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Chemistry 2 Unit 4: Acids and bases

Acids donate protons and bases accept them; pH measures acidity, and Ka and Kb describe the strength of weak acids and bases.

Covers: Arrhenius, Brønsted–Lowry and Lewis definitions, pH and pOH, strong and weak acids and bases, Ka and Kb, conjugate pairs, polyprotic acids, acid strength and structure

Study guide

Arrhenius acids produce H⁺ in water and Arrhenius bases produce OH⁻. Brønsted–Lowry acids donate protons (H⁺) and bases accept them; when an acid donates a proton, it becomes its conjugate base, and when a base accepts one, it becomes its conjugate acid. Lewis acids accept electron pairs and Lewis bases donate them. Water is amphoteric and self-ionizes: Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. pH = −log[H₃O⁺], pOH = −log[OH⁻] and pH + pOH = 14 at 25 °C. A pH below 7 is acidic, 7 is neutral and above 7 is basic. Each pH unit is a tenfold change in [H₃O⁺].

Strong acids ionize completely: HCl, HBr, HI, HNO₃, H₂SO₄ (first proton), HClO₄ and HClO₃. Strong bases include Group 1 hydroxides and heavier Group 2 hydroxides. For strong acids, [H₃O⁺] equals the acid concentration. Weak acids ionize only partly, described by the acid dissociation constant Ka = [H₃O⁺][A⁻] ÷ [HA]. A larger Ka means a stronger acid, and pKa = −log Ka. For a weak acid, [H₃O⁺] ≈ √(Ka × C) when ionization is small. Percent ionization increases as a weak acid is diluted. Weak bases, like ammonia, have a Kb, and for a conjugate pair Ka × Kb = Kw. The stronger an acid, the weaker its conjugate base.

Salts can be acidic, basic or neutral depending on their ions: the conjugate base of a weak acid (like acetate) makes a solution basic, and the conjugate acid of a weak base (like NH₄⁺) makes it acidic. Polyprotic acids, like H₂SO₄ and H₃PO₄, lose protons one at a time, with each Ka smaller than the last. Molecular structure affects acid strength: for binary acids, strength increases with bond length down a group (HI > HF); for oxyacids, more electronegative central atoms and more oxygen atoms increase strength (HClO₄ > HClO).

Small, highly charged metal cations such as Fe³⁺ and Al³⁺ make solutions acidic, because they pull electron density from the water molecules bound to them and release H⁺. Cations of Group 1 and the heavier Group 2 metals, and anions of strong acids like Cl⁻ and NO₃⁻, are neutral. For a weak diprotic acid with Ka₁ much larger than Ka₂, nearly all the H₃O⁺ comes from the first ionization, and the concentration of A²⁻ is approximately equal to Ka₂. An amphiprotic ion like HCO₃⁻ or H₂PO₄⁻ makes the solution acidic if its Ka is larger than its Kb, and basic if its Kb is larger.

In organic acids, the acidic hydrogen is the one bonded to oxygen in a –COOH group. Its conjugate base is stabilized by resonance. Hydrogens bonded to carbon are generally not acidic in water. Strong bases such as Ca(OH)₂ or Sr(OH)₂ release two OH⁻ ions per formula unit, so [OH⁻] is twice the base concentration. Find pOH first, then use pH = 14 − pOH.

Key ideas

• Brønsted–Lowry acids donate protons; bases accept them.
• pH + pOH = 14 at 25 °C.
• Strong acids ionize completely; weak acids have a Ka.
• Ka × Kb = Kw for conjugate pairs.
• Structure affects acid strength.

Formulas and rules

Common mistakes

• Thinking a weak acid is the same as a dilute acid; strength is about ionization, not concentration.
• Treating HF as a strong acid; it's weak.

Words to know

Arrhenius acid

A substance that produces H⁺ in water.

Brønsted–Lowry acid

A proton donor.

Brønsted–Lowry base

A proton acceptor.

Lewis acid

An electron-pair acceptor.

Lewis base

An electron-pair donor.

Conjugate base

What remains after an acid donates a proton.

Conjugate acid

What forms when a base accepts a proton.

Amphoteric

Able to act as an acid or a base.

Kw

The ion-product constant of water.

pH

The negative log of the hydronium concentration.

Strong acid

An acid that ionizes completely in water.

Weak acid

An acid that ionizes only partly in water.

Ka

The acid dissociation constant.

pKa

The negative log of Ka.

Kb

The base dissociation constant.

Percent ionization

The share of a weak acid that ionizes.

Polyprotic acid

An acid that can donate more than one proton.

Oxyacid

An acid containing oxygen bonded to a central atom.

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.