Question

Can somebody explain to me what enthalpy and entropy are without using definitions?

Answer 1

Enthalpy is a measure of energy content. It measures the amount of energy you can extract from a system when you keep the pressure and volume constant, i.e. when you take the energy out ONLY as heat. Sometimes it's therefore called "heat content."

Entropy is much more subtle. It's a measure of how "generic" a system is, to our macroscopic understanding. Here's an illustration: in a poker game some hands, e.g. A K Q J 10 of spades, are very valuable, and other hands, e.g. 2 of clubs, 3 of hearts, 8 of hearts, 10 of clubs, jack of spades are essentially worthless. Each unique hand is equally probable, of course, so the two hands I've mentioned are equally likely, if you deal a hand from a new deck. However, as a rule we don't care about the hand per se, but only how valuable it is in a game. So we lump all the valuable hands together and all the worthless hands together. Since there are many many more worthless hands than valuable hands, we say that a valuable hand (like the straight flush above) is much more likely than a worthless hand (like the worthless hand above).

It's that way with entropy. Some states of a system are extremely unusual and interesting -- a perfect crystal, a very pure substance, a carefully-crafted and structured composite material, a spiral of DNA, et cetera. Many other states of the same system are very "generic" and uninteresting -- a disordered scattering of atoms or ions, a mixture of many substances, a bowl of random carbon compounds containing the same atoms that could be used to make a DNA spiral. We tend to lump the "interesting" states and "uninteresting" states together. And, again, from that point of view there are far more "uninteresting" states than there are interesting states. We say the entropy of the uninteresting states is higher -- there are many more of them.

Since the universe has no perception of OUR definitions of interesting and uninteresting, it has no preference for one state versus another. One state is just as likely to evolve into another, regardless of which category it fits into. But since there are many many more uninteresting states, it stands to reason that an interesting state, if allowed to evolve, is much more likely to evolve into an uninteresting state than another interesting state. And an uninteresting state is very likely to evolve into another uninteresting state, rather than an interesting state. It's like the fact that if we throw away our straight flush in poker and demand 5 more cards, we are much more likely to get a worthless hand than another valuable hand. And if we throw away our worthless hand and ask for 5 more cards, we're much more likely to get another worthless hand than a valuable hand. Because there are way more worthless hands than valuable hands.

That forms the basis of the Second Law: systems are overwhelmingly likely to evolve towards less "interesting" states -- less ordered, more "generic," more mixed, and so forth.

There is one remaining puzzle, which is why we *have* interesting states -- e.g. living systems, solar systems, oceans full of life -- to evolve to much more uninteresting endpoints. It appears as if the universe started in an extremely unique and peculiar state, and has been evolving, ever since, towards a much more generic and boring arrangement of its components. Why should this be? Nobody actually knows yet.