A student tests two solutions, both with a pH of 3, using universal indicator. One solution contains hydrochloric acid and the other contains ethanoic acid. The student then adds a small piece of magnesium ribbon to each solution. Describe and explain what the student would observe when the magnesium is added, and explain why both solutions have the same pH despite being different types of acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrochloric acid is a strong acid. Ethanoic acid is a weak acid. Both solutions have been prepared so that they have a pH of 3.
Model answer (4 marks)
1. In the hydrochloric acid solution the magnesium ribbon reacts rapidly, producing a vigorous fizzing of bubbles.
2. In the ethanoic acid solution the reaction is much slower, with only a faint or delayed bubbling.
3. The rapid reaction in HCl occurs because HCl is a strong acid and is completely dissociated, giving a high concentration of free H⁺ ions that immediately attack the magnesium.
4. The slower reaction in CH₃COOH occurs because it is a weak acid and only partially dissociates, so fewer H⁺ ions are available to react with the magnesium at any instant.
5. Both solutions have the same pH because pH depends only on the concentration of free H⁺ ions; the solutions were prepared so that their [H⁺] is identical, even though the acids differ in strength.
2. In the ethanoic acid solution the reaction is much slower, with only a faint or delayed bubbling.
3. The rapid reaction in HCl occurs because HCl is a strong acid and is completely dissociated, giving a high concentration of free H⁺ ions that immediately attack the magnesium.
4. The slower reaction in CH₃COOH occurs because it is a weak acid and only partially dissociates, so fewer H⁺ ions are available to react with the magnesium at any instant.
5. Both solutions have the same pH because pH depends only on the concentration of free H⁺ ions; the solutions were prepared so that their [H⁺] is identical, even though the acids differ in strength.
Examiner tips
- Use the command words ‘describe’ (state what happens) and ‘explain’ (give the reason).
- Show the difference in reaction rate between a strong and a weak acid.
- Mention that pH is a measure of [H⁺] only, not of acid strength.
Common mistakes
- Confusing the rate of reaction with the amount of magnesium dissolved.
- Saying that the weak acid has a higher concentration of H⁺ ions rather than a lower one.
- Claiming that pH is a measure of acid strength instead of [H⁺] concentration.
Mark scheme (4 marks)
- The magnesium reacts faster / more vigorously in the hydrochloric acid than in the ethanoic acid (more bubbles / faster fizzing seen in hydrochloric acid)
- Hydrochloric acid is completely ionised in aqueous solution, so it has a higher concentration of H⁺ ions available than ethanoic acid at the same pH
- Ethanoic acid is only partially ionised in aqueous solution, so only a small number of H⁺ ions are released at any one time
- Both solutions have the same pH because pH measures the concentration of H⁺ ions in solution, and both solutions have been prepared to have the same H⁺ ion concentration / the same number of free H⁺ ions in solution at that moment
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe and explain" question →
- Decode the mark scheme abbreviations →