What does ΔG represent in ΔG = ΔH - TΔS?

What does ΔG represent in ΔG = ΔH - TΔS?

• A. Gibbs free energy change
• B. Enthalpy
• C. Entropy
• D. Temperature

Answer: (A)

ΔG represents the change in Gibbs free energy, the amount of energy available to do useful work when pressure and temperature are held constant. The equation ΔG = ΔH − TΔS shows how heat exchange (ΔH) and the system’s tendency to become more or less disordered (ΔS) compete: the temperature multiplies the entropy change to give how much energy is tied up in increasing disorder. When ΔG is negative, the process can proceed spontaneously under those conditions; when positive, it tends not to occur unless driven by external input; at equilibrium, ΔG is zero. A helpful way to see this is the melting example: melting ice requires energy input (ΔH is positive) but increases randomness (ΔS is positive); at higher temperature, the TΔS term can overcome ΔH, making ΔG negative and melting favorable. So the quantity described is the Gibbs free energy change. The other terms aren’t what ΔG alone represents: ΔH is the enthalpy change, ΔS is the entropy change, and T is the temperature.

ΔG represents the change in Gibbs free energy, the amount of energy available to do useful work when pressure and temperature are held constant. The equation ΔG = ΔH − TΔS shows how heat exchange (ΔH) and the system’s tendency to become more or less disordered (ΔS) compete: the temperature multiplies the entropy change to give how much energy is tied up in increasing disorder. When ΔG is negative, the process can proceed spontaneously under those conditions; when positive, it tends not to occur unless driven by external input; at equilibrium, ΔG is zero. A helpful way to see this is the melting example: melting ice requires energy input (ΔH is positive) but increases randomness (ΔS is positive); at higher temperature, the TΔS term can overcome ΔH, making ΔG negative and melting favorable. So the quantity described is the Gibbs free energy change. The other terms aren’t what ΔG alone represents: ΔH is the enthalpy change, ΔS is the entropy change, and T is the temperature.