AP Chemistry Unit 8 (Acids & Bases): Foundations for pH, Equilibrium, and Structure-Based Strength

Arrhenius acid

A substance that produces H+ in water (more realistically, increases H3O+ in aqueous solution).

Arrhenius base

A substance that produces OH− in water (increases hydroxide concentration in aqueous solution).

Brønsted–Lowry acid

A proton (H+) donor.

Brønsted–Lowry base

A proton (H+) acceptor.

Lewis acid

An electron-pair acceptor (often metal cations that accept lone pairs).

Lewis base

An electron-pair donor (a species that donates a lone pair to form a bond).

Hydronium (H3O+)

The form a proton takes in water; H+ quickly binds to H2O to make H3O+.

[H+] shorthand

In aqueous chemistry, [H+] is shorthand for the hydronium concentration, [H3O+].

Amphoteric (amphiprotic)

Able to act as either an acid or a base; water can donate H+ (forming OH−) or accept H+ (forming H3O+).

Conjugate base

The species formed when an acid donates a proton (H+).

Conjugate acid

The species formed when a base accepts a proton (H+).

Conjugate acid–base pair

Two species that differ by one proton (H+), such as HA/A− or B/BH+.

Equilibrium favors weaker acid/base

In proton-transfer reactions, equilibrium tends to favor formation of the weaker acid and the weaker base.

Acid/base strength (vs concentration)

Strength describes extent of ionization/dissociation in water; concentration describes how much solute is dissolved—these are not the same.

Strong acid

An acid that ionizes essentially completely in water (treated as ~100% to products in AP problems).

Weak acid

An acid that ionizes only partially in water and establishes an equilibrium with reactants present.

Strong base

A base that dissociates essentially completely to produce OH− (e.g., Group 1 hydroxides; heavier Group 2 hydroxides).

Weak base

A base that reacts only partially with water to form OH− and establishes an equilibrium.

Autoionization of water

The self-ionization equilibrium: 2H2O(l) ⇌ H3O+(aq) + OH−(aq).

Ion-product constant of water (Kw)

The equilibrium constant for water autoionization: Kw = [H3O+][OH−] = 1.0 × 10−14 at 25°C.

pH

A logarithmic measure of acidity: pH = −log[H3O+].

pOH

A logarithmic measure of basicity: pOH = −log[OH−].

pH + pOH = 14.00 (at 25°C)

The relationship between pH and pOH derived from Kw at 25°C; used to convert between them.

Acid ionization constant (Ka)

For HA + H2O ⇌ H3O+ + A−, Ka = ([H3O+][A−])/[HA]; larger Ka means a stronger weak acid.

Base ionization constant (Kb)

For B + H2O ⇌ BH+ + OH−, Kb = ([BH+][OH−])/[B]; larger Kb means a stronger weak base.