Chemguide: Support for CIE A level Chemistry

Learning outcome 5.3(c)

This statement asks you to be able to predict the sign of the entropy change during a particular process.

Before you go on, you should find and read the statement in your copy of the syllabus.

In statement 5.3(b) we looked at the entropy changes which occurred during various processes - physical and chemical.

This statement 5.3(c) just asks you to be able to predict whether entropy will increase or decrease during straightforward processes chosen by the examiners. You obviously can't do that if you aren't familiar with statement 5.3(b).

Some examples:

This is just a crystalline solid going into solution. The solid is highly ordered; the solution is disordered. There is an increase in entropy.

The water is changing from the highly disordered gas into a more ordered liquid. The entropy will fall.

There are three moles of gas on the left-hand side of the equation, but only one on the right. The starting materials are more disordered than the products, and so there is a decrease in entropy.

Notice that if the water had been formed as steam, you couldn't easily predict whether there was an increase or a decrease in entropy, because there would be three moles of gas on each side.

The presence of the five moles of liquid water on the left-hand side means that there will be far more disorder before the change than there is in the products. The copper(II) sulphate crystals formed will be very ordered. Entropy will decrease.

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© Jim Clark 2014