Entropy.. Amount of disorder?

Question

How do you determine the amount of entropy in groups of molecules?

Answer 1

There is no way to measure entropy directly... its presence as a factor was first theorized in connection with irreversible heat loss and its effects are most closely quantified by chemists. How chemists determine the amount that is there is pretty technical, but I'll give it a whack.

Basically, in order for a chemical reaction to go, it has to be energetically favourable overall. You wouldn't expect a ball to roll uphill, or even start rolling if it was stationary on a flat surface... it only rolls down. Chemistry works the same way.

Most of what makes a reaction 'go' is heat. You can burn sugar, wood, coal, or a lot of other things and you get much more heat out than in. Some reactions produce physical work too, they cause something to expand against pressure (which is how dynamite makes an explosion... it's just a VERY fast reaction that produces a lot of gas that pushes other stuff out of the way). But once tools became precise enough, it was found that once you added up all the heat and other forms of work out versus in, many reactions just didn't add up. Most famous among these are endothermic reactions - ones that make everything COLDER instead of hotter (like a cold pack).

Any value you get for entropy nowadays was probably produced by chemists in that way. It is the only variable in whether a reaction goes or not that is directly dependant on temperature, so the same reaction can be run different environments until the contribution from entropy is equivalent to that of heat, or it can be calculated from other known values of entropy (which is more common).

If you want more precise information than that, look into the 'Gibbs free energy equation' (link 1). Hope that helps!