Proton Transfer, Strong & Weak Acids (Cambridge (CIE) IGCSE Chemistry): Revision Note

Proton transfer, strong & weak acids

Proton transfer

Extended tier only 

• 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 

Proton transfer between hydrochloric acid and water

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

What is a strong acid?

• 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 solutions of a very low pH 

• Strong acids include HCl and H₂SO₄ 

• Example of a strong acid: hydrochloric acid

HCl (aq) → H⁺ (aq) + Cl^- (aq)

What is a weak acid?

• Weak acids partially dissociate (or ionise) in water and produce pH values which are closer to the middle of the pH scale, whilst still being below 7

• 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

• Another example of a weak acid is ethanoic acid which will react with alkalis such as sodium hydroxide to form ethanoate salts

ethanoic acid + sodium hydroxide → sodium ethanoate + water

CH₃COOH + NaOH → CH₃COONa + H₂O

Effect of concentration on strong and weak acids 

• 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

  • A concentrated weak acid only partly dissociates so the H⁺ concentration remains relatively low

Comparing strong and weak acids at the same concentration:

• A strong acid at a given concentration has a lower pH than a weak acid at the same concentration

• This is because the strong acid completely dissociates, producing a higher concentration of H⁺ ions

• A weak acid only partially dissociates, so fewer H⁺ ions are produced and the pH is higher (less acidic)

  • For example, at the same concentration, hydrochloric acid has a lower pH than ethanoic acid

• Weak acids typically give pH values in the range 3 – 6, not pH 1 or 2

Reactivity:

• Since strong acids produce more H⁺ ions at the same concentration, they are more reactive than weak acids

  • For example, hydrochloric acid reacts more vigorously with a metal or carbonate than ethanoic acid at the same concentration

Concentration vs strength:

• A dilute solution of a strong acid (e.g. HCl) can be more acidic than a concentrated solution of a weak acid (e.g. CH₃COOH), because the strong acid dissociates completely even when dilute