Carboxylic Acids
6 marks · NormalStudy the information about the carboxylic acid family and their behaviour as acids.
Give the name and molecular formula of the unbranched-chain carboxylic acid with three carbon atoms.
Explain why carboxylic acids, such as the one named in (a), are described as weak acids.
A student adds magnesium strip to a solution of this acid. The reaction is much slower than when magnesium is added to dilute sulfuric acid. Explain two reasons for this difference.
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(a) propanoic acid, C3H6O2
(b) Carboxylic acids are weak acids because they only partially (not fully) ionise in solution, so only a small fraction of their molecules release hydrogen ions.
(c) Magnesium reacts more slowly with propanoic acid than sulfuric acid because: 1. Propanoic acid is a weak acid and provides fewer hydrogen ions in solution for reaction. 2. Sulfuric acid is a strong acid, which is fully ionised, so it gives a higher concentration of hydrogen ions, making the reaction faster.
Common mistakes: Partial molecule for formula; Reason not linked to weak acid property
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Marking points
B1 (a) · 1 mark · Correctly names propanoic acid (3 carbons, unbranched)
B2 (a) · 1 mark · Correct molecular formula (C3H6O2)
B3 (b) · 1 mark · States the main reason (partial ionisation/dissociation) for weak acid
B4 (b) · 1 mark · Relates this property to hydrogen ion (H+) availability in solution
B5 (c) · 1 mark · First reason: fewer hydrogen ions from propanoic acid because it is weak
B6 (c) · 1 mark · Second reason: sulfuric acid is strong/fully ionised, so reaction is faster
Worked solution
a) The unbranched-chain carboxylic acid with three carbon atoms is propanoic acid. Its molecular formula is C3H6O2.
b) Carboxylic acids are described as weak acids because they only partially ionise (dissociate) when dissolved in water, so only some of their molecules release hydrogen ions.
c) 1. The reaction with magnesium is slower because propanoic acid is a weak acid and only gives a low concentration of hydrogen ions for the reaction. 2. Dilute sulfuric acid is a strong acid, fully ionised in solution, so it provides a much higher concentration of hydrogen ions, making the reaction faster.