Chemistry only

The word alcohol is most commonly associated with alcoholic drinks, unfortunately most alcohols are toxic and are definitely not suitable as drinks. The one alcohol which is less harmful is ethanol, although it still causes many people to do very silly things when taken in excess! Ethanol has been made for thousands of years as an alcoholic drink in a process called fermentation. As an example consider how beer is made.

To make beer, barley or sometimes oat grains are allowed to start germinating by being placed in water. After a few days natuarlly occurring enzymes break down the starch present in the grains into the sugar maltose. This process is called malting. The germinated barley grains are then dried by hot air . These dried grains are often called malt. Hops are then added to give the beer its bitter taste and aroma.

The barley grains are then lightly crushed in a process called milling and finally the grains are transferred to large tanks filled with hot water where the starch in the barley grains is converted into simple sugars by natuarlly occuring enzymes present in the barley. These simple sugars dissolve in the water to form a solution which is often called "sweet wort". This sweet wort is then transferred to copper vessels where it is boiled. The left over grains, often called brewers grains are sold as animal or cattle feed.

Once the sweet wort solution has cooled, it is transferred into large fermenatation tanks where the yeast is added. Yeast is a natural fungus which will produce an enzyme called zymase which will convert the simple sugars in the solution into carbon dioxide gas and the alcohol ethanol. This is fermentation.

The fermentation process is slow and can take days or even weeks. Fermentation is a natural process, it is a form of anaerobic respiration. An equation for fermentation is:

glucose_(aq) → ethanol_(aq) + carbon dioxide_(g)

C₆H₁₂O_6(g) → C₂H₅OH_(aq) + 2CO_2(g)

Making alcohol in the lab

Direct hydration

Alcohol (ethanol) used in alcoholic drinks such as beer, wine and spirits is made by fermentation. However alcohol can also be made a method called direct hydration. Using this method produces alcohol very quickly and being the only substance made there is no waste and no costs are needed in separating out unwanted substance. It is also a continuous process whereas traditional fermenatation is a batch process. This makes direct hydration much more cost effective and more efficient, however is a non-renewable process since the ethene needed for it comes from crude oil.

Alcohol used for industrial purposes such as solvents and in sanitisers will be made by the direct hydration method rather than fermentation. Direct hydration involves adding steam across the C=C in an alkene molecule, usually ethene. The reaction is shown below. A catalyst (phosphoric acid) and a high temperature (300⁰C) and high pressure (65 atmospheres) are both needed to get this reaction to occur smoothly.

Equations for this reaction are:

ethene_(g) + steam_(g) → ethanol_(g)

C₂H_4(g) + H₂O_(g) → C₂H₅OH_(g)

Key Points

• The alcohol ethanol can be made in a process called fermentation. Fermentation is a form of anaerobic respiration where enzymes produce by the fungus yeast convert glucose sugar into ethanol and carbon dioxide gas.
• Fermentation occurs quickest around 37⁰C. This is the optimum temperature for the enzymes produced by yeast to work most efficiently at.
• Alcohols can be made by a process called direct hydration. Here steam adds across a C=C in an unsaturated alkene in the presence of a catalyst and under a high pressure.

Check your understanding - Questions on fermentation.

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