Limitations of the first law of thermodynamics: There can be thermodynamic processes that are consistent with the first law but not observed in nature.
(1) According to the first law of thermodynamics, heat and work are interconvertible. But, the law does not tell us whether a particular process can or cannot occur. For example, we see a net transfer of energy (heat) from a hot body to a cold body but never the other way.
(2) The law does not rule out total (i.e., 100 %) conversion of heat into work or work into heat. However, this is impossible in practice. For example, it is impossible to design a heat engine that can completely convert heat into work, i.e., no heat engine is 100 % efficient. Similarly, it is impossible to remove heat from a system without doing some work on the system, i.e., the coefficient of performance of a refrigerator can never be infinite.
(3) Diffusion of two dissimilar inert gases is an irreversible process. For example, we never see the reverse process of a gaseous mixture separating into separate chambers, although such a process would not violate the first law.
