Alcohol production — AQA A-Level Chemistry
Test yourself on Alcohol production with AQA A-Level practice questions.
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Alcohol production explained
Industrially, an alkene is hydrated directly: ethene is mixed with steam and passed over a catalyst of concentrated phosphoric acid at about 300 °C and 60 to 70 atmospheres.
Read the full explanation
The acid is a catalyst, so it is regenerated and does not appear in the overall equation CH₂=CH₂ + H₂O → CH₃CH₂OH. The process is continuous, fast and gives a pure product with an atom economy of one hundred per cent, but only a small fraction of the ethene reacts on each pass, so unreacted gas is recycled and the overall yield stays high. With an unsymmetrical alkene the water adds to give the more substituted alcohol, so hex-1-ene gives mainly hexan-2-ol, and yield calculations in exams are usually built on that product.
Ethanol is produced industrially by fermentation of glucose. The conditions for this process.
Fermentation uses enzymes in yeast to convert glucose to ethanol in the absence of air: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Every condition can be justified. The temperature is held near 30 to 40 °C because below this the enzymes work too slowly, and much above it they are denatured and stop working altogether. Air is excluded because oxygen would allow the ethanol to be oxidised to ethanoic acid, turning the mixture to vinegar. The mixture is aqueous, since the enzymes need solution to act. It is a batch process, run at atmospheric pressure with simple equipment, and it stops when the ethanol concentration reaches roughly 15 per cent because the yeast can no longer survive.
Ethanol produced industrially by fermentation is separated by fractional distillation and can then be used as a biofuel.
Fermentation gives a dilute aqueous ethanol, so the ethanol has to be separated. Fractional distillation works because ethanol boils at 78 °C and water at 100 °C, so the vapour above the heated mixture is richer in ethanol. In the fractionating column that vapour repeatedly condenses and re-evaporates, each cycle enriching it further, and the distillate collected at about 78 °C is far purer than a single distillation would give. In the condenser, cooling water enters at the lower end and leaves at the upper end so the jacket stays full. The ethanol can then be burned as a biofuel, a fuel made from biomass, that is plant material grown recently rather than from fossil carbon.
Students should be able to: explain the meaning of the term biofuel justify the conditions used in the production of ethanol by fermentation of glucose write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel and give reasons why this statement is not valid outline the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst discuss the environmental (including ethical) issues linked to decision making about biofuel use.
A biofuel is a fuel derived from renewable biological sources. Ethanol is produced by fermentation of glucose using yeast under anaerobic conditions at ~35°C. The carbon neutral claim is tested with three equations: Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), Fermentation (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂), and Combustion (C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O, twice over). Six CO₂ in, six out, so on paper it is neutral. It fails in practice because fossil fuels are burned for transport and distillation. The hydration mechanism involves C=C attacking H⁺, forming a carbocation, water attacking with a lone pair, and H⁺ loss. Environmental and ethical issues must be discussed: land used for biofuel crops can cause deforestation, habitat loss, and raise food prices by displacing food crops.
Your focus
- State the reagent, catalyst and conditions for the industrial hydration of ethene and write the equation.
- Predict the major alcohol formed when an unsymmetrical alkene is hydrated.
- Calculate the mass of alcohol obtained from a given volume of alkene at a stated percentage yield.
Show all 13 objectives
- Write the balanced equation for the fermentation of glucose to ethanol.
- Justify the temperature and the exclusion of air used in fermentation, giving the consequence of departing from each.
- Compare fermentation with hydration of ethene in terms of rate, purity of product and raw material.
- Explain how fractional distillation separates ethanol from the aqueous mixture produced by fermentation.
- Label a distillation apparatus correctly, including the direction of flow of condenser water.
- Define the term biofuel and classify ethanol from fermentation against ethanol from ethene.
- Explain the meaning of the term biofuel and justify the conditions used for fermentation.
- Write and combine the photosynthesis, fermentation and combustion equations to test the carbon neutral claim.
- Outline the mechanism for the acid-catalysed hydration of an alkene, showing the catalyst regenerated.
- Discuss the environmental and ethical issues linked to decision making about biofuel use, such as land use and food supply.
Alcohol production exam tips
Marking Points
- one mark for steam as the reagent and concentrated phosphoric acid as the catalyst
- one mark for conditions of roughly 300 °C and 60 to 70 atmospheres
- one mark for the equation, with the acid shown as a catalyst rather than a reactant
- one mark for identifying the major alcohol from an unsymmetrical alkene, such as hexan-2-ol from hex-1-ene
- one mark for the equation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
- one mark for naming yeast, or the enzymes in yeast, as the catalyst
- one mark for a temperature in the range 30 to 40 °C justified on both sides, too slow below and enzymes denatured above
- one mark for the absence of air, to prevent oxidation of ethanol to ethanoic acid
- one mark for naming fractional distillation and the fractionating column
- one mark for stating that cooling water enters the condenser at the lower end and leaves at the upper end
- one mark for collecting the fraction at the boiling point of ethanol, about 78 °C
- one mark for defining a biofuel as a fuel produced from biomass or recently living material
- Define a biofuel as a fuel derived from renewable biological sources.
- Justify fermentation conditions, such as anaerobic to prevent oxidation or ~35°C to prevent enzyme denaturation.
- Write correctly balanced equations for photosynthesis, fermentation, and combustion to show the carbon dioxide absorbed equals the carbon dioxide released.
- Outline the three curly arrows of the hydration mechanism, ending with the loss of H⁺ to regenerate the catalyst.
- Discuss environmental and ethical issues of biofuels, such as deforestation or the 'food vs fuel' debate raising food prices.
Examiner Tips
- 💡In yield calculations, find moles of alkene first from mass and density, then apply the percentage.
- 💡Quote both temperature and pressure; one alone often does not score the conditions mark.
- 💡Say the catalyst is regenerated if asked why it is not in the equation.
- 💡Justify a temperature by saying what goes wrong at both a lower and a higher value; one side alone is half an answer.
- 💡Balance the fermentation equation by checking carbon first, then hydrogen.
- 💡Use the word anaerobic, or "in the absence of air", explicitly.
- 💡When labelling apparatus, mark water in at the bottom and water out at the top of the condenser every time.
- 💡Justify a fractionating column by saying the components have close boiling points.
- 💡Quote the collection temperature as a range around 78 °C rather than a single exact figure.
- 💡Multiply the combustion equation by two before comparing totals, since fermentation gives two ethanol molecules.
- 💡Give a specific energy input when arguing the claim fails; stating that machines are used is too vague to score.
Common Mistakes
- writing liquid water as the reagent rather than steam
- giving hexan-1-ol from hex-1-ene by adding the water the wrong way round
- including the acid catalyst among the reactants in the overall equation
- concluding that a low conversion on each pass means a low overall yield, when the alkene is recycled
- writing oxygen into the equation, when fermentation is anaerobic
- choosing a high temperature to speed the reaction up, ignoring enzyme denaturation
- saying the yeast is a catalyst and then describing it as used up
- calling fermentation a continuous process, when it is a batch process
- labelling the condenser with water entering at the top, so the jacket does not fill
- saying ethanol boils higher than water because its molecules are larger, ignoring hydrogen bonding in water
- expecting pure ethanol from one simple distillation of the fermented mixture
- calling the separation filtration or evaporation rather than fractional distillation
- Stating that carbon neutral means no carbon dioxide is produced at all; correct by explaining that CO2 absorbed equals CO2 released.
- Unbalancing the combustion equation by forgetting that ethanol already contains an oxygen atom; correct by counting the oxygen in ethanol.
- Leaving the final loss of H⁺ out of the hydration mechanism; correct by showing the O-H bond breaking to regenerate the catalyst.