Summary

This chapter traced one continuous idea — equilibrium as a state of equal, opposing rates, dynamic

rather than static — through an increasingly specific series of settings.

It began with physical equilibria: solid-liquid, liquid-vapour and solid-vapour phase changes,

and the dissolution of a solid or a gas in a liquid, each showing the same four general features

(closed system, constant macroscopic properties, dynamic balance, and a characteristic value at a

given temperature). The same dynamic character then extended to chemical equilibrium, formalized

through the Law of Mass Action into a single number, the equilibrium constant $K_c$ — with an

equivalent, pressure-based form $K_p$ related to it by $K_p = K_c(RT)^{\Delta n_g}$ — that summarizes

the position of any homogeneous or heterogeneous equilibrium (with pure solids and liquids always

omitted from the expression). Le Chatelier's principle then supplied a purely qualitative tool for

predicting how any such equilibrium responds to a change in concentration, pressure, volume or

temperature.

The second half of the chapter applied this same equilibrium framework to aqueous ionic systems.

Strong and weak electrolytes were distinguished by their degree of ionization $\alpha$, quantified

by Ostwald's dilution law, $\alpha \approx \sqrt{K_a/C}$. Water's own self-ionization,

$K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}$ at $298\ \text{K}$, underpinned the pH scale, while the ionization constants $K_a$ and $K_b$ of individual weak acids and bases —

linked for any conjugate pair by $K_a K_b = K_w$ — allowed the pH of any such solution to be

calculated directly, with polybasic acids ionizing stepwise through successively smaller

constants. Combining a weak acid or base with a salt of its conjugate gave a buffer solution,

whose pH follows the Henderson-Hasselbalch equation,

$\text{pH} = \text{p}K_a + \log([\text{salt}]/[\text{acid}])$, and which resists pH change precisely

because it holds a reservoir of both members of the conjugate pair. Hydrolysis of salts showed

that a salt's own aqueous pH depends on whether its parent acid and base were strong or weak, in all

four possible combinations. Finally, the same equilibrium logic applied to sparingly soluble ionic …