Carboxylic Acids
6 marks · NormalA bottle of vinegar is labelled as containing ethanoic acid.
State the name of the main acid found in vinegar.
Explain what is meant by the term 'weak acid', using ethanoic acid as an example.
Vinegar reacts with magnesium metal. Describe the reaction and give one reason why the reaction is slower compared to dilute hydrochloric acid.
State one observation you would make when magnesium reacts with vinegar.
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(a) ethanoic acid
(b) A weak acid is one that only partially ionises in water. Ethanoic acid does not release all of its hydrogen ions in solution, so only some molecules ionise.
(c) Magnesium reacts with ethanoic acid in vinegar to produce magnesium ethanoate and hydrogen gas. The reaction is slower than with hydrochloric acid because ethanoic acid is a weak acid and releases fewer hydrogen ions in solution.
(d) Fizzing (or bubbles) as hydrogen gas is released.
Common mistakes: Confusing strong and weak acid definitions; Incorrect observations (e.g., wrong gas)
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Marking points
B1 (a) · 1 mark · Correct name of main acid (ethanoic acid) in vinegar
B2 (b) · 1 mark · States that a weak acid only partially ionises (or dissociates) in solution
B3 (b) · 1 mark · Applies this idea specifically to ethanoic acid
M1 (c) · 1 mark · Correctly describes the reaction (magnesium + ethanoic acid → salt + hydrogen)
A1 (c) · 1 mark · Gives a valid reason for slower reaction (weak acid or fewer H+ ions released)
B4 (d) · 1 mark · States a valid observation (fizzing/bubbles)
Worked solution
a) The main acid in vinegar is ethanoic acid.
b) A weak acid is one that only partially ionises in water. Ethanoic acid only releases some hydrogen ions, so not all molecules ionise.
c) Magnesium reacts with ethanoic acid to produce magnesium ethanoate and hydrogen gas. The reaction is slower than with hydrochloric acid because ethanoic acid only partially ionises, so there are fewer hydrogen ions available to react.
d) Fizzing (bubbles) would be observed as hydrogen gas is released.