text: The van Laar equation defines the excess of this quantity based on component mole fractions.
full_question: The van Laar equation defines the excess of this quantity based on component mole fractions. In proteinfolding calculations, the phi value is a ratio of two forms of this quantity, preceded by two deltas. Thedefinition of exergy is nearly identical to this other quantity. Phase equilibrium occurs where two surfaces forthis property, in different phases, intersect. This quantity uses temperature and pressure as its naturalvariables. Any thermodynamic driving force must cause this quantity to decrease. At standard state, the cellvoltage times nF equals this quantity. It is defined as the enthalpy minus temperature times entropy. For 10points, name this quantity whose change is negative in spontaneous processes, which is symbolized G. 
first_sentence: The van Laar equation defines the excess of this quantity based on component mole fractions.
raw_answer: Gibbs free energy [or G until "symbolized" is read; prompt on free energy; prompt on chemical potential or mu] 