Chemistry (Chang), 9th Edition

Chapter 18: Entropy, Free Energy, and Equilibrium

Chapter Summary

1. Entropy is usually described as a measure of the disorder of a system. Any spontaneous process must lead to a net increase in entropy in the universe (second law of thermodynamics).

2. The standard entropy of a chemical reaction can be calculated from the absolute entropies of reactants and products.

3. The third law of thermodynamics states that the entropy of a perfect crystalline substance is zero at 0 K. This law enables us to measure the absolute entropies of substances.

4. Under conditions of constant temperature and pressure, the free-energy change ΔG is less than zero for a spontaneous process and greater than zero for a nonspontaneous process. For an equilibrium process, ΔG = 0.

5. For a chemical or physical process at constant temperature and pressure, ΔG = ΔH - TΔS. This equation can be used to predict the spontaneity of a process.

6. The standard free-energy change for a reaction, ΔG^°, can be calculated from the standard free energies of formation of reactants and products.

7. The equilibrium constant of a reaction and the standard free-energy change of the reaction are related by the equation ΔG^° = -RT ln K.

8. Many biological reactions are nonspontaneous. They are driven by the hydrolysis of ATP, for which ΔG^° is negative.