The mathematical expression for the change in free energy of a system is ΔG =ΔH - TΔS. Which of the following is correct? The mathematical expression for the change in free energy of a system is ΔG =ΔH - TΔS. Which of the following is correct? T is the temperature in degrees Celsius. ΔH is the change in entropy, the energy available to do work. ΔS is the change in enthalpy, a measure of randomness. ΔG is the change in free energy.