Free Energy Also called Gibbs Free Energy Thermodynamic function also related to spontaneity and is useful dealing with the temperature dependence of spontaneity. Symbolized by “G” and for a process that occurs at constant temperature, the change in free energy is given by: ΔG = ΔH - TΔS

Processes carried out at constant temperature and pressure are spontaneous only if ΔG is negative. That is, a process (at constant T and P) is spontaneous in the direction in which the free energy decreases (-ΔG means +ΔSuniv).

H, S, G and Spontaneity G = H - TS H is enthalpy, T is Kelvin temperature Value of H Value of TS Value of G Spontaneity Negative Positive Spontaneous Nonspontaneous ??? Spontaneous if the absolute value of H is greater than the absolute value of TS (low temperature) Spontaneous if the absolute value of TS is greater than the absolute value of H (high temperature)

Free Energy and Chemical Reactions Standard Free Energy Change (ΔGo) Go is the change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states. Go cannot be measured directly. Knowing ΔGo values for reactions can help predict the relative tendency of reactions to occur The more negative the value for Go, the farther to the right the reaction will proceed in order to achieve equilibrium. Equilibrium is the lowest possible free energy position for a reaction.

Calculating Free Energy Change For reactions at constant temperature: There are several ways to calculate ΔGo. Method #1: For reactions at constant temperature: G0 = H0 - TS0

Calculating Free Energy: Method #2 An adaptation of Hess's Law: Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ Cgraphite(s) + O2(g)  CO2(g) G0 = -394 kJ Cdiamond(s) + O2(g)  CO2(g) G0 = -397 kJ CO2(g)  Cgraphite(s) + O2(g) G0 = +394 kJ Cdiamond(s)  Cgraphite(s) G0 = -3 kJ

Calculating Free Energy Method #3 Using standard free energy of formation (Gf0): Gf0 of an element in its standard state is zero