Free Energy

    Free energy is symbolized by the letter G.  G can be defined as

                        G = H - TS

where H is enthalpy, T is the temperature in Kelvin, and S is the entropy.
    When a process is at a constant temperature, then the change of free energy (can be given in the equation

                 G = H - TS

This equation cwill tell us that a process is spontaneous when free energy is negative(-G).   When a process is at equilibrium, then G = 0.

S positve, H negative	Spontneous at all temperatures
S positive, H positive	Spontaneous at high temperatures ( where exothermicity is relatively unimportant)
S negative, H negative	Spontanous at low temperatures (where exothermicity is dominant)
S negative, H positive	Process no spontaeous at any tmperature (reverse process is spontaneous at all temperatures)

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