Which variable primarily affects the Gibbs energy of a substance in equilibrium?

The Gibbs energy of a substance in equilibrium is primarily affected by pressure because it plays a critical role in determining how a system behaves at equilibrium, particularly for reactions involving gases. In the context of the Gibbs free energy equation, G = H - TS (where G is Gibbs energy, H is enthalpy, T is temperature, and S is entropy), the pressure can influence both the enthalpy and entropy components, as these depend on the nature of the substances involved and their states.

At constant temperature and pressure, when dealing with reactions or equilibria, an increase in pressure often alters the concentrations of gaseous reactants and products. According to Le Chatelier's principle, changing the pressure will shift the equilibrium position in a direction that counteracts the change in pressure, thereby affecting the state of the system and its Gibbs energy.

While concentration also affects Gibbs energy, it is closely linked to pressure for gaseous systems through the ideal gas law. Thus, pressure has a more direct and significant impact on the Gibbs energy of a system at equilibrium, especially in reactions that involve changes in the number of moles of gas.