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Thermodynamics
Written by Tim Sheppard MBBS BSc. Last updated 11/11/10

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What are thermodynamics?

Put as simply as possible, thermodynamics means the study of the way in which heat is turned into energy, or the way that heat moves about. Understanding the term is helpful, but currently not essential, so long as you appreciate that when we talk about 'thermodynamics', we're probably going to be disucssing heat and energy.


What is enthalpy?

This is an important concept, and quite difficult to get your head round. However, once it clicks, it usually makes a lot of sense!

Imagine you burn something. It's giving off energy in the form of heat - you can feel it burn; you know that it is burning. This is called combustion, and it is a chemical reaction. It involves the substance that you're burning reacting with oxygen, to produce carbon dioxide and water.

Imagine that you're burning glucose. So glucose + oxygen → carbon dioxide + water. All of the chemicals there have bonds in them. If you want to turn the reactants (the things that are going to be reacting with each other) into the products (what comes out of the reaction afterwards), you need to break all the bonds that you have to begin with, and then make all the bonds that you want at the end. In the same way that if you have built a set of shelves and want to change it, you have to take it all apart and put it back in the way that you want it.

Consider the bonds are like magnets. When you pull them apart, it takes energy - you have to put energy in to pull them apart. When you put two magnets together, they make a clicking sound, i.e. they are releasing energy. It is the same with chemical bonds - it takes energy to break bonds, and energy is released when you make bonds. Each type of bond will require a different amount of energy to be broken, and this is called the enthalpy. The total enthalpy is the amount of energy stored in all of the bonds - that is, the amount of energy it would require to break all of the bonds.

If the energy released from making bonds is more than the energy required to make bonds, then overall the reaction will release energy - this is what happens when you burn something. So, when you burn glucose, the amount of energy required to break the bonds in glucose and oxygen is not as much as the amount of energy released when you form carbon dioxide and water.

This can be shown on an energy-time graph as in the animation on the left.

When the overall reaction gives off heat, the reaction is said to be exothermic and it has a negative enthalpy change because it releases energy. You can see in the graph that the energy at the end is lower down than the energy at the beginning, so the difference must be negative.

Conversely, if the overall reaction requires energy to be put in (i.e. the energy required to break the bonds is more than the energy released when the bonds are formed), the reaction is said to be endothermic and it has a positive enthalpy change.

For any reaction to start going, energy needs to be put in - at least to get the first bonds broken. This is called the activation energy. However, with an exothermic reaction, the making of the products will then give enough energy to keep the reaction going. However, for an endothermic reaction, energy must be put in all the time.

So the value of the enthalpy change gives an idea of the amount of energy going into, or coming out of, a reaction. Briefly forgetting about the activation energy, if the reaction has a very negative enthalpy change (i.e. exothermic, giving off heat) it would seem quite likely to occur, because it happily provides the energy it needs to break the bonds.

Enthalpy change is given the symbol ΔH


What is entropy?

Entropy is probably simpler to understand. Put simply, it is a measure of how disordered a system is - that is, if something is really 'messy', it will have a high entropy value; if it is really tidy, it will have a low entropy value. The key to understanding entropy is understanding what qualifies as messy or tidy, and what is considered a favourable enthalpy change.

A solid is generally considered to be very ordered, while a liquid is less ordered, and a gas is the most disordered. Similarly, if there are more molecules or particles around, that is considered more disordered - in the same way that if you took apart a car, it would look far more disordered than it does when it's all fitted together. Another way to make something more disordered it to dissolve it, because the particles will all split apart.

A favourable entropy change is one that results in less order - it is said (in the second law of thermodynamics) that the world is simply working towards randomness. Therefore a reaction will have a favourable entropy change if it causes a solid to turn into a liquid or gas, or a liquid to turn into a gas; if it turns something into a greater number of particles (such as the burning of glucose shown above), or if it dissolves a substance into something else.

To work out the entropy change, like working out the enthalpy change, the total entropy before and after have to be worked out. For instance, if you were working out the entropy change from burning glucose, you would work out the entropy in the reactants (i.e. the disorder of 1 glucose molecule and 6 oxygen molecules), and then work out the disorder of the products (6 carbon dioxide molecules and 6 water molecules). The difference between these two will give the entropy change. Using this method, a reaction is favourable if it has a positive entropy change, because this means the entropy has increased.

Entropy change is given the symbol ΔS


What is free energy?

This is the all important part - it tells you overall how likely a reaction is, by relating enthalpy to entropy. Personally I think that simply accepting the equation is about all you can do, because understanding the concept of 'free energy' is quite difficult. However, understanding what makes a reaction feasible is very helpful.

Free energy change is given the symbol ΔG and it is calculated using the equation on the left - that is, by subtracting from enthalpy change the product of temperature and entropy change.

As already discussed, a reaction would seem to be likely if it was very exothermic (very negative enthalpy change) and if it resulted in more disorder (positive entropy change). If you subtract a very positive number from a very negative number, then it will result in a very negative number; and so if a reaction has positive entropy change and negative enthalpy change, the change in free energy will be negative.

For a reaction to be feasible, ΔG must be negative. This means that even if a reaction is endothermic, it can occur if the entropy change is sufficiently positive. Similarly, even if a reaction results in more order (negative entropy change), it can occur if the enthalpy change is sufficiently negative. Reactions won't necessarily happen - sometimes the activation energy (the energy required to start off a reaction) is too high. However, provided the activation energy can be met, a reaction with negative ΔG will occur.

If one reaction is really unlikely (it is very endothermic, or has a negative entropy change) then it will have a positive ΔG and will not occur. However, if another reaction is happening at the same time which has a very negative ΔG, then it can help the first reaction out. (In the body, reactions that might be unfavourable are helped out by reactions that are very favourable, such as the hydrolysis of ATP).


Further Reading