Unit 8 · Acids and Bases

pH ● (core concept) — A measure of hydronium ion concentration: pH = −log[H₃O⁺].

pOH ● (core concept) — A measure of hydroxide ion concentration: pOH = −log[OH⁻].

Hydronium ion (H₃O⁺) ● (core concept) — The aqueous hydrogen ion; 'hydrogen ion' and 'hydronium ion' (H⁺(aq) and H₃O⁺(aq)) are used interchangeably, but H₃O⁺ is preferred.

Autoionization of water ● (core concept) — Water reacts with itself to produce H₃O⁺ and OH⁻: 2H₂O ⇌ H₃O⁺ + OH⁻.

Ion product of water (K_w) ● (core concept) — K_w = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C.

Neutral solution ● (core concept) — A solution in which [H₃O⁺] = [OH⁻], so pH = pOH. At 25°C, pK_w = 14 and a neutral solution has pH = pOH = 7.0. K_w (and thus neutral pH) changes with temperature.

Strong acid ● (core concept) — An acid that completely ionizes in water: HCl, HBr, HI, HClO₄, H₂SO₄, and HNO₃. The hydronium concentration equals the initial acid concentration.

Strong base ● (core concept) — A base that completely dissociates in water: Group 1 and Group 2 hydroxides. [OH⁻] equals the initial concentration for Group 1 hydroxides and is double the initial concentration for Group 2 hydroxides.

Weak acid ● (core concept) — An acid that only partially ionizes in water; the hydronium concentration is much less than the initial acid concentration.

Acid ionization constant (K_a) ● (core concept) — The equilibrium constant for a weak acid's reaction with water: K_a = [H₃O⁺][A⁻]/[HA]; pK_a = −log K_a. The pH of a weak acid solution can be found from the initial concentration and pK_a.

Weak base ● (core concept) — A base that only partially ionizes in water to produce hydroxide; most of the base molecules remain un-ionized.

Base ionization constant (K_b) ● (core concept) — The equilibrium constant for a weak base's reaction with water: K_b = [OH⁻][HB⁺]/[B]; pK_b = −log K_b.

Percent ionization ● (core concept) — The fraction of a weak acid or base that ionizes, expressed as a percentage; it can be calculated from the pK_a (or pK_b) and the initial concentration.

K_a × K_b = K_w ● (core concept) — For any conjugate acid-base pair, K_w = K_a × K_b, or equivalently pK_w = pK_a + pK_b.

Strong acid–strong base neutralization ● (core concept) — H⁺(aq) + OH⁻(aq) → H₂O(l) goes quantitatively; the pH of the mixture is determined by whichever reactant is in excess.

Weak acid + strong base mixtures ● (core concept) — They react quantitatively: HA + OH⁻ → A⁻ + H₂O. Excess weak acid leaves a buffer; equimolar amounts leave the conjugate base A⁻, which hydrolyzes water to give a slightly basic solution.

Weak base + strong acid mixtures ● (core concept) — They react quantitatively: B + H₃O⁺ → HB⁺ + H₂O. Excess weak base leaves a buffer; equimolar amounts leave the conjugate acid HB⁺, which gives a slightly acidic solution.

Weak acid + weak base mixtures ● (core concept) — They react to reach an equilibrium state: HA + B ⇌ A⁻ + HB⁺, rather than reacting quantitatively.

Salt hydrolysis ● (core concept) — The reaction of a conjugate base (or conjugate acid) of a weak acid (or base) with water, which is why equimolar weak-acid/strong-base or weak-base/strong-acid mixtures are not neutral.

Titration curve ● (core concept) — A plot of pH versus the volume of titrant added, summarizing the results of an acid-base titration.

Equivalence point (acid-base titration) ● (core concept) — The point where moles of titrant added equal moles of analyte originally present; the pH there is set by the major species — neutral for strong acid–strong base, but basic for weak-acid titrations and acidic for weak-base titrations because the conjugate hydrolyzes water.

Half-equivalence point ● (core concept) — The point halfway to the equivalence point in a weak acid or base titration, where [HA] = [A⁻]; the pH there equals the pK_a, so titration curves can be used to find pK_a.

Polyprotic acid titrations ● (core concept) — Titration curves of polyprotic acids show the number of acidic protons; the major species and the pK_a of each proton can be identified along the curve.

Structural factors in acid/base strength ● (core concept) — Acid and base strength can be inferred from molecular structure: electronegative atoms, inductive effects, and resonance stabilize the conjugate base and make the acid stronger.

Carboxylic acids ● (core concept) — A common class of weak acids, containing the –COOH group.

Nitrogenous bases and carboxylate ions ● (core concept) — Common weak bases include ammonia and other nitrogen-containing bases, as well as carboxylate ions (conjugate bases of carboxylic acids).

Weak conjugates of strong acids and bases ● (core concept) — Strong acids have very weak conjugate bases, stabilized by electronegativity, inductive effects, or resonance; strong bases have very weak conjugate acids.

pH vs. pK_a (predominant form) ● (core concept) — When the solution pH is below the acid's pK_a, the acid form (HA) predominates; when pH is above the pK_a, the base form (A⁻) predominates.

Acid-base indicator ● (core concept) — A substance that has different properties (such as color) in its protonated versus deprotonated form, so it responds to solution pH; choose an indicator whose pK_a is close to the titration's equivalence-point pH.

Buffer solution ● (core concept) — A solution containing large concentrations of both members of a conjugate acid-base pair; the conjugate acid neutralizes added base and the conjugate base neutralizes added acid, stabilizing the pH.

Henderson-Hasselbalch equation ● (core concept) — pH = pK_a + log([A⁻]/[HA]); it gives the pH of a buffer from the acid's pK_a and the ratio of conjugate base to conjugate acid concentrations.

Buffer capacity ● (core concept) — A buffer's ability to neutralize added acid or base. Raising the concentrations of both buffer components (keeping the ratio fixed) keeps the pH the same but increases capacity; a buffer with more conjugate acid resists added base better, and vice versa.

pH effect on solubility ● (core concept) — A salt's solubility is pH-sensitive when one of its ions is a weak acid, a weak base, or hydroxide; the effect is understood qualitatively with Le Châtelier's principle.