A weak acid is an acid that ionises only partially in water, so fewer hydrogen ions are released.
Malay: Asid lemah · Chinese: 弱酸
A weak acid is an acid that ionises only partially in water, so only some of its molecules split up and comparatively few hydrogen ions, H⁺, are released. Because there are fewer free H⁺ ions at the same molarity, a weak acid reacts more slowly, conducts electricity less well and gives a higher pH than a strong acid of the same concentration.
A worked example makes the partial ionisation clear. Ethanoic acid sets up an equilibrium, CH₃COOH ⇌ CH₃COO⁻ + H⁺, and at any moment most of it stays as whole molecules. The double arrow is the signal: it tells you the ionisation does not go to completion. Carbonic acid and other organic acids behave the same way. So a 1 mol dm⁻³ ethanoic acid contains far fewer than 1 mol dm⁻³ of hydrogen ions, unlike a strong acid of the same molarity.
The confusion to avoid is treating weak as if it meant dilute. Weakness is about the degree of ionisation; dilution is about how much water has been added. A weak acid can be concentrated, and a strong acid can be dilute. Concentrated ethanoic acid is still weak because only a fraction of its molecules ionise; a dilute hydrochloric acid is still strong because the little present is fully ionised. Marks are lost when a student writes “weak” to explain a low concentration, or “dilute” to explain slow reaction.
Within this chapter the weak acid is the mirror image of the strong acid, and the same distinction runs through weak and strong alkalis. Our teachers use side-by-side reactions of ethanoic and hydrochloric acid so the difference between strength and concentration becomes something you can see, not just recite.
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