Alkenes and Ethanol L.O: To know about alkenes and their chemical formulae, and to understand the methods of making ethanol and their advantages and disadvantages. Saturday, 17 November 2018

Starter : What are the names of these alkanes? : What are their chemical formulae? Methane Ethane Propane Butane C2H6 C4H10 CH4 C3H8

What is the difference between them? Alkanes and Alkenes

Definitions Saturated – all bonds of the carbon atoms are taken up by hydrogen atoms Alkane = any of the series of saturated hydrocarbons (e.g. methane, ethane, propane) Alkene = any of the series of unsaturated hydrocarbons containing a double bond (e.g. ethene, propene) Unsaturated – there are still free bonds on the carbon atoms

Alkanes Alkenes Structure So, alkenes are hydrocarbons which have a double covalent bond between two of the carbon atoms in their chain. Alkanes These carbon atoms have single bonds, with 3 hydrogens, and 1 carbon Alkenes

Copy the structures of ethene and propene in your book… Unsaturated Alkenes are known as unsaturated because you can add more atoms to them — the double bond can open up, allowing the two carbon atoms to bond with other atoms. The first two alkenes are ethene (with two carbon atoms) and propene (three carbon atoms)

Formulae All alkenes have the general formula: CnH2n They have twice as many hydrogens as carbons. You can use this general formula to recognise alkenes in your exam. If a molecule only has hydrogen and carbon atoms in it, and there are twice as many hydrogens as carbons, then it must be an alkene. You can also recognise alkenes from a displayed formula by looking out for the carbon-carbon double bond.

Quick Quiz Are these alkenes or alkanes? CH4 C4H8 C2H6 Methane Ethene Propane Propene C3H8 ALKANE ALKANE ALKANE ALKENE

Testing for Alkenes You can test for an alkene by adding the substance to bromine water. An alkene will decolourise the bromine water, turning it from orange to colourless. This is because the double bond has opened up and formed bonds with the bromine.

Steps of testing for alkenes Put some bromine water into a test tube. Add the substance that you want to test. Shake them together. If the substance added is an alkene, the bromine water will go colourless. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Fill the gaps! Copy this into your book! substance Bromine water colourless an alkene Bromine water

Making ethanol from ethene Ethene (C2H4) can be reacted with steam (H2O) in the presence of a catalyst to make ethanol. This is called a hydration reaction. At the moment this is a cheap process, because ethene's fairly cheap and not much of it is wasted. The trouble is that ethene is produced from crude oil, which is a non-renewable resource that could start running out fairly soon. This means using ethene to make ethanol will become very expensive.

Making ethanol from sugars The alcohol in beer and wine, etc. isn't made from ethene — it's made by fermentation. The raw material for fermentation is sugar. This is converted into ethanol using yeast. The word equation for this is:

Advantages This process needs a lower temperature and simpler equipment than when making ethanol using ethene. It also requires less energy and therefore results in less CO2 production, so fermentation is better for the environment. Another advantage is that the raw material is a renewable resource. Sugar is grown as a major crop in several parts of the world, including many poorer countries. The ethanol produced this way can also be used as quite a cheap fuel in countries which don't have oil reserves for making petrol.

Disadvantages The ethanol you get from this process needs to be purified and it isn't very concentrated, so if you want to increase its strength you have to distil it (as in whisky distilleries). There are also concerns that growing more sugar to make ethanol could lead to more deforestation.

Easier Questions Draw the chemical structure of propene. Explain why alkenes are described as unsaturated. Give two techniques that can be used to produce ethanol. Discuss the advantages and disadvantages of each of these techniques. 3.

Harder Questions A chemist has mixed up his bottles of propane and propene. When he adds bromine water to bottle A the resulting solution is colourless. When he adds bromine water to bottle B the resulting solution is orange. Which bottle contains propene and which contains propane? Which of these is an alkene? A: C5H12 B: C3H6Cl2 C: C4H8 D: C2H4O

Answers (Easier questions) Because more atoms can be added to them — they contain a double bond that can open up, allowing the two carbon atoms to bond with other atoms. Technique 1: By hydrating ethene with steam in the presence of a catalyst. Technique 2: By fermenting sugars. 3.

Answers (Easier Questions) E.g. making ethanol from ethene is cheap at the moment, but ethene comes from crude oil which is a non-renewable resource so when the crude oil starts to run out it will become very expensive. Ethanol made by hydration also occurs at a high temperature. Making ethanol by fermentation requires lower temperatures and simpler equipment and uses less energy than making ethanol from ethene. Sugars are also a renewable resource that won't run out so the production of ethanol by fermentation should remain cheap. But the ethanol produced by fermentation isn't very concentrated and it needs to be purified. Plus, growing more crops to make sugars for ethanol could lead to more deforestation.

Answers (Harder Questions) Bottle A contains propene because the solution turned bromine water colourless. Bottle B must therefore contain propane. C You can tell that C is the alkene because it's the only one that contains just carbon and hydrogen atoms AND has twice as many hydrogen atoms as carbon atoms.