The first law of thermodynamics enunciates that heat and work are mutually convertible, but the law does not concern itself with the extent of this convertibility. In other words, if one were to be guided by the first law alone, then every process involving the conversion of energy should be feasible, provided that energy is neither created nor destroyed. Thus according to this law, in every cyclic process work may be completely converted into heat or heat may be completely converted into work; heat may flow from a hot to a cold or from a cold to a hot body; a gas may expand from high pressure to low pressure or contract from low pressure to high pressure. It is known from our experience that not all of this is true. In natural processes, work is not completely converted into heat; heat flows from a hot to a cold body, but the reverse flow is not possible; a high-pressure gas expands to low pressure, but the opposite process does not occur. It is true that these spontaneous processes can be reversed, but not on their own. Some external agency is required to make the reverse process occur. A piece of ice when kept in the atmosphere absorbs heat and melts but it will not solidify and give back heat without any work being performed on it.
The examples cited above demonstrate that the first law has some limitations, and it cannot predict whether the system will spontaneously undergo a change, i.e., whether a certain process is possible or not. Experience shows that a spontaneous process can proceed in a particular direction only. The first law does not specify this direction. Neither does it provide any information regarding the conditions under which no net process can take place at all, i.e., when a system is in equilibrium. The first law merely states that work transfer during a cycle is equal to the heat transfer and does not impose any restriction on the direction of heat and the work transfer. It is the second law of thermodynamics which provides answers to the questions that have been raised. Like the first law, the second law is also a generalization of experience and is essentially a statement of the fact that certain processes which are wholly consistent with the first law nevertheless do not occur. Although the second law can be stated in a number of ways, all statements can be shown to generalize the knowledge that natural processes tend to go to a state of equilibrium. The second law totals our experiences with equilibria, just as the first law did for our experience with energy. The general enumerations of the second law, like the conservation-of-energy enumeration of the first law, are not immediately applicable to chemical problems. The following presentation of general statements of the law, will show that the law can be expressed in
a practically useful form.