2.1.4 Acids (a) & (b) formulae of common acids & strength of acids

Syllabus

Acids, bases, alkalis and neutralisation

(a) the formulae of the common acids (HCl, H₂SO₄, HNO₃ & CH₃COOH) & the common alkalis (NaOH, KOH & NH₃) & explanation that acids release H⁺ ions & alkalis release OH^– ions in aqueous solution

What does this mean?

Somethings you just have to learn!

Fortunately, if you've chosen A level Chemistry then you probably already know the following formulae.

If you don't, you absolutely have to learn them as examiners will refer to these compounds by name and expect you to be able to provide the formulae.

The basic definition of an Acid is a substance that can release H⁺ ions when it dissolves in water.

Having 1 H⁺ ion to release makes you monoprotic or monobasic

Having 2 H⁺ ions to release makes you diprotic or dibasic

Releasing all possible H⁺ ions on dissolving makes you a strong acid - totally ionised.

HNO_3(aq) → H⁺_(aq) + NO₃^-_(aq) - Complete ionisation = Strong Acid

CH₃COOH_(aq) ⇌ H⁺_(aq) + CH₃COO^-_(aq) - Partial ionisation = Strong Acid

It is possible for a substance to be acidic in water but not in other solvents.

eg HCl which ionises to form H⁺ ions in water but dissolves as a Covalent substance (HCl) without ionising in methyl benzene. So HCl isn't acidic in methylbenzene.

A base is a substance which can neutralise acids.

An alkali is a base that dissolves in water to release OH^- ions.

Strong bases completely ionises (dissociates) - releases all its OH^- ions.

Weak Bases dissolve in water but only ionise partly so don't produce all the OH^- ions possible

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