Ethanol from Sucrose: Is the Fermentation Finished?
A school project makes ethanol from sucrose. 500 g of sucrose is dissolved in 2.0 dm³ of water, yeast is added, and the mixture is kept at 30 °C in a sealed 5 dm³ vessel fitted with a tube leading into limewater. After 6 days gas production has almost stopped and the limewater is milky. The mixture is filtered and the filtrate is distilled, collecting everything that boils below 90 °C; 180 cm³ of distillate is collected and its mass is 160.2 g. A sample burns with a blue flame and turns acidified potassium dichromate from orange to green. The candidate claims: 'Fermentation is complete because gas production stopped, and the distillate is pure ethanol because it burns with a blue flame.' Mr sucrose = 342; density of ethanol = 0.789 g/cm³.
1. Calculate the maximum volume of ethanol that could be obtained from 500 g of sucrose, and use the density of the distillate to estimate the percentage of that maximum actually collected.
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Fermentation: C12H22O11 + H2O → 2C6H12O6, then C6H12O6 → 2C2H5OH + 2CO2. So 1 mol of sucrose can give 4 mol of ethanol.
n(sucrose) = 500 / 342 = 1.46 mol, so n(ethanol) = 4 × 1.46 = 5.85 mol. m(ethanol) = 5.85 × 46.0 = 269 g; V = 269 / 0.789 = 341 cm³ of pure ethanol.
Density of the distillate = 160.2 / 180 = 0.890 g/cm³, which is greater than 0.789 g/cm³, so the distillate is a mixture, mainly ethanol and water. Taking x as the ethanol volume fraction: 0.789x + 1.000(1 − x) = 0.890, giving x = 0.52. So the distillate contains about 94 cm³ of ethanol.
Percentage of the theoretical maximum = (94 / 341) × 100 ≈ 28%. Even allowing for distillation losses, this is far below what complete conversion would give, so most of the sucrose was not converted or the ethanol was lost.
2. Evaluate the claim that fermentation is complete. Include a test that would provide direct evidence.
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The claim is not supported by the observations. Gas production stopping can mean the sugar has been used up, but it can equally mean the yeast has stopped working: ethanol is toxic to yeast, and fermentation usually slows and stops at roughly 15% v/v ethanol. The yeast may also have run out of nutrients or been affected by a temperature rise or by acid produced. Limewater turning milky only proves that carbon dioxide was produced at some stage, not that all the sucrose has gone.
Direct test: filter off the yeast and warm the liquid with Benedict's solution. A brick-red precipitate shows reducing sugar (glucose or fructose), so fermentation is incomplete. Sucrose itself is not reducing, so if unhydrolysed sucrose is suspected the sample should first be boiled with dilute acid, cooled and neutralised before testing. The low yield of about 28% of theory points the same way: a completed fermentation would give a much larger fraction of the calculated ethanol.
3. Explain why the distillate is not pure ethanol, and suggest one further test and one purification step that would support your answer.
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The density gives the answer: pure ethanol is 0.789 g/cm³, but the distillate is 0.890 g/cm³. Water (1.00 g/cm³) is the obvious impurity, and it distils over with the ethanol; simple distillation cannot remove it completely because ethanol and water form a minimum-boiling mixture at about 95% ethanol.
A blue flame is not proof of purity either: methanol and other alcohols also burn with a pale blue flame, and the flame only shows that a flammable liquid is present.
Further tests: measure the boiling point. Pure ethanol boils at 78 °C; a higher or drifting boiling point indicates water. Adding a little anhydrous copper(II) sulfate, which turns from white to blue, confirms the presence of water (it does not test for other impurities).
Purification: fractionally distil with a thermometer and collect the fraction boiling at about 78 °C, then dry the distillate with a drying agent such as freshly heated calcium oxide and distil once more.