The beauty of the equation defining the free energy of a system is its ability to determine the relative importance of the enthalpy and entropy terms as driving forces behind a particular reaction. The change in the free energy of the system that occurs during a reaction measures the balance between the two driving forces that determine whether a reaction is spontaneous. As we have seen, the enthalpy and entropy terms have different sign conventions. The entropy term is therefore subtracted from the enthalpy term when calculating G o for a reaction. Because of the way the free energy of the system is defined, G o is negative for any reaction for which H o is negative and S o is positive.
G o is therefore negative for any reaction that is favored by both the enthalpy and entropy terms.
We can therefore conclude that any reaction for which G o is negative should be favorable, or spontaneous. Conversely, G o is positive for any reaction for which H o is positive and S o is negative. Any reaction for which G o is positive is therefore unfavorable. Free energy calculations become important for reactions favored by only one of these factors. Click here to check your answer to Practice Problem 6.