In thermodynamics, what does a negative ΔG indicate?

A negative ΔG, or Gibbs free energy change, signifies that a reaction is spontaneous under constant temperature and pressure conditions. Spontaneity in this context means that the reaction can occur without the need for external energy input.

The Gibbs free energy equation, ΔG = ΔH - TΔS, integrates both enthalpy (ΔH) and entropy (ΔS) changes of the system, along with the temperature (T) in Kelvin. When ΔG is negative, it indicates that the total energy available for work after accounting for entropy changes is favorable, thus encouraging the reaction to proceed towards the products.

This is a crucial concept in thermodynamics; spontaneous reactions are characterized by their ability to move towards equilibrium naturally, and they can occur at varying rates. Hence, a negative ΔG effectively confirms that the forward reaction is thermodynamically favored and potentially occurs spontaneously, setting it apart from scenarios where ΔG is zero (indicating equilibrium) or positive (indicating nonspontaneity).