 So, since we found it fairly easy to understand why ice versus liquid water forms under different conditions, let's try something slightly harder and interpret that in terms of free entity. What if I had two water molecules? And here they are in vacuole and then I bring them closer together and at some point they're going to be so close that they form a hydrogen bond in water. So the question is what happens here? There are a couple of things here. I am definitely forming one hydrogen bond here so that there is somehow a change here in delta E that is delta E H bond. And that's going to be negative because we're forming a bond. There is also a change in entropy and that's a bit harder to reason about. I'm just going to hand wave. So if each water molecule has a freedom corresponding to one 3D rotating water molecule, well I'm taking this molecule and it's losing, under normal conditions each water would participate in two hydrogen bonds and that would completely lock it in. So maybe this water molecule loses half of its rotational freedom and that water molecule also loses half of its rotational freedom. So that is perhaps roughly 2 times 0.5 delta S water rotation. Just hand waving. The question here now is that under what conditions will a hydrogen bond form? Because the hydrogen bonds don't, in some cases we can boil water so occasionally a hydrogen bond will break. And this you can generalize to any bond really. You should be able to do this with our normal friend F equals E minus TS and reason about under what conditions and what is true when a hydrogen bond does form. So I'm going to help you a little bit long-awaiting and I'm going to ask you to answer three questions. And to make this easier I'm just going to give you two alternatives. The first question I already asked for you, sorry, answered for you. The delta E of this process is that smaller than zero or greater than zero. And you see my reasoning there was well it's a chemical bond, a chemical bond is favorable otherwise it wouldn't be formed. So the change in energy of this process is definitely smaller than zero. The second part though this T delta S is that smaller than zero or larger than zero. And third if you're comparing the absolute values of the change in energy versus the change in the entropy term. Notice how I sometimes say entropy and sometimes entropy term in the term I include the temperature. Which one is larger there? Hit pause for a second and think about that before I tell you the answer. So I bet it wasn't that easy and the reason why that isn't that easy is that you're probably taking two large steps here. You hopefully started from this equation. If don't that's the first lesson. Don't try to guess and hand away because then you fall in the same trap I did as a student. Trust your equations. The other point is that it's dangerous to go after the absolute value of the equation but you can't say what the absolute entropy is in various things. It's much better to focus on relative changes. I already mentioned this in class right. Don't think about F. Think delta F. What is the change of the process? And I'm going to get hit by all these water so let's move it down. If I'm talking about the change of the process you also have to define what is the starting point? What is the end point and in what direction is the arrow pointing? If you haven't even decided that and I bet 90% of you didn't, you're just hand waving and guessing. You're not doing physics. So go back and try to do it once more now. Use this equation and do it properly. Define what your start state is. Define what your end state is. Look at the relative differences and solve things one by one. So you're back. Okay. I'm going to do this and I hope you heard. We already said that delta E was smaller than zero. So what happened in terms of entropy? Well, T is always positive because we're starting at zero as absolute zero. So the T delta S term when we are forming a hydrogen bond as I already mentioned that we are going from a state where both molecules are completely free and then we are restricting them more. And if they're becoming more restricted, that is a more ordered system. And if the system is more ordered, that is a system that has lower entropy. So T delta S is also smaller than zero. And you're going to see that for most things you look at actually these two terms tend to have the same sign. If they had different signs, well, it would be trivial and you could guess instantly what's going to happen or not. So what then determines if this reaction is going to happen? This term is negative minus another term that is also negative. Whether delta F is going to be negative, meaning this reaction will happen, that is going to require that this term is more negative than this one is. And if a hydrogen bond forms, the magnitude of this term, we have to gain more in terms of the binding energy than we lose in terms of the entropy. So the absolute value in the case where the hydrogen bond does form must be that the absolute value again of the term, that must be larger than the absolute value of the entropy term. So the gain we get here, the absolute value of the change, because no, the term itself is negative. But the absolute value we gain is more than the energy term we lose in the entropy. It wasn't so difficult when you did it step by step, right? So don't do that mistake again. Don't try to jump to your conclusions, trust your equation, and you need to be particularly careful about the signs, and the signs are related to the way your arrows are pointing. So do that homework first.