Thermodynamics and Equilibrium

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Thermodynamics (from the Greek thermos meaning heat and dynamis meaning power) is a branch of physics that studies the effects of changes in temperature, pressure, and volume on physical systems at the macroscopic scale by analyzing the collective motion of their particles using statistics.

Heat is something that is observed by everyone. Warmth and coolness, combustion and oxidation - but Modern thermodynamics is concisely the understanding that heat is the result of the motion of molecules.

Thermodynamics became the unification of the concept of heat with the motion of molecules from investigations of the problem of how to increase the efficiency of early steam engines. It resulted in the understanding of statistical principles of molecules that are profound.

For the purposes of an instructive text, it will be useful to study story of efficiency in heat engines along with examples from physical chemistry in parallel.


The Heat Engine[edit]

It turns out that all engines and many other devices that do work are also heat engines. That is they can be seen as functioning by transferring heat from one place to another or generating heat while generating heat.

It not only turns out that engines are the subject of thermodynamics, but also chemical reactions too.

A Simple Steam Engine[edit]

To get started, we will focus only on steam engines and see that modern thermodynamics can be seen from this example.

We'll start with a steam engine designed to pump water. You have a closed vessel of water over a flame - when the water boils, steam is produced and goes from the water vessel through a hose to a cylinder. When the steam expands into the cylinder, it will push the cylinder along from a closed to open position, from a small volume to a larger volume.

We'll put a stick on the end of the cylinder and use that to turn a wheel on a pump to pump the water. When the cylinder has gone to its furthest open position (i.e. it has expanded enough) we'll also have the wheel release the steam and push the cylinder back to its starting (or closed) position, close the valve and allow more steam to come in, we need the momentum of the wheel to close the piston. When the piston has returned to its original position, the valve moves to allow in more steam and the cycle starts again.

Looking at this arrangement, we can see that what is driving the piston is the hot steam. When the steam is hotter, the piston will drive with more force and pump more water.

The reason that this engine works is the difference in the temperature of the steam to that of the outside world. We often forget that if the temperature outside the piston were the same as inside,

A Combustion Engine[edit]

Chemical Thermodynamics[edit]

The Four Laws of Thermodynamics[edit]

Conservation of Energy[edit]

Heat is a form of energy and energy is conserved. This may seem obvious to some of us - Because heat can come from chemical reactions, like the flame from combustion, it can appear to come from nowhere.

Entropy[edit]

The spontaneous flow of heat from one body to another always goes in the same direction, from warmer to colder bodies. When we understand that heat is the result of molecular motion, we can express the flow of thermal energy as the expansion of entropy.

Zero Temperature[edit]

A profound result of the understanding of thermal energy as molecular motion is that there is a condition where there is minimal molecular motion (zero point motion), known as absolute zero. However, owing to the uncertainty principle, a molecule will never be absolutely still.

Chemical thermodynamics[edit]

Phase properties of water[edit]

Phase properties of other substances[edit]