ABSTRACT
Spontaneous processes In everyday life, a huge number of spontaneous processes occur. Ice melts at 20∞C, a leaf falls from a tree to the ground, but the reverse processes do not happen spontaneously. Indeed, in general it can be said that if a process occurs spontaneously in one direction it cannot also occur spontaneously in the opposite direction. In considering what changes accompany spontaneous processes it is logical to assume that a spontaneous process occurs so as to lower the energy of the system. This certainly helps explain why things fall and do not rise again spontaneously. However, this is not the complete answer. It cannot
explain why, for example, the expansion of an ideal gas, at constant temperature, does not involve a change in its internal energy (Sections O1 and O2). Also, when ice melts the internal energy of the system actually increases. Hence, as energy change cannot predict the direction of spontaneous reactions, a further thermodynamic parameter is required; this is entropy (S).
