Spontaneous Reactions & Gibbs Free Energy

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  • 0:00 What Is a Spontaneous Process?
  • 1:56 Enthalpy and Entropy
  • 3:07 Gibbs Free Energy and…
  • 5:07 Predicting a…
  • 7:47 Lesson Summary
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Lesson Transcript
Instructor: Nicola McDougal

Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.

In this lesson, we will learn about spontaneous reactions and how we can use Gibbs free energy to predict them. We will remind ourselves about enthalpy and entropy and discuss the importance of temperature and spontaneity.

What Is a Spontaneous Process?

Picture this. You are in a warm nightclub drinking a cold glass of cola with plenty of ice. You put your drink on the bar to go dance, and when you come back to take a sip 20 minutes later, the cola is warm. Yuck! All of the ice has melted.

Now that should be no surprise to you because you instinctively know if you leave ice out of the freezer it will melt. You didn't have to do anything other than leave the ice on the bar. This is an example of a spontaneous reaction.

Now, take a look at this picture. Here you are trying to push a big pile of jello uphill. It is tough to push the jello up, and you have to really put your back into it. If you take your hand off for even a second, the jello just wants to fall back down again. To get that jello uphill, you have to physically push it there. This is an example of a nonspontaneous reaction.

A spontaneous reaction is a process that could occur by itself without any work from outside. Our jello example is a nonspontaneous reaction because this is a process that can only occur when work is put in from the outside. It is important to know that for a spontaneous reaction, the opposite reaction is always nonspontaneous. We say that spontaneity tells us the direction the reaction will take but nothing about how fast it will be.

Now this is a very important point. It is very important that you do not think that something spontaneous is something that happens quickly, this simply isn't true. Some spontaneous reactions do happen very quickly, for example an explosion, while other spontaneous reactions happen much more slowly. A rusting car is a good example of a very slow, but spontaneous reaction.

Enthalpy and Entropy

Let us quickly remind ourselves about two important thermodynamic quantities.

Enthalpy (H) is a measure of how much energy is released or absorbed during a chemical reaction. Energy, in the form of heat, is released in an exothermic reaction, and the change in enthalpy is negative (-H). Exothermic reactions are thermodynamically favorable. On the other hand energy, in the form of heat, is absorbed in an endothermic reaction, and this time the change in enthalpy is positive, (+H). Endothermic reactions are thermodynamically unfavorable.

The second property is the measure of entropy (S). This is a measure of disorder or randomness in the system. In nature, a messy room is far more favored than a neat, ordered room, and when disorder increases, we have a +S. Systems with large positive entropy values are thermodynamically favorable.

Together, enthalpy and entropy can help us to understand whether a reaction is spontaneous or not.

Gibbs Free Energy and Spontaneity

While we can figure out the spontaneous direction of some reactions, it isn't always so obvious and particularly not for many chemical reactions. So it is nice to know if there is an easy way to tell us if a process will spontaneously occur. And, this is where Gibbs free energy comes in. This new quantity describes spontaneity in a very simple equation. It is an important point and you must learn this equation:

delta G = delta H - TdeltaS

This rhyme may help you remember:

Delicious Gymnasts Excelled Diplomatically Hence Meaty Tigers Delegated Solemnly

We can see Gibbs free energy is calculated from the changes in enthalpy and entropy, as well as the temperature at which the reaction is carried out at. The change in Gibbs free energy represents the amount of total energy change that is available, or free, to do useful work. This is why free energy is so important; we want to get energy out of a spontaneous process and not have to put energy in.

We really do not want to be pushing jello up hill!

From this equation, we could easily calculate values of Gibbs free energy, but the key point for this lesson is that now we have a single quantity whose mathematical sign will tell us easily whether a reaction is spontaneous. In other words, can I get useful work out of this reaction?

For a spontaneous reaction, the sign of delta G is always negative. So, for a spontaneous reaction, you are looking for a free energy of less than zero. If you end up with a free energy of more than zero, then you have a non-spontaneous reaction.

Predicting a Spontaneous Reaction

It is very important that you can predict if a reaction will be spontaneous. You don't need numbers to do this; you just need to understand the impact of all of the terms in the Gibbs equation. Remember that a spontaneous reaction has a negative value for delta G and a value less than 0.

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