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Oxidation of alcohols This package has been written to help you to have a better understanding of the oxidation of ethanol. You can help to improve the quality of your drawings by assessing the work of some of the 2001 A Level students. Start

Task 1: Students were asked to draw a labelled apparatus that could be used to oxidise ethanol to ethanal only. They should have included any special precautions necessary and assumed that the ethanal would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C (6 marks) Drawing apparatus that works! Student J Which student was correct? Student m answer on to task 2

Ethanol is oxidised first to ethanal To be sure that further oxidation is prevented, the ethanal should be distilled from the reaction mixture as soon as it is formed. 3CH 3 CH 2 OH (l) + Cr 2 O 7 2- (aq) 3CH 3 CHO (aq) + 2Cr 3+ (aq) + 7H 2 O (l) + 8H + (aq) must be acidified The potassium dichromate(VI) Student m on to task 2 back to task 1

Task 2: Students were then asked to draw a labelled apparatus that could be used to fully oxidise ethanol without separating it from the reaction mixture. Special precautions were again to be considered. The ethanoic acid would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C. (5 marks) Drawing apparatus that works! Which students diagram is better? Student J Student C back to task 1answer

So to oxidise ethanol directly to ethanoic acid, a reflux apparatus must be used 3CH 3 CHO (aq) + Cr 2 O 7 2- (aq) + 8H + (aq) 3CH 3 COOH (aq) + 2Cr 3+ (aq) + 4H 2 O (l) Overall, excess oxidising agent must be used to make sure all the ethanol & ethanal is oxidised to ethanoic acid Student J now see if you can assess some students diagrams

How many marks you would give for each of these diagrams? Explain your allocation of marks in the empty boxes. (Press escape to type, then click to continue.) Drawing apparatus that works! Task 1: Draw a labelled apparatus that could be used to oxidise ethanol to ethanal only. Include any special precautions necessary. Assume that the ethanal would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C (6 marks) Student G Ethanol & potassium dichromate answer

Drawing apparatus that works! Task 1: Draw a labelled apparatus that could be used to oxidise ethanol to ethanal only. Include any special precautions necessary. Assume that the ethanal would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C (6 marks) Answer ethanol & potassium dichromate (1) no acid used (essential for the oxidation process) safe heating (1) student cant be sure that the ethanal wouldnt be contaminated with ethanol gas syringe is not gas tight, & if it was, the bung might come out ! is 22 o C really hot enough? Student G Ethanol & potassium dichromate

Drawing apparatus that works! Task 1 continued: Draw a labelled apparatus that could be used to oxidise ethanol to ethanal only.(6) answers Student M Student m

Drawing apparatus that works! Task 1 continued: Draw a labelled apparatus that could be used to oxidise ethanol to ethanal only.(6) Answer ethanol & acidified potassium dichromate (2) safe heating (1) ethanal condensed but will it evaporate again? is 21 o C really hot enough? Answer ethanol & acidified potassium dichromate (VI) (2) controlled & safe heating of reaction mixture (1) thermometer ensures that distillation can be stopped before unreacted ethanol is collected (1) ethanal condensed and collected as a liquid (2) On to task 2 Student M Student m

Drawing apparatus that works! Task 2: Draw a labelled apparatus that could be used to fully oxidise ethanol without separating it from the reaction mixture. Include any special precautions necessary. The ethanoic acid would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C. (5 marks) How many marks you would give for each of these three diagrams? Explain your allocation of marks in the boxes. Student C answer

Drawing apparatus that works! Task 2: Draw a labelled apparatus that could be used to fully oxidise ethanol without separating it from the reaction mixture. Include any special precautions necessary. The ethanoic acid would boil off at a temperature higher than 21 o C. Ethanols Bp. is 79 o C. (5 marks) Answer ethanol & acidified (1) potassium dichromate (VI) (1) excess oxidising agent should be used the heating method is dangerous in this case because of a flying bung and then a possible fire ! what is the thermometer for ? How many marks you would give for each of these three diagrams? Explain your allocation of marks in the boxes. Student C

Drawing apparatus that works! Task 2 continued: Draw a labelled apparatus that could be used to oxidise ethanol completely to ethanoic acid.(5) answers Student B Student J

Drawing apparatus that works! Task 2 continued: Draw a labelled apparatus that could be used to oxidise ethanol completely to ethanoic acid.(5) Answer ethanol & acidified (1) potassium dichromate(VI) (1) excess oxidising agent should be used attempt to reflux safely (1) Answer ethanol & excess (owtte) (1) acidified (1) potassium dichromate (VI) (1) safe heating (1) under reflux (1) ensures conversion of ethanol to ethanoic acid (careful heating by Bunsen is acceptable) Student B Student J see diagram below for how to draw a reflux condenser ! ethanol will escape

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