A level Chemistry · Edexcel 9CH0 · Topic 12A
Strong and weak acids
Key points
- Brønsted–Lowry: acids donate H⁺, bases accept H⁺. In HCl + H₂O → H₃O⁺ + Cl⁻, Cl⁻ is the conjugate base of HCl.
- Strong acids (HCl, HNO₃, H₂SO₄) dissociate fully; weak acids (ethanoic acid) dissociate only partly: CH₃COOH ⇌ CH₃COO⁻ + H⁺.
- Strength (degree of dissociation) is not the same as concentration (amount per dm³).
- Acid dissociation constant: ; larger (smaller ) means a stronger acid.
- Strong and weak acids of equal concentration need the same volume of alkali to neutralise, but differ in pH and reaction rate with metals.
- Enthalpy of neutralisation is less exothermic for weak acids because energy is used to dissociate them.
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Acids and bases are defined by what happens to protons. This sub-topic shows how to spot conjugate pairs, why "strong" and "concentrated" mean different things, and how K_a puts a number on the strength of a weak acid.
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