Direction of the Spontaneous Processes

Direction of the spontaneous processes is determined by the second law of thermodynamics. Under this law, such processes are always accompanied by the increase in entropy of the system to values appropriate to its equilibrium state. 

Let us imagine a volume of water, of which one half is heated and the other half is not. Thus, solution equilibrium is disrupted. Spontaneous process first of all will level off temperatures. It is accompanied by the heat AQ exchange between the hot and cold portions of the solution. Finally, temperature difference will disappear without performing any useful work. But at that entropy of the heated portion decreases by value-AQ/T, and of the cold portion increases by +AQ/T, . As T, total entropy of the entire system (water) increases by 

Similarly, any spontaneous process is accompanied by an increase in the systems entropy, namely, i.e., disorderliness. H O dipoles are most orderly in a piece of ice. At transition of ice into the Uquid state entropy of the solution increases by a factor of 1.5, and at the transition into the vapour state, again by the factor of 4. 

Because of this, always, at any irreversible spontaneous process, due to heat dissipation, dS 0. In conditions of a reversible process, i.e., stable thermodynamic equilibrium, heat does not dissipate, and entropy value does not change. Such processes, at which dS = 0, are called adiabatic. Overall, for all spontaneous processes is valid inequation 

At one and the same heat expenditure the work of a reversible process is always greater than the work of an irreversible process. Therefore, the work of a reversible process characterizes the maximum work, which may be obtained on account of heat expenditure Q, and that is why it is called maximum work. 

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