The thermodynamic driving force of a reaction is which of the following?

• A. Gibbs Free Energy
• B. Enthalpy
• C. Entropy
• D. Internal Energy

Answer: (A)

The driving force for a reaction under typical lab conditions is captured by the Gibbs free energy change. Spontaneity at constant temperature and pressure is determined by ΔG: if ΔG is negative, the reaction tends to proceed on its own; if ΔG is positive, it tends not to occur without input of energy; and if ΔG is zero, the system is at equilibrium.

Gibbs free energy combines enthalpy and entropy through the relation ΔG = ΔH − TΔS. This shows why both heat flow and disorder matter, but in a single quantity that tells you whether a process will occur without outside help. Enthalpy is part of the story because it tells you about heat exchange, and entropy is about disorder, but neither alone decides spontaneity under constant T and P. Internal energy changes (ΔU) relate to total energy changes, but the Gibbs free energy is the relevant quantity for predicting spontaneity in these common conditions.