Alkanes and alcohols
Students should be familiar with the fact that chemists group organic compounds together into different families with similar properties, in the same way that they use the periodic table to group elements with similar properties together. Students will have already been introduced to the chemistry of alkanes and alkenes.
Alcohols are another ‘family’ of organic compounds, with ethanol being the best known member of the group. Structurally, they are like alkanes but one of the H’s is replaced with an –OH group. They have some similar properties to alkanes, e.g. they burn, giving carbon dioxide and water. Students will know that alcohols are flammable but may not have considered the use of alcohols as a fuels.
Students need to be able to compare and contrast the different methods for the production of ethanol: by fermentation, from biomass or synthesised from ethene. There are a number of discussion points which can be raised with students. For example, the fact that fermentation can be done with very simple technology, allowing people in the developing world to produce ethanol from crops.
Students should be able to consider and evaluate the cost, yield and concentration of alcohol production using the different methods.
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Links and Resources
Ethanol can be made by at least three methods, which give useful environmental comparisons between industrial processes. In this activity students look at the traditional method for making ethanol - the fermentation of sugars with yeast at optimum temperature (25–50°C).
Glucose ------> ethanol + carbon dioxide
They can the compare this process with the other larger industrial methods of producing ethanol.
Key learning points are:
The yeast respires anaerobically and its enzymes act as a catalyst to convert sugar into carbon dioxide and ethanol. This must be carried out in the absence of air, as ethanoic acid is produced if oxygen is present.
Ethanol is separated from the mixture of ethanol and water by distillation. This method of production is a batch process and is used to make alcoholic drinks. As yeast can only survive to a concentration of 10–15% alcohol, production stops at this point and the reaction vessel must be emptied, cleaned and then refilled with fresh starting materials, i.e. a batch.
Batch processes take longer and cost more than a continuous process, but may be unavoidable. Most of the world’s ethanol is produced by fermenting crop biomass such as sugar cane, sugar beet, rice and maize. The sugars in the biomass need to be broken down into simple sugar molecules before fermentation can take place with yeast.
This resource includes two methods for ethanol production and provides a useful environmental comparison between industrial processes.
Activity 3 (pages 27, 36 & 37) is the fermentaion method which uses distillation to puring the ethanol.
Activity A4 introduces the production of ethanol from E.coli bacteria - this is a research activity. Students can research this method of ethanol production and compare/contrast with the other two methods that they have already explored.
Genetically modified E.coli bacterium can be used to convert water biomass into ethanol. To get round yeast’s limitations of only converting simple sugars such as glucose to ethanol, a genetically modified bacterium called KO11 has been developed. It converts the large complex polymers made from sugars, such as hemicellulose, in biomass to ethanol. This makes it cost effective to use biomass such as wood waste corn stalks and rice husks that would not be economic for yeast fermentation.
The greener industry website provides an overview of all the processes which can be used as a summary.
The properties of alcohols is straightforward to demonstrate and for students to investigate. The heat energy produced through the combustion of alcohols and alkanes can be compared using calorimetry. The alcohol gun is a spectacular demonstration of ethanol as a fuel, and illustrates the principle of the internal combustion engine: a plastic bottle is fitted with spark electrodes, filled with ethanol vapour and corked. The vapour is ignited with a spark and the cork is fired across the room with a small explosion.
These experiments and demostrations could be used as a prompt for discussion about the use of ethanol as a 'green' fuel.
This case study can be used as a starter activity to intiate a discussion about the use of ethanol as a 'green' fuel. It also highlights a possible career as an energy consultant, which makes use of this chemistry knowledge.
The Great Biofuels Debate resources might prove useful for managing the group discussion.
The Ethanol fact Sheet produced by the National Non-Food Crops Centre can be used to help students prepare for the discussion. This provides some useful additional information about ethanol and its uses.
The Catalyst article on Biofuels can also provide informationa bout the use of Ethanol as a fuel.
This experiment can be used to demonstrate that ethanol can be dehydrated to form ethene which is used to make a range of polymers. It is the reverse of the reaction mentioned earlier where ethene is used in the production of ethanol.
When carrying out the demonstration be careful to avoid water being sucked back into the hot boiling tube - an extra pair of hands would be useful with this experiment.
This SEP Booklet provides many engaging practicals and activities to highligh the range of ploymers and plastics that can be made from organic monomers.
The easiest polymerisation reaction to exlpain to students is the addition polymerisation of ethene to make polyethene. The RSC Addition Ploymerisation reaction can be demostrated to the students. These reactions involve monomers that have a double bonds somewhere along the carbon chain.
Condensation Polymerisation and Making Nylon - the 'Nylon Rope Trick' from the RSC can be demostrated to students. The monomers that react to form these polymers do not have a double bond and therefore results in the loss of a small molecule e.g. water. In the manufacture of Nylon the reaction produces HCl as a vapour. It is best to only use very small quantities of the two reactants as this will make quite a long strand of Nylon. Make sure you wash the Nyon with plenty of water before you allow the students to touch it.