A spontaneous process is one that proceeds by itself. Such a process may not occur without the addition of a certain amount of energy, such as the combustion of a fuel. However, once it has begun, the reaction will proceed until the reactants are exhausted or the system reaches a state of equilibrium. A non-spontaneous process is the opposite of a spontaneous process. It will only proceed if energy is supplied to the system. However, the moment the energy supply is removed, the reaction will cease. The amount of useful work that can be obtained from a spontaneous reaction is limited by the difference in free energy between the products and reactants. If the free energy of a system is negative, the reaction at constant temperature and pressure can produce useful work, and is said to be spontaneous. If the free energy change of a system...