Question

Under what circumstances can ?G be negative if ?G'

Answer 1

For a reaction at constant temperature and pressure, ?G in the Gibbs free energy is:

The sign of ?G depends on the signs of the changes in enthalpy (?H) and entropy (?S), as well as on the absolute temperature (T, in kelvin). ?G changes from positive to negative (or vice versa) where T = ?H/?S.

For heterogeneous systems where all of the species of the reaction are in different phases and can be mechanically separated, the following is true.

When ?G is negative, a process or chemical reaction proceeds spontaneously in the forward direction.

When ?G is positive, the process proceeds spontaneously in reverse.

When ?G is zero, the process is already in equilibrium, with no net change taking place over time.

We can further distinguish four cases within the above rule just by examining the signs of the two terms on the right side of the equation.

When ?S is positive and ?H is negative, a process is always spontaneous

When ?S is positive and ?H is positive, the relative magnitudes of ?S and ?H determine if the reaction is spontaneous. High temperatures make the reaction more favorable, because exothermicity plays a small role in the balance.

When ?S is negative and ?H is negative, the relative magnitudes of ?S and ?H determine if the reaction is spontaneous. Low temperatures make the reaction more favorable, because exothermicity is important.

When ?S is negative and ?H is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous.