If a reaction is exothermic, shouldn’t that be enough to determine favorability? Actually, no!

Even if a reaction is highly exothermic, the level of order it requires may make it statistically improbable.

Concept: Explaining what entropy is.

Video Transcript

All right guys, so now I want to talk about what I would consider the most confusing term of the Gibbs free energy equation and that is entropy. Entropy, generally stated, we said that it was a measure of disorder in the system. But that's a really confusing definition. I would rather go with an easier definition. What that is is that entropy is the tendency of a system to take its most probable form.
That means that if we have two different states, one is highly ordered one is not as ordered, statistically, it's more likely to be in the disordered state. That's what entropy has to do with. It has to do with probability really.
What that means is that even if a reaction is highly exothermic, like if you talk about bond association energies and enthalpy, that has to do with exothermic. But the level of order that it requires might make it statistically improbable. So basically what we do with entropy is we have to analyze is this going to be statistically more probable or statistically less probable. That is what entropy is.
Remember that we defined that entropy says that a positive, I mean, I'm sorry – a negative value is going to be more ordered because remember that basically, a positive value means that it's more disordered. That means that your entropy is getting bigger. So if you have a negative value, that means your entropy is getting smaller or it's getting more ordered.
It turns out that we're never going to have to calculate the entropy in terms of calculating the entropy of the surrounding environment or the entropy of a system because of the fact that we don't have really the tools to analyze that in orgo one. But what you are going to be asked to do is you're going to be asked to analyze if something is going to have a positive delta S or a negative delta S. That's what we want to do. We don't want to figure out the exact number. We just want to figure out is this going to be higher entropy or lower entropy.
Now there is one situation where you might calculate delta S and that's if you're given every other variable. If you're given the T and the delta H and the delta G, then sure, you could just use algebra to figure out delta S. But I'm just saying that in the absence of this being just a simple algebra problem you're not going to be asked just to calculate what the delta S is of an environment.
So let's talk about these three phenomenon that make reactions more probable or make delta S go up. And it turns out that all of them are going to be favored by high temperature. So that should be really clear in a little bit when I go back to the equation.

Entropy (ΔS) is the tendency of a system to take its most probable form.

  • Negative values (-) indicate less probable = Unfavored
  • Positive values (+) indicate more probable = Favored

There are 3 common ways to make reactions more probable (increase ΔS)

  • They all become more likely as we add heat to the reaction (increase T)

1. Increasing the Number of Molecules

Reactions that create extra molecules are more probable since there are more ways to arrange them.

2. Phase Transition

Transformation of solid to liquid or liquid to gas is more probable since the molecules will have a greater vibrational freedom.

3. Increasing Molecular Freedom of Motion

Converting cyclic molecules to acyclic molecules are more probable since it increases freedom of rotation.