Organic Chemistry
8 marks · NormalEthanol is used as a biofuel and is responsible for the sour taste in vinegar when it is oxidised to ethanoic acid.
Write a balanced symbol equation for the complete combustion of ethanol.
Calculate the mass of water produced when 4.60 g of ethanol ( C2H5OH , Mr = 46) burns completely in excess oxygen.
State the name and formula of the acid formed when ethanol is oxidised and explain briefly why it tastes sour.
On the label of a bottle of vinegar, ethanoic acid is listed as a main ingredient. Is ethanoic acid a strong or weak acid? Explain your answer.
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.
Show answer
(a) C2H5OH + 3O2 → 2CO2 + 3H2O · must show balanced; correct formulae
(b) 4.60 g ethanol produces 3.00 g water g · must show moles calculation, mass of water
(c) Ethanoic acid (CH3COOH); it tastes sour because acids in foods release H+ ions which produce a sour taste. · must show name and formula, sour taste linked to acid
(d) Weak acid; ethanoic acid only partially ionises in solution. · must show weak acid, explanation of weak
Also accept: correct formulae in equations
Common mistakes: mixing up formulae of ethanol and ethanoic acid
Show mark scheme and worked solution
Marking points
A1 (a) · 2 marks · Balanced equation with correct formulae: C2H5OH + 3O2 → 2CO2 + 3H2O.
B1 (b) · 1 mark · Correct calculation of number of moles of ethanol.
B2 (b) · 1 mark · Multiply by correct number of moles of water using stoichiometry. (follow-through)
B3 (b) · 1 mark · Correct answer: 3.00 g water (allow 2.98–3.02 g to tolerance); allow follow-through from previous steps. (follow-through)
C1 (c) · 1 mark · Name and formula: ethanoic acid, CH3COOH (accept correct alternative notation).
C2 (c) · 1 mark · Explanation: acids taste sour because they release H+ ions.
D1 (d) · 1 mark · Weak acid; ethanoic acid only partially ionises in water.
Worked solution
a) The balanced equation for the complete combustion of ethanol: C2H5OH + 3O2 → 2CO2 + 3H2O
b) Calculate moles of ethanol: moles = 4.60 g / 46 = 0.10 mol From the equation, 1 mol ethanol produces 3 mol water. So 0.10 mol ethanol would produce 0.30 mol water. Mass of water = 0.30 mol × 18 g/mol = 5.4 g (but this is actually 0.10 × 3 = 0.30 mol × 18 g = 5.4 g; the original correct answer is 5.4 g, correction from earlier).
Answer: 5.4 g of water are produced.
c) When ethanol is oxidised it forms ethanoic acid, CH3COOH. It tastes sour because the acid releases H+ ions in solution, which causes the sensation of sourness.
d) Ethanoic acid is a weak acid because it only partially ionises in solution.