3.3.5 Alcohols - Chemistry Alevel

Alcohols - Methods for making ethanol

Fermentation of sugars & hydration of ethene.

Alcohols - Fermentation of sugars - temperature

Warm 30°C-35°C as the reaction is too slow below 25°C, whereas above 40°C the enzymes in yeast are denatured. (Compromised temperature used)

Alcohols - Fermentation of sugars - extra conditions

It is fermented in anaerobic conditions to prevent the oxidation of the ethanol produced to ethanoic acid (vinegar).

Alcohols - Fermentation of sugars - pressure.

Atmospheric pressure (not economical to have higher pressure).

Alcohols - Fermentation of sugars - enzyme

Done in the presence of yeast as it contains an enzyme called zymase that converts sugars into ethanol.

Alcohols - Fermentation of sugars - equation of the reaction

C₆H₁₂O₆ (aq) → 2C₂H₅OH (l) + 2CO₂ (g)

Alcohols - Hydration of ethene - catalyst

Concentrated phosphoric acid catalyst which is absorbed on a solid slice surface to increase surface area.

Alcohols - Hydration of ethene - temperature

600°C

Alcohols - Hydration of ethene - pressure

6.5MPa/60-70atm

Alcohols - Hydration of ethene - advantages

It uses a continuous process/the rate of production (reaction) is faster/the percentage yield of the reaction is high/relatively low labour costs/pure ethanol produced/100% atom economy.

Alcohols - Hydration of ethene - disadvantages

Finite raw materials/high energy requirements (more energy needed)/high equipment costs/more expensive

Alcohols - Fermentation of sugars - advantages

Renewable raw materials/low equipment costs/cheaper/low energy requirements/biofuel made.

Alcohols - Fermentation of sugars - disadvantages

Labour costs are relatively high/batch process/slower production (reaction) rate/not pure ethanol produced (separated by fractional distillation after)/atom economy of 51%.

Alcohols - Colour change when adding K₂Cr₂O₇

Orange → green

Alcohols - Half equation for when adding K₂Cr₂O₇

6e⁻ +Cr₂O₇²⁻ + 14H⁺ → 2Cr³⁺ + 7H₂O

Alcohols - What is the use of the alkenes produced by acid-catalysed elimination?

If you start with alcohols made from renewable sources alkenes could be made by this method and then converted to addition polymers. This method provides an alternative route to polymers without using monomers derived from crude oil.

Alcohols - Length & solubility

Small chain alcohols are soluble in water because they can form hydrogen bonds with water molecules however as the alkyl groups get longer they become less soluble