Nicky has taught a variety of chemistry courses at college level. Nicky has a PhD in Physical Chemistry.
Ever heard the phrase 'pushing Jell-O uphill on a hot day'? This describes a hopeless task. In this lesson, we will predict hopeful and hopeless reactions. Or put scientifically, predicting spontaneous (hopeful) and non-spontaneous (hopeless) reactions.
What Is Spontaneity?
Take a look at the following activities, and based upon what you already know, let us predict what will happen.
First, let us think about the boulder and what happens to it on the hill. On the left of the screen, it is at the top of the hill:
If we give it a push just to get it moving, the boulder rolls all the way to the bottom. It doesn't need us to help it along once it starts. The boulder rolls down the hill on its own.
In this image we are trying to get the boulder to roll uphill:
It simply won't roll uphill on its own. The only way this can happen is to put in a huge amount of effort or energy to push it up. As soon as we stop pushing, the boulder rolls back down the hill again.
From your own knowledge of how things work, I am guessing you predicted that the boulder would tend to roll downhill on its own. But it simply will not do the reverse process, which is to roll back uphill. The only way the boulder could go back up the hill is to put energy in to get it back up.
Here's our second activity. Here, we have a wood fire.
Initially, to get a fire to burn we have to provide a spark, in this case a match. But once alight, the fire will burn all by itself. We don't have to keep holding a match to it to keep it burning. The fire will only stop burning when it runs out of wood. Or if I physically do something to put it out.
On the right of the screen above, we have the reverse of burning. The products of combustion are carbon dioxide and water, and here we are trying to recreate the wood and the oxygen again.
Intuition tells you that you can't do that. Once the wood is burned, that is it; you cannot recreate the wood again simply by reversing the reaction. Again, you know that the wood readily burns on its own (once it's lit), but the reverse process does not happen.
What do both examples have in common? In both cases, there is one direction that will occur by itself and the other that will not. This is spontaneity.
A reaction that is spontaneous is a process that could occur by itself without any work from outside. The opposite reaction is ALWAYS non-spontaneous. It cannot occur without significant outside intervention.
Spontaneity tells us the direction the reaction will take, but nothing about how fast it will be. For example, you have seen that wood burning is a spontaneous reaction; however, without an initial flame, it would be so slow we wouldn't notice it. It is very important that you do not think that something spontaneous is something that happens quickly; that simply is not true. Some spontaneous reactions do happen very quickly, while other spontaneous reactions happen much more slowly.
Spontaneity and Temperature
Here are two more examples of spontaneous activities; this time, think about the conditions that make them spontaneous.
So, what do we see here?
On the left, we see that it is a very warm beach and the solid ice is spontaneously melting to liquid. The temperature is 25 degrees Celsius, and we know that water is always liquid above 0 degrees C. There is no way that we could reverse this reaction at this temperature; water will always be liquid.
But in the picture on the right, we have the same block of ice, but this time the temperature is much colder and the solid ice remains frozen. At -15 degrees Celsius water is solid ice, and if we had liquid water, it would spontaneously freeze. Again, there is no way that we could reverse this reaction at this temperature; water will always be solid ice.
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These examples show us that the spontaneity of a process is affected greatly by the temperature. Whether water is liquid, ice or a gas depends on what the temperature is. Again, the reverse reaction at a given temperature is always non-spontaneous.
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 that 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 was discovered by an American scientist called J. Willard Gibbs who defined a new mathematical relationship that describes spontaneity:
G = H - TS
He made a link between free energy (G), enthalpy (H), temperature (T) and entropy (S).
Because we are usually looking at how a system has changed, we can rewrite this equation slightly.
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. From this equation we can easily calculate values of Gibbs free energy, and we do this in a different lesson.
But the key point for this lesson is that we now 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.
An easy way to remember this is to learn the following rhyme:
When delta G is less than zero, then down the hill the boulder will go. When delta G is more than nil, no way that boulder goes up the hill!
In this lesson, you have learned that a spontaneous reaction is one that takes place without any work from the outside. You also know that the reverse reaction of a spontaneous reaction is always non-spontaneous. Spontaneity tell us the direction of the reaction, but not how fast it goes. Gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. G = H - TS. And, finally the sign of delta G is ALWAYS negative for a spontaneous reaction.
The above topics were designed to expand your ability to:
Characterize a spontaneous reaction
Write the Gibbs free energy formula
Understand how enthalpy, entropy and temperature are related to the spontaneity of a reaction
Recollect the sign of delta G for spontaneous reactions
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