Define Gibbs free energy.

Gibbs free energy is defined as a potential that assesses the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. This concept is pivotal in thermodynamics because it helps predict the spontaneity of a process. If the change in Gibbs free energy (∆G) is negative, the process can occur spontaneously. In contrast, if ∆G is positive, the process is non-spontaneous, meaning that external work would be needed to drive it. It combines the system's enthalpy and entropy, offering a clear criterion for determining the feasibility of reactions and phase changes under equilibrium conditions.

The other options do not accurately capture the essence of Gibbs free energy. For instance, defining it as a measure of all thermal energy overlooks the specific thermodynamic context in which Gibbs free energy operates. Similarly, while it might relate to heat loss in some situations, characterizing it merely as a calculation of heat loss during a chemical reaction does not encompass its broader implications in assessing work potential. Moreover, indicating that it is a metric used to determine system pressure changes misrepresents the Gibbs free energy concept, as it is more aligned with thermodynamic stability and spontaneous processes rather than just changes in pressure.