Eli5 How can a reaction proceed by entropy?

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A spontaneous reaction is one that requires no external and continuous input of energy. It occurs when G<0. Very straightforward. When G>0, the reaction is not spontaneous ok. As a reaction reaches equilibrium, G trends towards while S is maximized. Ok. Very clear understandable.

My confusion begins with understanding how entropy can drive a reaction.

So lets assume a endothermic reaction: A + B –> CD + EF. Great! This reaction proceeds spontaneously because the products have a higher entropy than the reactants and the magnitude of entropy * T is higher than enthalpy.

Even though I am able to categorize this reaction as spontaneous. Why does this reaction occur spontaneously??? Obviously, the molecules involved in this reaction (specifically A and B) have no clue that the products have more microstates. Why do the reactants react in the first place than? What is the exact mechanism involved.

Why do exothermic reactions typically proceed spontaneously (with an initial energy inputted). I don’t understand this at all. I don’t want to have to just rely on the math and lack an intuitive understanding.

Edit: I wanted to add that I understand entropy to be a function of probability. Example: if I flip a coin 1000 times, it is more likely that I get 500 heads and tails. But just because the products have more arrangeable states does not explain why the reaction proceeds in the first place. For example, using the coin analogy, even though there are a lot of microstates with 500 heads and tails, why does the coin flip in the first place (if that makes sense).

Edit 2: My only working idea right now is that because products have higher entropy, the chances of the products interacting to form reactants is less likely. But this does not explain something like diamond spontaneously turning into graphite so I am reluctant to believe it.

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Here’s how I think of it. Increasing entropy is not a fundamental principle of physics, but rather an emergent one given the initial conditions of the universe.

The universe began in an ordered state. Think of the universe as a blank snow field that no person or animal has walked across first thing in the morning. Because of this, pretty much any reaction that happens is going to increase the disorder of the universe. Any person who walks across the snowfield is going to make it more “messy.”

Now the thing is, disordered states are (effectively) infinitely more likely than ordered states. So when stuff happens, it increases the disorder/entropy of the universe. If one person walks across that snow field, it’s going to make it more “messy.” If a second person walks across, the odds of them “undoing” the first person’s mess is so small it’s zero, so the next person will make it more messy, and the third, and so on.

Now if we were to examine the universe many eons in the future, and it looks like the snow field after millions of people have marched across it and there isn’t a single square inch of clean snow left. Now entropy no longer increases. Now you’re just as likely to “unmessy” the snow field as you are to “messy” it when you walk across it, because it’s starting in a completely messy state.

So, you see, entropy doesn’t drive the universe. Things don’t happen *because* of entropy. Rather, we started with the universe in a very unlikely condition and as stuff happens, it heads to a more probable condition (the high entropy state).

All reactions can happen in both directions, as long as there’s enough activation energy around to kick them into motion. For *most* reactions, though, there’s a strongly favored direction. That direction is totally independent of activation energy, it depends on entropy. Entropy is all about statistics/probability…if one direction is entropically favoured it’s far more likely to happen than the other.

Diamond to graphite has a high activation energy…entropically, it really wants to turn into graphite, but there’s not enough energy around (typically) to drive the reaction. But stick a blow torch on the diamond (provide activation energy) and you get graphite in short order.

Spontaneous reactions are the ones that are entropically favoured *and have no or ambient temperature activation energy*. They basically happen instantly because the moment the molecules run into each other they react and the entropically favoured direction is the statistically far more likely result.

> For example, using the coin analogy, even though there are a lot of microstates with 500 heads and tails, why does the coin flip in the first place (if that makes sense).

The coins are being flipped constantly by random molecular collisions, with the energy to react coming from their respective thermal motion. (Or in the case of some spontaneous decomposition, from the internal energy of the molecule.)