Carboxylic Acids
5 marks · EasyCarboxylic acids are weak acids and react with metals to produce salts.
State the name of the unbranched-chain carboxylic acid with four carbon atoms.
Write its molecular formula.
Describe what is meant by a 'weak acid'.
Explain why carboxylic acids react more slowly with magnesium than hydrochloric acid.
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(a) Butanoic acid
(b) C4H8O2
(c) A weak acid is only partially ionised in solution, so only some of its molecules release hydrogen ions; most remain as intact acid molecules. · must show partial ionisation, many molecules remain unionised
(d) Carboxylic acids are weak acids, so they provide fewer hydrogen ions than strong acids like hydrochloric acid and react more slowly with magnesium. · must show weaker acid so slower reaction, fewer/free hydrogen ions
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Marking points
B1 (a) · 1 mark · Correct name: butanoic acid
B2 (b) · 1 mark · Correct molecular formula: C4H8O2
B3 (c) · 1 mark · States that weak acids are only partially ionised in solution
B4 (c) · 1 mark · Mentions that most acid molecules remain un-ionised/undissociated
B5 (d) · 1 mark · Describes that the reaction is slower because carboxylic acid provides fewer hydrogen ions (is a weak acid)
Worked solution
a) The unbranched-chain carboxylic acid with four carbon atoms is butanoic acid.
b) Its molecular formula is C4H8O2.
c) A weak acid is only partially ionised in solution; only a small fraction of its molecules release hydrogen ions (H+), with most molecules staying as intact acid.
d) Carboxylic acids react more slowly with magnesium because they are weak acids and have fewer free hydrogen ions, so the reaction rate is lower than with strong acids like hydrochloric acid.