Introduction to Gibbs free energy | Applications of thermodynamics | AP Chemistry | Khan Academy

Gibbs Free Energy is denoted by G, and its change by ΔG. The formula for ΔG is ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. A negative ΔG indicates a thermodynamically favored forward process (spontaneous), while a positive ΔG indicates the reverse process is favored (non-spontaneous in the forward direction). If ΔG is zero, the system is at equilibrium.

When a superscript naught (ΔG°) is added, it refers to the change in Gibbs Free Energy under standard state conditions. For solids and liquids, the standard state is the pure substance at one atmosphere pressure. For gases, it's the pure gas at one atmosphere pressure. For solutions, it's a one molar concentration. If substances are in their standard states, the equation becomes ΔG° = ΔH° - TΔS°. If ΔG° is less than zero, the reaction is thermodynamically favored in the forward direction under standard conditions.

The video demonstrates how to calculate ΔG° for the synthesis of hydrogen fluoride gas. Given ΔH° = -537.2 kJ/mol and ΔS° = 13.7 J/K mol at 25°C, first convert the temperature to Kelvin (298 K) and ΔS° to kilojoules (0.0137 kJ/K mol). Plugging these values into the formula yields ΔG° = -541.3 kJ/mol. Since ΔG° is negative, the forward reaction is thermodynamically favored under standard conditions, meaning hydrogen and fluorine gas will combine to form more hydrogen fluoride.