Section 3 d: ethanol
3.9 describe the manufacture of ethanol by passing ethene and steam over a phosphoric acid catalyst at a
temperature of about 300°C and a pressure of about 60–70 atm
3.10 describe the manufacture of ethanol by the fermentation of sugars, for example glucose, at a
temperature of about 30°C
3.11 evaluate the factors relevant to the choice of method used in the manufacture of ethanol, for example
the relative availability of sugar cane and crude oil
3.12 describe the dehydration of ethanol to ethene, using aluminium oxide
Ethanol is a member of the homologous series of alcohols. Its
molecular formula is C2H5OH; its displayed formula is:
Ethanol is used as a fuel, a solvent and in alcoholic drinks.
Manufacture of ethanol: catalytic addition of steam and fermentation.
Hydration of ethene:
word equation
ethene + water (steam)

ethanol
balanced symbol equation
C2H4 +
H2O  C2H5OH
Conditions: 300 C, 60-70 atmospheres of pressure and phosphoric acid as catalyst.
Fermentation
Fermentation is a reaction during which sugar is
converted into ethanol and carbon dioxide by yeast which
are single-celled organisms. Yeast has the necessary
enzyme (biological catalyst) which catalyses this reaction.
This reaction is also known as anaerobic respiration as no
oxygen is involved in the reaction.
Respiration is a reaction that gives out energy so the
yeast ferments the sugar to obtain energy for its life
processes; carbon dioxide and ethanol are the waste
products of this reaction. The ethanol can be obtained
through fractional distillation of the mixture.
The equations for fermentation are:
word equation
sugar (e.g. glucose)
enzyme in yeast
ethanol
+
carbon dioxide
balanced symbol equation
C6H12O6 (aq)  2C2H5OH (aq) + 2CO2 (g)
Conditions: sugar in water, yeast and a temperature of 30 C to 40 C.
As yeast is a living organism the optimum temperature for fermentation is 35 C. At higher temperatures
the reaction nearly stops as the enzyme is broken down or denatured.
Section 3 d ethanol
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After a few days of fermentation, the process stops as the high alcohol content (maximum is around
14%) kills off the yeast. The aqueous alcohol can be concentrated further by fractional distillation which
is what is done in the production of spirits such as whisky, brandy, rum and so on.
Comparing both methods of manufacturing ethanol
Factor to
consider
Hydration of ethene
Availability of
resources
Countries where land is scarce or not
easy to cultivate and have oil reserves,
hydration of ethane will be the favoured
process.
Reaction
conditions
300 C, 60-70 atmospheres of pressure
and phosphoric acid as catalyst. This
means a lot more energy needed.
Faster
Rate of
reaction
Rate of
production
Quality of
product
Continuous process, keeps going
Ethanol is the only product (pure); a
higher yield.
Fermentation
In countries with no crude oil reserves
ethanol is more likely to be made by the
fermentation method; in particular if the
climate allows sugar cane to grow well
and there is plenty of land available
sugar in water, yeast and a temperature
of 30 C to 40 C.
Slow as takes several days
Batch process: needs to be stopped at
time to collect ethanol and started again.
Ethanol is part of a mixture that still
contains water, yeast and other
sediments; needs further purification to
get ethanol – lower yield.
Dehydration of ethanol
This is basically the reverse reaction to the catalytic addition of steam to ethene as dehydration means a
reaction in which a water molecule is removed. The apparatus is shown below. This reaction is also
sometimes referred to as the cracking of ethanol.
This is basically the reverse
reaction to the catalytic
addition of steam to ethene
as dehydration means a
reaction in which a water
molecule is removed. The
apparatus is shown below.
This reaction is also
sometimes referred to as
the cracking of ethanol.
Conditions: heat and a
catalyst such as aluminium
oxide or porcelain.
(from http://www.practicalchemistry.org/experiments/dehydration-of-ethanol-to-form-ethene,237,EX.html
visited on 10/1/2011)
Section 3 d ethanol
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