G is a measure of the non-expansion work that a system can perform on the surrounding in a given process, and ∆G is the maximum non-expansion work that can be extracted from the system. Furthermore, ∆G can determine the spontaneity of a process. Principally, a process that releases more energy than it receives is spontaneous, and vice versa. Therefore, the absolute value of T ∆S sys should be larger than T ∆S sur as we discuss in the following.