Chemistry › Acids and bases
Acids and bases
Unit C11 · 4 lessons.
Pure water at 50 °C has a pH of 6.6 and is still neutral. An acid is a proton donor, pH is calculated for strong acids and bases with Kw and for weak acids with Ka and pKa, and the pH is followed through a titration to choose an indicator. Buffers combine all of it, explaining how a solution resists a change in pH and what pH it holds.
The approximation for a weak acid, [H⁺]² = Ka[HA], assumes so little of the acid dissociates that its concentration is unchanged, and the mark scheme expects that assumption stated. Kw also changes with temperature, so a question set at another temperature needs its own Kw before the pH of a base is worked out.
Questions on it can come up on AQA Papers 1 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1, 2 and 4; the past-paper pages say what else each of those papers covers.
Specification sections: AQA 3.1.12.1, 3.1.12.2, 3.1.12.3, 3.1.12.4, 3.1.12.5, 3.1.12.6 · OCR A 5.1.3 · Edexcel 12.1 to 12.24 · CAIE 7.2, 25.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Brønsted–Lowry acids and bases, pH and strong acids
Brønsted–Lowry acids as proton donors and bases as proton acceptors, conjugate acid–base pairs, strong and weak acids, strength against concentration, pH = −log[H⁺] and its inverse, the pH of strong monoprotic acids and of sulfuric acid, basicity (monobasic, dibasic, tribasic), ionic equations for acids with metals, carbonates and oxides, the effect of dilution, and the pH after mixing a strong acid with a strong base, with either one in excess.
- Weak acids, Ka and pKa; Kw and strong bases
The ionic product of water Kw and its value at 298 K, how Kw and the pH of neutral water change with temperature, the pH of strong bases and their dilution, weak bases such as ammonia, the acid dissociation constant Ka and its units, pKa and the pKa calculation from Ka or from a measured pH, the two approximations used for weak acids and when they fail, the pH of a weak acid from Ka, the concentration of a weak acid from its pH, and pKw and pOH for other boards.
- Titration curves and choosing an indicator
pH curves for strong acid–strong base, weak acid–strong base, strong acid–weak base and weak acid–weak base titrations, the start pH, vertical section and equivalence pH of each, the half-neutralisation point where pH = pKa, how indicators work and how to choose one, pKIn, why no indicator suits a weak–weak titration, calibrating a pH meter, and required practical 9.
- Buffer solutions and buffer calculations
Buffer solutions defined, acidic buffers from a weak acid and its salt or by partial neutralisation, basic buffers from ammonia and ammonium chloride, how a buffer resists added acid and added alkali, the pH of an acidic buffer from Ka and the ratio of acid to salt, the pH after a small amount of acid or base is added, the amount of salt needed for a chosen pH, the Henderson–Hasselbalch equation, the carbonic acid–hydrogencarbonate buffer in blood, and, for Cambridge, the solubility product Ksp with the common ion effect.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
- Measuring pH curves
Calibrate a pH meter, follow a titration with it, plot the pH curve, and read the equivalence volume and the pKa from it.
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