Thermodynamics/Definitions/Limitations

Although thermodynamics is a very powerful tool, we must be aware of the limitations that exist to its application.

The most important applications of thermodynamics are based on one fact:"all natural systems go to equilibrium unless an external force is applied"

And thermodynamics tells us what the system's properties are at equilibrium (see the alternative definition of thermodynamics). In other words, thermodynamics tells us where the system is going.

But it does NOT tells how to get to equilibrium, or even if we can GET to equilibrium.

To think of it in another way we could say that thermodynamics tells us we can go from New York to London. But it does not tells us that we cannot do so by walking.

Examples
A more concrete example is with carbon. Most people know that carbon can exist as either diamond or graphite. Thermodynamics tells us that the most stable form, at room temperature and normal atmospheric pressure is graphite. But of course when we buy a diamond it does appear to change. Actually, it is changing but at an extremely slow rate. It takes millions of years for it to happened.

Many processes in science and engineering have factors that limit the process. Some examples of these are:
 * The ability to remove heat from a process. This can affect the rate of the process or the amount of products produced.
 * Aerobic bacteria need oxygen to process wastewater. The rate at which the oxygen dissolves in water limits the amount of waste the bacteria can "eat" the waste. This is why you use aerators in sewage lagoons.

Conclusions
The basic point is this: If thermodynamic arguments show that a process cannot work, it will not work (we cannot violate the laws of thermodynamics). However, if thermodynamics shows it may be possible, then we must look at other aspects of the process before we can make final conclusions.

This statement sums it well:"If a thermodynamics says no, then it will not work; if thermodynamics says yes, then it might work"

Next Steps
That ends the introduction, We are now ready to go onto the core: the Laws of Thermodynamics.

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