Gibbs free energy: A thermodynamic concept that combines both enthalpy and entropy changes to determine whether a process or reaction will be spontaneous or nonspontaneous.
Reversible reaction: A reaction that can proceed in both the forward and reverse directions. If the reverse reaction is more favorable, then the overall reaction may be nonspontaneous.
Standard conditions: Specific reference conditions (e.g., temperature, pressure) at which thermodynamic properties are measured. A chemical reaction might be spontaneous under normal conditions but become nonspontaneous under different conditions.