Suggested solutions to questions
Sheet 1: Preparation of 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 (Cr2O72-) changes to the green
chromium(III) ion (Cr3+)
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 0C 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 cm3 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
Na2Cr2O7.2H2O.
Moles of sodium dichromate = Mass/ molar mass
= 10/298 mol
= 0.034 mol
7. Given that 3 moles C2H5OH require 1 mole Na2Cr2O7.2H2O for complete reaction, show clearly that
sodium dichromate is the limiting reactant.
3C2H5OH Ξ 1 Na2Cr2O7.2H2O
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?
*Can be used instead of Benedict’s Solution. Fehling's solution is used to test for reducing sugars, i.e.
saccharide molecules which contain an aldehyde group (-CHO).
Sheet 2. Preparation of 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 (Cr2O72-)( Cr7+). changes to the green chromium(III) ion
(Cr3+).
(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 cm3 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
Na2Cr2O7.2H2O.
Moles of sodium dichromate = Mass / molar mass
= 9 g / 298 g mol-1
= 0.03 mol
(7) Given that from the balanced equation 3 C2H5OH ≡ 2Na2Cr2O7, show clearly that the ethanol is the
limiting reactant.
2 mol Na2Cr2O7 ≡ 3mol C2H5OH
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) 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 0C and 118 0C.
(9) Anti-bumping granules are to ensure smooth boiling