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The Relationship Between Enthalpy (H), Free Energy (G) and Entropy (S)

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  • 0:02 What Is Free Energy?
  • 2:05 Enthalpy & Entropy Revisited
  • 3:09 Why Are Reactions Spontaneous?
  • 3:58 The Effect of Temperature
  • 7:50 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 video lesson, we'll study free energy (G) and its relationship to enthalpy, entropy and temperature. You'll also learn why free energy (G) is the single most useful criterion for predicting the spontaneity and direction of a chemical reaction.

What Is Free Energy?

Chemical reactions are all around us. They are even inside our body. In fact, the human body is a mass of thousands of chemical reactions. If you are like me, the one you are really familiar with is eating and digesting, scientifically called cellular respiration. I love this chemical reaction. This is where we eat delicious food that our body breaks down to form the products of carbon dioxide and water. This chemical reaction gives out energy that we can use.

The amount of available energy depends on the type of food we eat and how much potential energy is available in its chemical bonds. If we don't use all the energy given out, then the extra energy is stored by the body as fat. The available energy that allows us to run and do useful work is called Gibbs free energy, or sometimes just Gibbs energy. Put simply, Gibbs free energy is the amount of energy left over after a chemical reaction has taken place. It is important that we have this extra, leftover energy; otherwise, we wouldn't be able to do work.

We can use our understanding of Gibbs free energy as an easy way to predict whether a reaction is spontaneous and calculate how much free energy is available for us to use. Spontaneity is the direction in which the reaction will take place without outside interference. In other words, it doesn't need us to keep putting in work or energy to keep it happening.

The Gibbs free energy equation we will be working with is Delta or change in G is equal to change in enthalpy minus temperature multiplied by the change in entropy. This is a very important equation for you to remember, so be sure to commit it to memory. G equals H minus TS. Use 'Green Eagles Hit Moaning Televisions Suddenly' to help you.

Enthalpy and Entropy Revisited

As we can see from our equation, 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. Let us quickly remind ourselves about these 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. 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.

The second property is the measure of entropy (S), which 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 +S.

Why Are Reactions Spontaneous?

Now it turns out that for a spontaneous reaction, the sign of Delta G must be negative. Knowing this, what effect does enthalpy and entropy have on spontaneity? Let us first look at enthalpy: an exothermic reaction is more likely to be spontaneous; if Delta H is negative, this makes it more likely that Delta G will also be negative and, therefore, spontaneous. And now, let's look at entropy: if a reaction causes an increase in randomness of the system, it is more likely to be spontaneous; a positive Delta S term will also mean that Delta G is more likely to be negative.

The Effect of Temperature

Got it? Now, if only it were that simple! Let me throw a wrench in the works. We know from everyday experience that water freezes spontaneously, and we recall that solid ice is more ordered than liquid and so the entropy of the system has gone down. The Delta S term is negative, so why is this reaction spontaneous?

I can hear you shouting at the video, but it has to be at freezing temperature for this to happen. And you are absolutely right because this is where the effect of temperature comes in. You can see from the equation that it can have a big effect on the entropy term.

So, now we can see that we cannot simply say that if entropy change is negative, the reaction is always non-spontaneous. It depends. And there are four possible situations a reaction can have, which are summarized in the table below:

Effect of Temperature on Spontaneity of a Reaction

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