Proton Transfer, Strong & Weak Acids (Cambridge International IGCSE Chemistry)

Spec point — 7.1.3 · Define acids as proton donors and bases as proton acceptors Define a strong acid as an acid that is completely dissociated in aqueous solution and a weak acid as an acid that is partially dissociated in aqueous solution State that hydrochloric acid is a strong acid, as shown by the symbol equation, HCl(aq) → H+ (aq) + Cl–(aq) State that ethanoic acid is a weak acid, as shown by the symbol equation,CH3COOH(aq) ⇌ H+ (aq) + CH3COO–(aq)

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