Suggested Solutions to student questions(soap) 1. How do you explain the cleansing action of soap? The ionic part of the soap molecule is water-soluble while the non-polar hydrocarbon part is soluble in oil and grease. Soap therefore allows otherwise insoluble substances such as oil or grease to become soluble in water. 2. Why is a reflux apparatus used in this experiment? To ensure that the hydrolysis of the fat is as complete as possible. If the mixture was boiled in an open vessel the ethanol would be lost from the mixture. 3. Name the compound CH 2 (OH)CH(OH)CH 2 (OH). Propane-1,2,3-triol (Glycerol). 4. Draw the structure of a fat from which soap can be made. C 15 H 31 COO CH 2 C 15 H 31 COO CH C 15 H 31 COO CH 2 5. Glycerol is a by-product of the reaction in which soap is made. This does not distil over with the ethanol after the reaction is complete, and remains dissolved in the hot water when added to brine to precipitate the soap. Explain why. Glycerol (CH 2 (OH)CH(OH)CH 2 (OH)) has a much higher boiling point than either ethanol or water. With three hydroxyl groups per molecule, there is considerable hydrogen bonding between its molecules. There is also considerable hydrogen bonding between glycerol molecules and water molecules when glycerol is dissolved in water, and this accounts for glycerol remaining in solution when added to brine. 1
Suggested solutions to student questions(ethene) 1. Why is it desirable to push the glass wool into the tube after the ethanol has been added? To ensure that all of the ethanol is soaked up. 2. Why should the ethanol not be heated strongly? Strong heating of the ethanol will cause it to evaporate too quickly and escape from the tube before it can be dehydrated. 3. Would you expect all the test tubes of gas collected to contain equally pure samples of ethene? Explain your answer. The first test tubes of gas collected will contain air and ethanol vapour as well as ethene. Subsequent test tubes of gas collected will contain ethene only. 4. Why is it very important to remove the delivery tube from the water as the Bunsen burner is turned off? To avoid cold water being sucked back from the trough onto the hot glass of the reaction tube. 5. In each of steps 1,2 and 3 in the investigation of properties, give a reason for the results observed. Step 1: The limewater is turned milky by the carbon dioxide formed by the combustion of ethene. Steps 2 and 3: The decolourisations indicate unsaturation. Solutions to student questions(ethanal) 1. What colour change happened in the reaction vessel during the reaction? Name the species responsible for each colour. During the reaction the orange dichromate ion (Cr 2 O 7 2- ) changes to the green chromium(iii) ion (Cr 3+ ) 2
2. Why is it necessary to turn off the Bunsen burner or heating element before the reaction is started? The reaction would become too vigorous, and some unreacted ethanol and other material is likely to be forced over into the conical flask. Once the mixture in the reaction vessel has been brought to the boil, further heating is not required as the oxidation of ethanol is an exothermic reaction, and gentle boiling can be maintained by regulating the flow from the dropping funnel. 3. What is likely to happen if excess sodium dichromate was used by mistake? More ethanoic acid will be produced reducing the yield of ethanal 4. Why is ethanal collected in a vessel embedded in a beaker of ice water? Ethanal boils at 21 0 C and must be kept cold in order to prevent losses through evaporation. 5. Assuming the density of ethanol is 0.80 g cm -3, calculate the number of moles of ethanol used in the experiment. Mass of ethanol = volume x density 10 cm 3 x 0.80 g cm -3 = 8 g Moles of ethanol = Mass / Molar mass = 8 g / 46 = 0.17 mol 6. Calculate the number of moles of sodium dichromate used in this experiment, given that its formula is Na 2 Cr 2 O 7.2H 2 O. Moles of sodium dichromate = Mass/ molar mass = 10/298 mol = 0.034 mol 7. Given that 3 moles C 2 H 5 OH require 1 mole Na 2 Cr 2 O 7.2H 2 O for complete reaction, show clearly that sodium dichromate is the limiting reactant. 3C 2 H 5 OH Ξ 1 Na 2 Cr 2 O 7.2H 2 O 3
So 0.034 mol of dichromate will only oxidise 3 x 0.034 mol of ethanol = 0.102 mol The amount of ethanol actually present (0.17mol) is greater than 0.102 mol. Sodium dichromate is therefore the limiting reactant. 8. What use is made of the Fehling's solution test in the Leaving Certificate Biology course? Fehling's solution is used to test for reducing sugars, i.e. saccharide molecules which contain an aldehyde group (-CHO). Suggested solutions to student questions(ethanoic acid) (1) Explain the term reflux distillation. Reflux distillation involves condensing the vapour from a boiling liquid in such a way as to return the condensed material to the reaction vessel. In this way a reaction may be carried out at quite a high temperature while preventing the loss of any of the reactants or products. (2) Explain why it was thought necessary in stage 5 of the preparation to add the alcohol-water mixture through the condenser. When the alcohol water mixture meets the acidified dichromate solution a strongly exothermic reaction occurs. Without the presence of the condenser the reagents could spray out. (3) Name three possible impurities present in the final distillate. The distillate may contain, along with the ethanoic acid, some water, ethanal and ethyl ethanoate. (4) What colour change happened in the reaction vessel during the reaction? Name the species responsible for each colour. During the reaction, the orange dichromate ion (Cr 2 O 7 2- ) changes to the green chromium(iii) ion (Cr 3+ ). 4
(5) Assuming the density of ethanol is 0.80 g cm -3, calculate the number of moles of ethanol used in the experiment. Mass of ethanol = volume x density = 2 cm 3 x 0.80 g cm -3 = 1.6 g Moles of ethanol = Mass / Molar mass = 1.6 g / 46 g mol -1 = 0.035 mol (6) Calculate the number of moles of sodium dichromate used in this experiment, given that its formula is Na 2 Cr 2 O 7.2H 2 O. Moles of sodium dichromate = Mass / molar mass = 9 g / 298 g mol -1 = 0.03 mol (7) Given that from the balanced equation 3 C 2 H 5 OH 2Na 2 Cr 2 O 7, show clearly that the ethanol is the limiting reactant. 2 mol Na 2 Cr 2 O 7 3mol C 2 H 5 OH So 0.03 mol of dichromate would oxidise 1.5 x 0.03 mol of ethanol. = 0.045 mol of ethanol 0.035 mol (of ethanol) < 0.045 mol Ethanol is the limiting reactant. (8) Given that from the balanced equation 1 C 2 H 5 OH 1 CH 3 COOH, calculate the theoretical yield in grams of ethanoic acid. 1 mol C 2 H 5 OH 1 mol CH 3 COOH 0.035 mol C 2 H 5 OH 0.035 mol CH 3 COOH Mass of ethanoic acid = moles x molar mass 0.035 mol x 60 g mol -1 = 2.1 g (9) If the actual yield of ethanoic acid were required, further purification of the distillate would be necessary. Explain how this might be carried out. Redistil the aqueous ethanoic acid collecting the fraction that boils between 116 0 C and 118 0 C. 5
Suggested answers to student questions (clove oil) (1) Explain why the clove oil could not be distilled directly from the cloves. Clove oil contains component molecules whose boiling points are quite high. Heating them to their boiling point would be destructive. (2) Explain the principles by which steam distillation works. Some organic compounds are immiscible with water. Usually these compounds have a low vapour pressure. After mixing them with water, however, the mixture will distil when the sum of the two vapour pressures reaches atmospheric pressure. It follows, then, that this must happen below the boiling point of water. This process is known as steam distillation. (3) Suggest a reason why the clove oil is much more soluble in non-polar solvents (than in water). The component molecules found in clove oil are often relatively non-polar and will therefore be more soluble in the non-polar solvent than in water. 6