Strengths of acids and bases
The Brønsted–Lowry theory says an acid is a proton donor and a base is a proton acceptor. A strong acid (HCl, H₂SO₄, HNO₃) ionises completely. A weak acid (CH₃COOH, H₂CO₃) ionises only slightly and has an acid dissociation constant K_a. In water the ionic product of water is constant: K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.
| Solution | Working | pH |
|---|---|---|
| 0.010 mol/dm³ HCl (strong) | [H⁺] = 0.010 | 2.00 |
| 0.10 mol/dm³ ethanoic acid (K_a = 1.8 × 10⁻⁵) | [H⁺] = √(1.8 × 10⁻⁵ × 0.10) = 1.3 × 10⁻³ | 2.87 |
| 0.010 mol/dm³ NaOH (strong) | pOH = 2, pH = 14 − 2 | 12.00 |
Buffer solutions
A buffer keeps the pH almost constant when small amounts of acid or alkali are added. An acidic buffer is a weak acid with its salt (ethanoic acid and sodium ethanoate). Added H⁺ is removed by the ethanoate ions (A⁻ + H⁺ → HA); added OH⁻ is removed by the acid (HA + OH⁻ → A⁻ + H₂O). Blood is buffered at pH 7.4 by carbonic acid and hydrogencarbonate ions.
Choose the pKₐ of the weak acid and the ratio of salt to acid. See the pH of the buffer.
Hydrolysis of salts
| Salt of… | Example | Solution is… | Reason |
|---|---|---|---|
| Strong acid + strong base | NaCl | Neutral (pH 7) | Neither ion reacts with water |
| Strong acid + weak base | NH₄Cl | Acidic | NH₄⁺ gives H⁺ to water |
| Weak acid + strong base | CH₃COONa | Alkaline | CH₃COO⁻ takes H⁺ from water |
| Weak acid + weak base | CH₃COONH₄ | Close to neutral | Both ions react |
Titration curves and indicators
Run your own titration and find the end point.
| Titration | pH at the equivalence point | Suitable indicator |
|---|---|---|
| Strong acid + strong base | 7 | Almost any: methyl orange or phenolphthalein |
| Weak acid + strong base | Above 7 | Phenolphthalein (range 8.3–10) |
| Strong acid + weak base | Below 7 | Methyl orange (range 3.1–4.4) |
| Weak acid + weak base | Near 7, no sharp change | None suitable |
Key terms
- Brønsted–Lowry acid
- A proton donor.
- Buffer
- A solution that resists changes in pH when small amounts of acid or base are added.
- Ionic product of water
- K_w = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.
- Equivalence point
- The point in a titration at which the reactants are in exact stoichiometric amounts.
Summary
- Acids donate protons; strong acids ionise completely; weak acids have K_a.
- pH = −log[H⁺]; pH + pOH = 14.
- Buffers: pH = pKₐ + log([salt] ÷ [acid]).
- Choose the indicator whose range matches the pH jump at the equivalence point.