Chapter 17 (Acids & Bases)

Arrhenius acid

A substance that increases H⁺ concentration in water.

Arrhenius base

A substance that increases OH⁻ concentration in water.

Bronsted–Lowry acid

Proton donor (gives H⁺).

Bronsted–Lowry base

Proton acceptor (takes H⁺).

Conjugate acid

Formed when a base gains a proton (H⁺).

Conjugate base

Formed when an acid loses a proton (H⁺).

Strong acid

Completely dissociates in water; produces a large [H⁺].

Weak acid

Partially dissociates; establishes equilibrium; has Ka.

Strong base

Completely dissociates to produce OH⁻ (like NaOH, KOH).

Weak base

Partially accepts H⁺; establishes equilibrium; has Kb.

Common strong acids

HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄.

Common strong bases

Group 1 hydroxides + heavy Group 2 hydroxides (Ca(OH)₂, Sr(OH)₂, Ba(OH)₂).

Autoionization of water

Water acts as both acid and base: H₂O ⇌ H⁺ + OH⁻.

Kw value at 25°C

Kw = 1.0 × 10⁻¹⁴ = [H⁺][OH⁻].

pH

−log[H⁺].

pOH

−log[OH⁻].

Relationship between pH and pOH

pH + pOH = 14 at 25°C.

[H⁺] from pH

[H⁺] = 10^(−pH).

[OH⁻] from pOH

[OH⁻] = 10^(−pOH).

Acid dissociation constant (Ka)

Equilibrium constant for a weak acid dissociation.

Base dissociation constant (Kb)

Equilibrium constant for a weak base.

Relationship between Ka and Kb

Ka × Kb = Kw.

Strength of acid vs conjugate base

Strong acids have weak conjugate bases; weak acids have stronger conjugate bases.

Percent ionization

(% ionization) = ([H⁺]eq / [HA]initial) × 100.

Weak acid pH formula

Use ICE table or [H⁺] ≈ √(Ka × [HA]).

Weak base pH formula

Use ICE table or [OH⁻] ≈ √(Kb × [B]).

Polyprotic acid

Acid that can donate more than one H⁺ (H₂SO₄, H₃PO₄).

Ka1 vs Ka2

for polyprotic acids: Ka1 > Ka2 > Ka3.

When to ignore x in weak acid problems

If x / initial concentration < 0.05.

Strong acid reaction with water

HA → H⁺ + A⁻ (no equilibrium).

Weak acid reaction with water

HA ⇌ H⁺ + A⁻ (equilibrium forms).

Strong base reaction with water

MOH → M⁺ + OH⁻.

Weak base reaction with water

B + H₂O ⇌ BH⁺ + OH⁻.

Common weak acids

HF, CH₃COOH, HCN, NH₄⁺.

Common weak bases

NH₃, CH₃NH₂, anions of weak acids.

What determines acid strength?

Stability of conjugate base: more stability = stronger acid.

Electronegativity and acid strength

Across a period: more electronegative = stronger acid.

Bond strength and acid strength

Weaker H–A bond = stronger acid.

Oxyacid strength rule

More oxygens = stronger acid; higher electronegativity of central atom = stronger acid.

pKa relationship to acid strength

Lower pKa = stronger acid.