IGCSE · Practice questions · Alcohols

Brew it or build it.

Six original Cambridge-style questions on making and using ethanol.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two routes, one product.

Fermentation: glucose, yeast, 25–35 °C, no oxygen; gives CO₂ too.
Hydration: ethene + steam, 300 °C, 6000 kPa, acid catalyst.
Extended: renewable but slow and impure vs fast and pure but non-renewable.
Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Recall

[3 marks]

Describe the conditions needed to make ethanol by fermentation of aqueous glucose.

Yeast is added. ✓
Temperature of 25–35 °C. ✓
Oxygen (air) is kept out: anaerobic conditions. ✓

Recall

[3 marks]

Write the balanced equation for the manufacture of ethanol from ethene, and state the conditions used.

C₂H₄ + H₂O → C₂H₅OH (water as steam). ✓
About 300 °C and 6000 kPa (60 atm). ✓
Acid catalyst (phosphoric acid). ✓

Calculation

[3 marks]

Calculate the maximum mass of ethanol that can be made by fermenting 45.0 g of glucose, C₆H₁₂O₆. (A_r: C 12, H 1, O 16)

M_r glucose = 180, so moles = 45.0 ÷ 180 = 0.250 mol. ✓
1 mol glucose gives 2 mol ethanol, so 0.500 mol ethanol. ✓
M_r ethanol = 46, so mass = 0.500 × 46 = 23.0 g. ✓

Explain

[4 marks]

(Extended) A country has large areas of farmland but no crude oil. Explain which method of making ethanol it should use, giving two advantages and one disadvantage of that method.

Fermentation, because it does not need crude oil. ✓
Advantage: the raw material (sugar from crops) is renewable. ✓
Advantage: low temperature, so low energy costs and simple equipment. ✓
Disadvantage: slow batch process giving impure ethanol that must be distilled (or: uses land that could grow food). ✓

Calculation

[3 marks]

Calculate the volume of carbon dioxide, at room temperature and pressure, produced when 9.2 g of ethanol burns completely. (Molar gas volume 24 dm³)

Moles of ethanol = 9.2 ÷ 46 = 0.20 mol. ✓
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, so 0.40 mol CO₂. ✓
Volume = 0.40 × 24 = 9.6 dm³. ✓

Explain

[2 marks]

Explain why the fermentation mixture is not heated above about 35 °C, and why air is kept out.

Above about 35 °C the enzymes in the yeast are denatured, so fermentation stops. ✓
With air, the yeast respires aerobically and the ethanol can be oxidised to ethanoic acid. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.