Organic Chemistry
8 marks · NormalThis question is about oxidation reactions and the behaviour of carboxylic acids with metals.
Describe how ethanol can be oxidised in the laboratory using potassium dichromate(VI) and dilute sulfuric acid. State what observable change is seen in the reaction mixture.
After oxidation, a sample of the product is added to a test-tube containing magnesium ribbon. Describe two observations you would make and write a balanced symbol equation for the reaction.
Explain why ethanoic acid reacts more slowly with magnesium than hydrochloric acid does.
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(a) Ethanol is heated with potassium dichromate(VI) in dilute sulfuric acid under reflux. The solution changes colour from orange to green as the dichromate(VI) is reduced and ethanol is oxidised to ethanoic acid. · must show heating with K2Cr2O7 and dilute H2SO4, reflux or heating, colour change orange to green
(b) Effervescence (fizzing) and the magnesium dissolves forming a colourless solution. Symbol equation: Mg + 2CH3COOH → (CH3COO)2Mg + H2. · must show fizzing or bubbles (hydrogen produced), magnesium dissolving, correct symbol equation
(c) Ethanoic acid is a weak acid so it only partially ionises, providing fewer H+ ions; hydrochloric acid is a strong acid and fully ionises, providing more H+ ions, so it reacts faster with magnesium. · must show ethanoic acid weak, partial ionisation, fewer H+ ions, hydrochloric acid strong, full ionisation
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Marking points
A1 (a) · 1 mark · Ethanol heated with potassium dichromate(VI) and dilute sulfuric acid under reflux or similar.
A2 (a) · 1 mark · Correctly states ethanol oxidised to ethanoic acid.
A3 (a) · 1 mark · Orange to green colour change (or equivalent description).
B1 (b) · 1 mark · Effervescence or fizzing observed (hydrogen gas formed).
B2 (b) · 1 mark · Magnesium dissolves/disappears and a colourless solution forms.
B3 (b) · 1 mark · Symbol equation: Mg + 2CH3COOH → (CH3COO)2Mg + H2
C1 (c) · 1 mark · Mentions ethanoic acid is a weak acid / partially ionised / produces fewer H+ ions.
C2 (c) · 1 mark · Mentions hydrochloric acid is strong / fully ionised / produces more H+ ions, so reaction is faster.
Worked solution
a) Ethanol is oxidised in the lab by heating it with an oxidising agent: potassium dichromate(VI) in dilute sulfuric acid, often under reflux. During the reaction, the orange solution changes to green as the dichromate(VI) is reduced and ethanol is converted to ethanoic acid.
b) When the product (ethanoic acid) is added to magnesium ribbon: - Effervescence (fizzing) is seen as hydrogen gas is formed. - Magnesium ribbon dissolves, leaving a colourless solution of magnesium ethanoate. Equation: Mg(s) + 2CH3COOH(aq) → (CH3COO)2Mg(aq) + H2(g)
c) Ethanoic acid reacts more slowly with magnesium than hydrochloric acid because it is a weak acid, so it only partially ionises and produces fewer H+ ions in solution. Hydrochloric acid is a strong acid and produces more H+ ions, so the reaction is faster.