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The limitations of the First Law of Thermodynamics are: The first law of thermodynamics fails to give the feasibility of the process or change of state that the system undergoes. It fails to explain the direction of heat flow. It doesn't say the process is a spontaneous or non-spontaneous process.

The first law establishes the relationship between work done and heat absorbed while doing that work. However, it fails to specify the direction of heat flow, which is a significant limitation of this law. Example- We cannot extract heat from a cube of ice by cooling it to low temperatures.

The Onsager reciprocal relations have been considered the fourth law of thermodynamics. They describe the relation between thermodynamic flows and forces in non-equilibrium thermodynamics, under the assumption that thermodynamic variables can be defined locally in a condition of local equilibrium.

In an isothermal process all heat is converted into work while in an isobaric process heat is converted into work as well as internal energy. However, the PV-graph shows that the maximum work is done in an isobaric process.