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With fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly being considered as a possible alternative for the future.

Biofuels are produced from natural products, often plant biomass containing carbohydrate. As biofuels are produced from plants, they are renewable and theoretically carbon neutral.

Some biofuels are produced by using microorganisms to anaerobically ferment carbohydrate in the plant material - as is the case with bioethanol and biogas production (each process uses different microorganisms).

Bioethanol

When ethanol is made by fermentation, sugar is converted into ethanol and carbon dioxide if conditions are anaerobic. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen.

In some countries, such as Brazil, the source of sugar is sugar cane – which yeast can directly ferment into ethanol. In other countries, plants such as maize are used. As maize contains starch rather than sugar, the enzyme amylase must first break down the starch into sugar before the yeast can ferment it into ethanol.

The ethanol produced by yeast only reaches a concentration of around 15 per cent before the ethanol becomes toxic to the yeast. In order to make it sufficiently concentrated to be burnt as a fuel, the ethanol must be distilled.

Disadvantages of bioethanol

There are some disadvantages to growing biofuel crops (such as sugar cane and maize) to be used as bioethanol.

• The demand for biofuel crops means greater demand on rainforest land.
• Crops grow slowly in parts of the world that have lower light levels and temperatures, so growing biofuel crops in these countries would not satisfy the demand for fuel.
• For bioethanol to be burnt in a car engine, some engine modification is needed. Modern petrol engines can use petrol containing up to ten per cent ethanol without needing any modifications, and most petrol sold in the UK contains ethanol.
• Although biofuels are in theory carbon neutral, this does not take into account the carbon dioxide emissions associated with growing, harvesting and transporting the crops, or producing the ethanol from them. Therefore, overall, more carbon dioxide is emitted than is absorbed, which means that it contributes to global warming.
• Some people morally object to using food crops to produce fuels. For example, it could cause food shortages or increases in food prices.

Formation of ethanoic acid

When ethanol is oxidised by microbes, it forms a compound called ethanoic acid (CH₃COOH), which is a form of carboxylic acid. This is the acid found in vinegar.

Other alcohols can also be oxidised in a similar way. For example, methanol can be oxidised to form methanoic acid. As well as the gain of oxygen through the oxidation process, hydrogen atoms have been reduced.

Test for alcohols

When alcohols are gently heated with a mixture of potassium dichromate(VI) and sulfuric acid, the mixture changes colour from orange to green. This chemical test was originally used to test for the presence of alcohol on people’s breath if they were suspected of drink-driving.