Alcohols: Alcohol Production

Question 1

What is the standard industrial method of producing alcohol?

Answer 1

• Alcohols are produced industrially by hydration of alkenes (ethene) in the presence of an acid catalyst
• CnH2n + H20 ⇌ CnH2n+1OH
• Catalyst H+ from phosphoric acid (H3PO4)
• Can also be produced from fermentation of glucose

Question 2

Describe the hydration reaction of ethene to produce ethanol and its conditions

Answer 2

• CH2CH2(g) + H2O(g) → CH3CH2OH(g)
• Ethene + Water → Ethanol
• Phosphoric acid catalyst (H3PO4)
• At 570K
• 6-7 MPa pressure / 60atm

Question 3

Describe the mechanism for the reaction of ethene with steam

Answer 3

• A pair of electrons from double bond bonds to an H+ from acid
• A lone pair of electrons from an oxygen in a water molecule bonds to carbocation (oxygen in H2O has 2 lone pairs)
• Oxygen has 3 bonds and positive charge, so water loses an H+
• Alcohol is formed (and H+)

Question 4

Describe the reaction for the fermentation of glucose

Answer 4

• C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g)
• Glucose → Ethanol + Carbon dioxide
• 310K or (30-40°C)
• Yeast
• Anaerobic conditions
• 1 atm pressure
• Glucose in aqueous solution

Question 5

Why are these specific conditions needed for the fermentation of glucose?

Answer 5

• 30-40°C

• Optimum temperature for yeast enzyme
• Enzyme will denature at higher temperatures and reaction rate will be too slow at lower temperatures

• 1 atm pressure
- Reactants are not gases so pressure does not have effect on rate of reaction

• Anaerobic conditions
- If oxygen is present this may oxidise the ethanol to ethanoic acid

Question 6

How would you separate the mixture of water and ethanol formed during fermentation?

Answer 6

• Fractional distillation

- Ethanol produced from this method is very impure

Question 7

Is the source for fermentation of glucose renewable or non-renewable?

Answer 7

• Renewable
• Sugar is the source
• Starch from sugar cane or sugar beets is refined into sugars
• Ethanol made from renewable sources are often called bioethanol (type of biofuel)
• Biofuels are fuels made from biological (living) raw materials

Question 8

Is the source for hydration of ethene renewable or non-renewable?

Answer 8

• Non-renewable

- Crude oil is the source

Question 9

Compare the two ethanol production methods

Answer 9

• Rate of reaction

• Hydration of ethene - very fast
• Fermentation - very slow

• Quality of product

• Hydration - pure
• Fermentation - impure, needs further processing via fractional distillation

• Raw material

• Hydration - crude oil, finite
• Fermentation - sugars, renewable

• Process/costs

• Hydration - continuous process, so expensive equipment needed; low labour costs
• Fermentation - batch process, cheap equipment; high labour costs

Question 10

Why is the fermentation of glucose a batch process?

Answer 10

• When solution reaches 15% ethanol, the yeast denatures

- Reaction must be stopped before solution reaches this point

Question 11

Describe the environmental advantages and disadvantages linked to biofuels

Answer 11

• Advantages

• Made from renewable energy source - more sustainable than crude oil
• Considered carbon neutral - produce CO2 when burned, absorb CO2 during growth and photosynthesis
• Biofuels limit the demand for fossil fuels, helping to reduce increases in fuel prices

• Disadvantages

• Land for crops may be obtained via deforestation
• Purifying and transporting materials requires energy - likely from fossil fuels
• Fertilisers can pollute waterways

Question 12

Describe the ethical and economic issues linked to biofuels

Answer 12

• Advantages

• ‘Carbon neutral’
• Produces more agricultural jobs than hydration of ethene

• Disadvantages

• May deplete food supply and therefore increase food prices
• Most current car engines would be unable to run on biofuel with high ethanol concentrations

Question 13

What does carbon neutral mean?

Answer 13

• A process in which the net carbon dioxide emission is zero
• Biofuels are not carbon neutral
• Fossil fuels are used in their production e.g making fertilisers for the growing plants and machinery used to harvest the crops

Question 14

Give examples of reactions that support the idea that biofuels are carbon neutral

Answer 14

• 6CO2 + 6H2O → C6H12O6 + 6O2

• Carbon dioxide + Water → Glucose + Oxygen
• Photosynthesis

• C6H12O6 → 2CH3CH2OH + 2CO2

• Glucose → Ethanol + Carbon dioxide
• Fermentation

• 2CH3CH2OH + 6O2 → 4CO2 + 6H2O
- Complete combustion of ethanol

• The amount of CO2 released is equal to the amount of CO2 absorbed when forming the glucose - making it ‘carbon neutral’