Gibb's Free Energy and Spontaneity

Spontaneity (horrible word choice, important idea)

Some reactions happen easily and some reactions require us to force them to occur.

For instance, hydrogen and oxygen react violently together to form water (like the Hindenburg). Water can be decomposed to release hydrogen and oxygen, but we can only make that work by forcing an electric current through a solution of water and some ions.

So nature "likes" making water but not taking it apart.

We say that a reaction nature "likes" is spontaneous, while a reaction nature "dislikes" is non-spontaneous. Warning: This is NOT what most people mean when they use the word spontaneous. A better term would be favored.

Here's a real definition: A spontaneous (or favored) reaction is one that continues without outside interference once it is started.

Some important things to note:

• Spontaneous reactions must be started - All reactions require SOME energy to be added (activation energy)

• Non-spontaneous reactions can (and do) occur all the time, they just require some constant "push"

To make those points clearer, we can think about forest fires and photosynthesis.

Forest fires are spontaneous in the chemistry sense of the word. That means that once a forest fire starts it is VERY difficult to stop it. Without huge efforts, the fire burns day and night, non-stop. Being spontaneous does not mean that forests randomly burst into flames.

Photosynthesis is non-spontaneous in the chemistry sense. Although it happens all around us (grass, trees, crops, cacti, etc.) it needs the constant "push" of sunlight to work. Photosynthesis shuts down every night.

So, what makes a reaction spontaneous (favored)?

The Driving Forces of Nature

Warning: What follows is a simplified discussion of the ideas of thermodynamics. Understanding this does not make you an expert, but it will give you a general sense of what is going on and why.

There are 2 different factors that (together) determine whether or not a reaction is spontaneous (favored). They are Heat (more correctly known as Enthalpy) and Entropy.

Let's tackle them one at a time.
Entropy (messiness) - Nature favors reactions that make the world messier. For a (fuller) discussion of this idea, go here. Thinking of forest fires, trees are MUCH more organized than the carbon dioxide, water vapor and ash that they become in a fire.

Enthalpy (heat) - Since all reactions need heat to occur, a reaction that gives off heat will "feed itself activation energy". For more information on the heat of a reaction go here. Again thinking about forest fires, the first tree that burns gives off enough heat to "light" the next tree and so on.

So, any reaction that is exothermic AND makes things messier will be spontaneous (favored by nature).

At the same time, any reaction that is endothermic and makes things neater will be non-spontaneous (not favored by nature).

What about reactions in which nature "disagrees with itself"?
There are many reactions that occur  in which heat is released (exothermic reaction) but things get neater, or in which heat is needed (endothermic) but things get messier.

A simple example of this is the melting of ice. In order to melt ice, heat must be added. However, in the process a very organized crystal becomes a loose puddle of water.

So, does ice melt?

The answer is (obviously) depends on the temperature. At high temperatures (above 0^oC) ice melts spontaneously (nature favors melting the ice). Below that temperature, the ice does not melt (nature does not favor melting the ice).

The person who put idea into a mathematical formula was named J. Willard Gibbs, who gave us the formula:

In this formula, ΔG is called the Gibb's Free Energy, ΔH is the heat of the reaction, and ΔS is the change in entropy of the reaction.

The equation is set up to give a negative value for ΔG if the reaction is spontaneous (favored) and a positive value for ΔG if the reaction is non-spontaneous (not favored).