Carboxylic Acids
7 marks · HardAnswer the following about unbranched-chain carboxylic acids and their reactions.
Give the name and molecular formula of the unbranched-chain carboxylic acid with one carbon atom.
Carboxylic acids react with magnesium carbonate. State one observation you would make during this reaction.
Write a balanced chemical equation (using molecular formulae) for the reaction between ethanoic acid and magnesium carbonate.
Using your knowledge of acid reactions, explain why the reaction between ethanoic acid and magnesium carbonate is slower than the reaction between hydrochloric acid and magnesium carbonate.
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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(a) Methanoic acid, CH2O2
(b) Bubbles/fizzing/effervescence; carbon dioxide gas formed
(c) MgCO3(s) + 2CH3COOH(aq) → (CH3COO)2Mg(aq) + CO2(g) + H2O(l)
(d) Ethanoic acid is a weak acid and only partially ionises in water, giving fewer H+ ions. Hydrochloric acid is a strong acid and provides more H+ ions, so the reaction with magnesium carbonate is faster with hydrochloric acid.
Also accept: Fizzling or bubbling for observation; Effervescence, release of CO2
Common mistakes: Name: formic acid (should be methanoic acid); Equation unbalanced; Saying both acids react at the same rate
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Marking points
B1 (a) · 1 mark · Gives correct name: methanoic acid.
B1 (a) · 1 mark · Gives correct molecular formula: CH2O2.
B1 (b) · 1 mark · Observation: bubbles, fizzing, or evolution of carbon dioxide gas.
M1 (c) · 1 mark · States correct reactants and products for reaction.
A1 (c) · 1 mark · Balanced molecular equation with correct states.
M1 (d) · 1 mark · States ethanoic acid is a weak acid/partially ionises; hydrochloric acid is strong/fully ionises.
A1 (d) · 1 mark · Links lower concentration of H+ ions to slower reaction rate.
Worked solution
a) The unbranched-chain carboxylic acid with one carbon atom is methanoic acid. Its molecular formula is CH2O2.
b) On reaction with magnesium carbonate, bubbles/fizzing/effervescence occurs due to carbon dioxide gas being produced.
d) Ethanoic acid is a weak acid—it only partially ionises in water, releasing fewer H+ ions, so reacts more slowly with magnesium carbonate than hydrochloric acid, which is a strong acid and fully ionises to provide more H+ ions.