Entropy is covered in the AHL part of the course: Topic 15.1 in the energetics / thermochemistry unit.

The entropy change (symbol = S) in a chemical reaction is determined by as follows:

total entropy change = total entropy of products – total entropy of the reactants

You will have probably been taught that entropy is a measure of disorder. Order goes to disorder. The universe is expanding (and therefore getting more disordered). Liquid water has a greater entropy than solid water (ice) as it is more disordered. The particles in water slip and slide over each other, they move around. And whilst the structure of water is not as ordered as say an ionic solid, it is more ordered than the liquid form.

In the grander scheme of things, this fits in with Gibb’s free energy equation:

Gibb’s free energy change = enthalpy change – (temperature [in Kelvin] × entropy change)

If the Gibb’s value is negative, the reaction will be spontaneous at that temperature.

But what is actually is entropy?

The idea of entropy being a measure of disorder is a generalisation (that said, this idea is fine for IB) but there are some anomalies. Look at earth – life is complex. Plant’s photosynthesize and take disordered water and oxygen to produce ordered carbohydrates. It seems as if entropy is going in the wrong direction as this video implies:


(By the way, the video film has been reversed!)

Entropy can be thought of as being the way in which energy is distributed or even dispersed. A hot cup of coffee cools down as the heat energy flows from the hot cup into the cooler room. The energy of the molecules of coffee decreases but importantly, the energy of the molecules in the room increases – ever so slightly.

And this all ties in nicely with the second law of thermodynamics (something you may have studied in physics). The second law of thermodynamic states that the total entropy of an isolated system can never decrease over time.

But how can we explain the paradox above, whereas life on earth seems to go against entropy as life brings order from disorder? The answer is simple enough. Even though it seems as if life brings order, energy is still being dispersed. Life produces amongst other things heat energy. This energy will heat up the molecules in the atmosphere, increasing the entropy of them and thus satisfying the second law of thermodynamics.

I’ve also found a rather amusing video on YouTube about entropy. I hope you enjoy it!