Proton Transfer, Strong & Weak Acids

Exam code: 5070

Written by: Ashika|Reviewed by: Caroline Carroll|Updated 2 July 2026

The earlier definition of an acid and a base can be extended
In terms of proton transfer, we can further define each substance in how they interact with protons

Acids are proton donors as they ionise in solution producing protons, which are H⁺ ions
These H⁺ ions make the aqueous solution acidic

Bases are proton acceptors as they accept the protons which are donated by the acid

Diagram showing the role of acids and bases in the transfer of protons - here water acts as a base as it accepts a proton

Acids can be either strong or weak, depending on how many H⁺ ions they produce when dissolved in water
Strong acids completely dissociate (or ionise) in water, producing a high amount of H⁺ ions and producing solutions with a very low pH
Strong acids include HCl and H₂SO₄
Example of a strong acid: hydrochloric acid

HCl (aq) → H⁺ (aq) + Cl^- (aq)

Weak acids partially dissociate (or ionise) in water, producing fewer H⁺ions in solution than strong acids and producing solutions with pH values which are closer to the middle of the pH scale, whilst still being below 7
Weak acids include organic acids such as ethanoic acid, CH₃COOH
For weak acids, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water
Example of a weak acid: propanoic acid

CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^-

The equilibrium lies to the left, indicating a high concentration of intact acid molecules, with a low concentration of H⁺ ions in the solution

A concentrated solution of an acid is one that contains a higher number of acid molecules per dm³ of solution
It does not necessarily mean that the acid is strong though, as it may be made from a weak acid which does not dissociate completely
For example a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH₃COOH do

Build on this topic