What happens to the sign of standard free energy of formation when a reaction is reversed?

When a chemical reaction is reversed, the standard free energy of formation for that reaction changes sign. This is rooted in the second law of thermodynamics and the properties of Gibbs free energy.

Gibbs free energy (G) is a measure of the spontaneity of a reaction, with negative values indicating that a process can occur spontaneously in the forward direction. If a reaction releases free energy when proceeding forward (i.e., has a negative free energy change), reversing that reaction means we would be imposing a positive change in energy, indicating that the reaction now requires energy input to proceed. Therefore, the standard free energy of formation for the reversed reaction becomes positive if it was originally negative and vice versa.

This characteristic aligns with the fundamental principle: the free energy change (ΔG) for the reverse reaction is equal in magnitude but opposite in sign to that of the forward reaction. Hence, the correct understanding is that when a reaction is reversed, the sign of the standard free energy of formation indeed changes.