Year 12 Chemistry Tutorial 9.2.C – Ethanol Answers 1.
(a) (b) (c) (d) (e) (f) 2.
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(a) The larger the molecular mass of the molecule, the less soluble it will be (they are in an inverse relationship). (b) Hydrogen bonding between water molecules and the hydroxy (-­‐OH) group of the alcohol. (c) As the chain length increases, the molecule becomes more non-­‐polar and less polar. For this reason they are less soluble in water. (a) Ethanol is a very polar molecule because of the presence of the hydroxy group. However, it is also non-­‐polar because of the short hydrocarbon chain. For these reasons, ethanol will dissolve in water (polar) and also hexane (non-­‐polar). Water is a completely polar molecule. It will, therefore, only dissolve in ethanol (which can be polar) and not hexane (which is only non-­‐polar). (b) Ethanol is a widely used solvent because it is able to dissolve both polar and non-­‐polar substances. (a) Petrol is made from crude oil which is a non-­‐renewable resource because it is being used up faster than it is being produced. Ethanol can be made from cellulose by fermenting the glucose units which make up the cellulose polymer. Cellulose is the principle component of biomass and since we can consume biomass at the same rate that we replant it, it is considered a renewable resource. (b) Two advantages are that it does not produce sulfur dioxides (cleaner burning fuel) when combusted and that it can be made from renewable resources. Two disadvantages are that car engines would need to be modified to run on a pure ethanol based fuel and that greater engine wear occurs with high ethanol-­‐petrol blends because of the need for higher engine compression ratios. 5.
(a) Glucose, yeast, water and nutrients for the yeast (found in barley, fruit etc.) (b) A temperature of about 37°C and the absence of air. (c) The cellulose must first be chemically broken down into glucose monomer units in a hydrolysis reaction which occurs when the glycosidic bonds between the monomers are broken. 6.
This is the amount of heat energy released when either one gram or one mole a compound is combusted in the presence of excess oxygen. 7.
59°C 8.
(a) 7.7 x 10 kJ mol (b) The value obtained by the students was roughly half of the accepted value. Not all of the heat produced by the spirit burner would have been absorbed by the calorimeter. Some of the heat would have been lost to the surroundings. Also, heat would have been lost from the water to the air because their was no lid. (c) The students should have conducted their experiment in a closed system so that no heat would be lost to the surroundings. They could also have used an insulating material with a lid, such as a foam cup, to prevent heat loss from the water. 9.
Both petrol and ethanol combust in air to produce heat. Petrol has a higher heat of combustion than ethanol, however, petrol contains impurities which produce pollutant exhaust gases such as sulfur dioxide. Ethanol produces only carbon dioxide and water vapour. To use pure ethanol as a fuel, car engines would need to be modified to cope with the higher compression ratios needed. Ethanol can be a renewable resource while petrol is not. 2
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10. Adding ethanol to petrol for use in car engines has both advantages and disadvantages, however, the advantages outweigh the disadvantages. Ethanol burns more efficiently and completely than petrol and produces less toxic emissions. Spark plugs do not need to be replaced as often as less carbon is deposited in the engine. Ethanol has a lower ignition temperature than petrol and so air-­‐fuel combustible mixtures more readily ignite. Ethanol-­‐petrol blends reduce carbon monoxide emission by 25-­‐30% as well as reducing ozone formation. Also, 10% ethanol-­‐petrol blends increase the octane rating of lead-­‐free fuels. On the other hand, engines using ethanol petrol blends require higher engine compression ratios and this increases the wear on the engine. Since this is the only real disadvantage, it would seem that the protection of the environment and reserves of fossil fuels is far more important than the economic cost of having to replace engines more frequently. 11. biomass is crushed and ground ⇓ dilute HCl is added to hydrolyse the cellulose and break it down into glucose units ⇓ mixture is filtered to yield an acidic solution of sugars ⇓ calcium hydroxide is added to neutralise acid ⇓ sugar solution is fermented with yeast in the absence of oxygen ⇓ mixture is distilled to purify the ethanol. Page 2