Limits of the First Law of Thermodynamics

Spontaneous processes, like a rock falling to the ground or sodium reacting with chlorine, happen without any external work and often involve a decrease in system energy. Most spontaneous processes release energy, but not all. Some processes can absorb energy and still be spontaneous.

For example, certain endothermic processes, like the dissolution of sodium chloride in water, happen spontaneously despite an increase in energy.

This shows that the first law of thermodynamics, which focuses only on energy changes, cannot fully predict the spontaneity of a process.

As a result, the second law becomes necessary as it provides direction for energy transfer and predicts the process's spontaneity.

Also, the second law helps in calculating the maximum fraction of heat that can be converted into work during a process.

It introduces entropy – a macroscopic property that reaches its maximum in the equilibrium state of an isolated system.

Entropy arises from the total energy dispersed in an isolated system, providing insights into spontaneous and non-spontaneous processes.