Cracking the AP Chemistry Exam

Part IV

Content Review for the AP Chemistry Exam

Chapter 7

Big Idea #5: Laws of Thermodynamics and Changes in Matter

ENTROPY

The entropy, S, of a system is a measure of the randomness or disorder of the system; the greater the disorder of a system, the greater its entropy. Because zero entropy is defined as a solid crystal at 0 K, and because 0 K has never been reached experimentally, all substances that we encounter will have some positive value for entropy. Standard entropies, S°, are calculated at 25°C (298 K).

You should be familiar
with several simple rules
concerning entropies.

1. • Liquids have higher
entropy values than
solids.

2. • Gases have higher
entropy values than
liquids.

3. • Particles in solution
have higher entropy
values than solids.

4. • Two moles of a
gaseous substance
have a higher
entropy value than
one mole of a
gaseous substance.

The standard entropy change, ∆S°, that has taken place at the completion of a reaction is the difference between the standard entropies of the products and the standard entropies of the reactants.

∆S° = ΣS°_products − ΣS°_reactants