The quantum casino: tutorials

The Second Law of Thermodynamics

The First Law of Thermodynamics is the Law of Conservation of Energy and the Third Law states that the entropy of pure substances approaches zero as the temperature approaches absolute zero. These laws have been paraphrased as:

you can’t win, you can only break even;
you can only break even at absolute zero; and
you can’t reach absolute zero.

Since processes take place by chance alone, and this leads to increasing randomness, we can say that in any spontaneous process (one that takes place of its own accord and is not driven by outside influences) entropy increases. This is called the Second Law of Thermodynamics and is probably the most fundamental physical law. So processes that involve spreading out and increasing disorder are favoured over those where things become more ordered – students may have noticed this in their bedrooms!

The word feasible is also used to mean the same as spontaneous. The terms have nothing to do with the rate of a process. So a reaction may be feasible (spontaneous) but occur so slowly that in practice it does not occur at all. The reaction of diamond (pure carbon) with oxygen to form carbon dioxide is feasible (spontaneous) but we do not have to worry about jewellery burning in air at room temperature – ‘diamonds are forever’!.


Look at the two videos. You will have no difficulty in deciding which one is being played in reverse – randomly–arranged fragments do not spontaneously arrange themselves into an ordered configuration.

So the idea of entropy increasing seems to explain why a crystal of salt (sodium chloride) will dissolve in a beaker of water of its own accord (spontaneously). The resulting solution in which sodium and chloride ions are thoroughly mixed with water molecules is more random than a beaker of pure water and a crystal of solid sodium chloride in which the ions are in a highly ordered crystal lattice. It also seems to explain our initial query –

why Mg(s) + 2HCl(aq) H₂(g) + MgCl₂(aq) occurs,

but H₂(g) + MgCl₂(aq) Mg(s) + 2HCl(aq) does not.

In the first case, the production of a gas from a solid clearly involves an increase in entropy while the reverse has a decrease.

Note. There is some local increase in order on forming the solution as (disordered) water molecules cluster round the Na⁺and Cl^- ions but this is outweighed by the large increase in disorder as the sodium chloride lattice breaks up.

The Second Law of Thermodynamics