AS Chemistry: Acids and Bases — Revision Notes

Condensed for the final weeks. For the full explanation, use the Acids and Bases: Brønsted-Lowry Theory study guide.

Brønsted-Lowry definitions

Acid = proton donor. Base = proton acceptor.

Conjugate pairs differ by exactly one H⁺:

HCl  +  H2O  ->  H3O+  +  Cl-
acid1   base2    acid2   base1

Conjugate pairs:  HCl / Cl-      and     H2O / H3O+

Identify pairs by finding the two species that differ by one proton — one on each side of the equation.

Amphoteric species can act as either: water, HCO₃⁻, HSO₄⁻, amino acids.

The definition is behavioural, not compositional — it describes what a substance does, not what class it belongs to. This is why ammonia counts as a common alkali even though its own formula contains no OH⁻: dissolved in water, NH₃ accepts a proton from H₂O to form NH₄⁺ and OH⁻.

Strong vs weak — and the trap

	Strong	Weak
Dissociation	Complete	Partial (equilibrium)
Examples	HCl, HNO₃, H₂SO₄; NaOH, KOH	Ethanoic, carbonic, citric; NH₃

Strong/weak describes degree of dissociation. Concentrated/dilute describes amount per volume. A weak acid can be concentrated; a strong acid can be dilute. These are independent properties — the single most tested confusion in the topic.

Observable differences between a strong and a weak acid of the same concentration: the strong acid reacts faster with a reactive metal (more free H⁺ immediately available), has a lower pH, and conducts electricity better (full dissociation produces more ions in solution).

Titration curves — the four shapes

Combination	pH at start	Vertical section	Equivalence pH
Strong acid + strong base	~1	Long, pH 3–11	7
Strong acid + weak base	~1	Shorter, pH 3–7	< 7 (acidic)
Weak acid + strong base	~3	Shorter, pH 7–11	> 7 (alkaline)
Weak acid + weak base	~3	No vertical section	~7

The equivalence point is not always pH 7 — it depends on the salt formed. Weak acid + strong base gives a salt that hydrolyses to give an alkaline solution.

Choosing an indicator

An indicator is suitable if its pH range falls entirely within the vertical section of the curve.

Indicator	Range	Use for
Methyl orange	3.1–4.4	Strong acid + weak base
Phenolphthalein	8.3–10.0	Weak acid + strong base
Either	—	Strong acid + strong base
Neither	—	Weak acid + weak base — no sharp change

That last row is the exam favourite: with a weak acid and weak base there is no vertical section, so no indicator works and a pH meter must be used.

Neutralisation and salt formation

Neutralisation is the reaction of H⁺(aq) with OH⁻(aq) to form water:

H+(aq) + OH-(aq) -> H2O(l)

The positive ion left from the acid and the negative ion left from the base combine to form a salt — e.g. hydrochloric acid + sodium hydroxide → sodium chloride + water. At AS Level, pH is described qualitatively only (below 7 acidic, 7 neutral, above 7 alkaline) — numerical Ka, pKa and pH calculations are A2 content.

Half-equivalence point — A Level only

Not assessed at AS: the AS outcome on indicators states that pKa values will not be used, and pKa is defined and used only at A Level (syllabus 25.1.3). Included here so the curve makes sense when you meet it again at A Level.

At exactly half the equivalence volume of a weak acid–strong base titration, [HA] = [A⁻], so at A Level you will use pH = pKa to find Ka from a titration curve. At AS, read titration curves qualitatively: the starting pH, the vertical section, the equivalence point and the choice of indicator.

Exam traps

• Confusing strong/weak with concentrated/dilute.
• Assuming the equivalence point is always pH 7.
• Choosing an indicator whose range lies outside the vertical section.
• Forgetting that a conjugate base of a weak acid is itself a base strong enough to raise pH.
• Writing the conjugate pair as differing by H rather than H⁺.
• Attempting to calculate pH numerically at AS — Ka, pKa and pH calculations are A2 content; at AS, pH is qualitative only.

Self-test

1. Identify both conjugate pairs in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
2. Distinguish a weak acid from a dilute acid.
3. Why is the equivalence pH above 7 for a weak acid with a strong base?
4. Which indicator suits ethanoic acid titrated with sodium hydroxide, and why?
5. On a weak acid–strong base titration curve, why is the equivalence point above pH 7?
6. Give one observable difference between a strong acid and a weak acid of the same concentration.
7. Explain why ammonia is classed as a Brønsted-Lowry base despite containing no hydroxide ion in its own formula.

Answers: 1. NH₃/NH₄⁺ and H₂O/OH⁻. 2. Weak means only partially dissociated into ions; dilute means a small amount of acid per unit volume — a weak acid can be concentrated. 3. The salt formed contains the conjugate base of the weak acid, which hydrolyses in water to produce hydroxide ions. 4. Phenolphthalein — the vertical section runs roughly pH 7–11, and phenolphthalein’s range (8.3–10.0) lies entirely within it. 5. At equivalence the solution contains the salt of a weak acid, whose anion is a base strong enough to raise the pH above 7. 6. The strong acid reacts faster with a reactive metal, has a lower pH, and conducts electricity better, because full dissociation gives more free ions. 7. Dissolved in water, ammonia accepts a proton from H₂O to form NH₄⁺ and OH⁻ — the Brønsted-Lowry definition is behavioural (what a substance does), not based on its own formula.