Explain why hydrochloric acid is described as a strong acid, and why ethanoic acid is described as a weak acid. Include what is meant by the term 'weak acid' in your answer.

Hydrochloric acid is a strong acid because it completely dissociates in water:

HCl → H⁺ + Cl⁻

All of the HCl molecules give H⁺ ions, so the concentration of H⁺ equals the initial concentration of HCl.

Ethanoic acid is a weak acid because it only partially dissociates:

CH₃COOH ⇌ CH₃COO⁻ + H⁺

An equilibrium is established between the undissociated acid and its ions, so only a fraction of the acid molecules produce H⁺.

A weak acid is therefore one that establishes such an equilibrium and does not give all of its H⁺ ions.

At the same concentration, the fully dissociated HCl provides a higher H⁺ concentration than the partially dissociated CH₃COOH.

Consequently, HCl has a lower pH than ethanoic acid when both are at equal concentrations.