First law of thermodynamics states that during any cycle that a system undergoes, the cyclic integral of the heat equals cyclic integrals of the work. The first law, however, places no restriction on the direction of flow of heat and work. A cycle in which a given amount of heat is transferred from the system and an equal amount of work is done on the system satisfies the first law just as well as a cyclic in which the flow of heat and work are reversed. However, we know from our experience that a proposed cycle that does not violate the first law does not ensure that the cycle will actually occur. It is a kind of experimental evidence that led to the formulation of the second law of thermodynamics. Thus a cycle will occur only if both the first law and second law of thermodynamics are satisfied.

Consider a hot water glass left in a cooler room that eventually cools off. This process satisfies the first law of thermodynamics as the amount of energy lost by hot water equals amount of heat gained by the surroundings. If we consider a reverse process, i.e., hot water getting even hotter in a cooler room as a result of heat transfer from the room air. It is obvious that this process never takes place. Furthermore, this process does not violet first law of thermodynamics as long as amount heat lost by air equals the amount of heat gained by hot water

Gasoline is  used as a car drives up hill, but the fuel in gasoline tank cannot be restored to it’s original level when car coasts down the hill. However, this process as well does not violate first law of thermodynamics.

These arguments clarify that processes proceed in a certain directions and not in the reverse direction. The first law of thermodynamics places no restriction on the direction of a processes and satisfying the first law of thermodynamics does not ensure that the process can actually occur. This inadequacy of the first law of thermodynamics to identify whether a process can take place in remedied by introducing another general principle, called second law of thermodynamics. The reversed processed discussed in above examples violet the second law of thermodynamics.