Fermentation is the conversion of sugars such as glucose into ethanol and carbon dioxide by enzymes in yeast:C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g) The glucose comes from crops such as sugar cane, sugar beet or maize.Conditions
Yeast, whose enzymes catalyse the reaction.
A temperature of about 35 °C. Too cold and the reaction is slow; too hot and the enzymes are denatured.
Anaerobic conditions (air excluded). If oxygen gets in, the ethanol is oxidised to ethanoic acid (vinegar).
Glucose in aqueous solution.Fermentation stops when the ethanol concentration reaches about 15%, because the ethanol then kills the yeast. The ethanol is separated from the mixture by fractional distillation.Advantages: uses a renewable resource (crops); low temperature and atmospheric pressure, so little energy is needed; simple equipment.
Disadvantages: a slow batch process; the product is impure and must be distilled; land used for fuel crops cannot grow food.Worked example: what mass of ethanol can be made from 90 g of glucose? (Mr: glucose 180, ethanol 46)
Moles of glucose = 90 ÷ 180 = 0.50 mol.
From the equation, 1 mol of glucose gives 2 mol of ethanol, so 1.0 mol of ethanol forms.
Mass of ethanol = 1.0 × 46 = 46 g.
Yeast, whose enzymes catalyse the reaction.
A temperature of about 35 °C. Too cold and the reaction is slow; too hot and the enzymes are denatured.
Anaerobic conditions (air excluded). If oxygen gets in, the ethanol is oxidised to ethanoic acid (vinegar).
Glucose in aqueous solution.Fermentation stops when the ethanol concentration reaches about 15%, because the ethanol then kills the yeast. The ethanol is separated from the mixture by fractional distillation.Advantages: uses a renewable resource (crops); low temperature and atmospheric pressure, so little energy is needed; simple equipment.
Disadvantages: a slow batch process; the product is impure and must be distilled; land used for fuel crops cannot grow food.Worked example: what mass of ethanol can be made from 90 g of glucose? (Mr: glucose 180, ethanol 46)
Moles of glucose = 90 ÷ 180 = 0.50 mol.
From the equation, 1 mol of glucose gives 2 mol of ethanol, so 1.0 mol of ethanol forms.
Mass of ethanol = 1.0 × 46 = 46 g.