaction with oxygen from the atmosphere, forming carbon dioxide and water:

CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l) 

ΔG° = (1 mol)[ΔGf° for CO₂(g)] + (2 mol)[ΔGf° for H₂O(l)] – (1 mol)[ΔGf° for CH₄(g)] – (2 mol)[ΔGf° for O₂(g)] = (1 mol) (–394.4 kJ/mol) + (2 mol) (–237.0 kJ/mol) – (1 mol) (–50.8 kJ/mol) – (2 mol) (0) = –817.6 kJ 

The negative value of ΔG° indicates that the reaction is spontaneous. This matches ourexperiences in everyday life, where we have seen that natural gas burns spontaneously.