Reaction with metals
2 marks · EasyExplain why carboxylic acids such as ethanoic acid have a higher pH than hydrochloric acid of the same concentration.
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Carboxylic acids only partially ionise in water (they are weak acids), so they produce fewer hydrogen ions (H⁺) than hydrochloric acid, which fully ionises (is a strong acid). Therefore, the pH of carboxylic acids is higher (less acidic) at the same concentration.
Must show: Partial ionisation for weak acid, Complete ionisation for strong acid, Lower H+ concentration means higher pH
Also accept: Ethanoic acid is a weak acid and does not produce as many hydrogen ions as hydrochloric acid, so pH is higher.
Common mistakes: Saying the acids have equal pH or not mentioning ionisation
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Marking points
B1 · 1 mark · States that carboxylic acids (like ethanoic acid) are weak acids and only partially ionise in water.
B2 · 1 mark · Explains that this results in fewer H⁺ ions (lower concentration of hydrogen ions), so the pH is higher than a strong acid such as hydrochloric acid, which fully ionises.
Worked solution
Ethanoic acid is a weak acid, which means it only partially ionises in water. This results in fewer hydrogen ions in solution compared to a strong acid such as hydrochloric acid, which fully ionises. Therefore, for the same concentration, ethanoic acid has a higher pH than hydrochloric acid.