The change in Gibbs Free Energy for a reaction is defined as the difference between the Gibbs Free Energy of products and reactants. Which expression correctly represents this change?

• A. G(reactants) - G(products)
• B. G(products) + G(reactants)
• C. G(products) - G(reactants)
• D. G(reactants) - G(products) plus constants

Answer: (C)

The change in Gibbs free energy for a reaction at constant temperature and pressure is defined as ΔG = G_products − G_reactants. This subtraction tells you how much free energy is gained or lost as reactants are converted into products. If ΔG is negative, the reaction can proceed spontaneously under those conditions; if positive, it requires energy input; if zero, it sits at equilibrium.

So the expression that matches this definition is G(products) − G(reactants). The other options either reverse the order and flip the sign, add the energies instead of taking a difference, or include extra constants that aren’t part of the basic definition.