Reaction with metals
2 marks · HardDescribe two ways you could show that propanoic acid is a weak acid.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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1. Propanoic acid has a higher pH than a strong acid of the same concentration. 2. It only partially turns blue litmus paper red / does not fully ionise in water (only partially ionises).
Also accept: 1. pH is between about 3–5, higher than strong acid. 2. It reacts slowly with metals/carbonates compared to a strong acid.
Common mistakes: Just says 'propanoic acid is weak' without evidence.; States only an observation for a strong acid.
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Marking points
B1 · 1 mark · Any valid explanation: pH is higher than strong acid of the same concentration.
B2 · 1 mark · Any valid explanation: weak acid only partially ionises, reacts slowly, or only partially turns indicator.
Worked solution
1. The pH of propanoic acid is higher than that of a strong acid at the same concentration.
2. Propanoic acid only partially ionises in water (it is not fully ionised), or it turns blue litmus paper red less strongly than a strong acid.