Ethanol: manufacture, combustion and oxidation

Last time

Before we start: last time, in Addition polymers, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Sugar, yeast and petrol

In Brazil many cars run on ethanol made from sugar cane. In a laboratory, the same ethanol can be made from the gas ethene. Two routes, one product. Choosing between them is a chemistry question, and the product can be burned, used as a solvent, or oxidised to vinegar.

Ethanol

Ethanol is C 2 H 5 OH. It is the simplest alcohol that people drink, and it is a colourless liquid at room temperature. It is a covalent compound. Its Mr is 46. It is flammable, and it dissolves both many organic substances and water.

Uses of ethanol

It is a solvent in perfumes, paints, inks and medicines. It is a fuel, either alone or blended with petrol. It burns cleanly. It is the alcohol in alcoholic drinks, and it is used to make other chemicals.

Combustion

Ethanol burns completely in plenty of oxygen to give carbon dioxide and water. The equation: C 2 H 5 OH plus 3O 2 gives 2CO 2 plus 3H 2 O. The reaction is exothermic. Bioethanol, made from plants, is renewable and nearly carbon neutral.

Fermentation

Yeast converts glucose, from sugar, into ethanol and carbon dioxide. C 6 H 12 O 6 gives 2C 2 H 5 OH plus 2CO 2. Conditions: a warm temperature of about 30 degrees, and no oxygen. Too hot, and the enzymes in the yeast are destroyed. Too much oxygen, and the ethanol is oxidised to ethanoic acid instead.

Hydration of ethene

Ethene reacts with steam. C 2 H 4 plus H 2 O gives C 2 H 5 OH. Conditions: a phosphoric acid catalyst, 300 degrees Celsius and about 60 to 70 atmospheres of pressure. The ethene comes from cracking petroleum. It is a non-renewable resource.

Comparing the routes

Fermentation uses renewable sugar, a low temperature and low energy. But it is slow, makes a dilute and impure solution, and works in batches. Hydration is fast, continuous and gives pure ethanol. But it needs high temperature and pressure, and non-renewable ethene. Fermented ethanol is purified by fractional distillation.