Edexcel Jan 2012 Paper 3 Q5 (with explained solutions) |

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Edexcel Jan 2012 Paper 3 Q5

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PMT 5 Propanoic acid may be prepared by oxidizing propan-1-ol in acidic conditions. CH3CH2CH2OH X + H2SO4 CH3CH2COOH Procedure 1. Pour 10 cm3 of distilled water into a boiling tube and add 12 g of oxidizing agent X. Shake the mixture and leave X to dissolve. 2. Pour 3 cm3 of propan-1-ol into a round-bottom flask and add 10 cm3 of distilled water and a few anti-bumping granules. Set up the apparatus for heating under reflux. 3. Add 4 cm3 of concentrated sulfuric acid, drop by drop, to the propan-1-ol. While the mixture is still warm, add the solution of oxidizing agent X, drop by drop. The energy released from the reaction should cause the mixture to boil without external heating. 4. When all of the solution of X has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape. 5. Distil the mixture in the flask using the apparatus shown below. Collect 56 cm3 of distillate, which is an aqueous solution of propanoic acid. heat (a) Suggest, by name or formula, a suitable oxidizing agent, X, for this reaction. (1)12 *P39305A01216*<br />
(b) What colour change does X undergo when it oxidizes propan-1-ol? Fromto(c) Draw a labelled diagram showing the apparatus for heating under reflux. (d) Give two reasons why the escape of vapour in step 4 should be prevented. PMT (1) (2) (2) Reason 1Reason 2(e) How does the reflux apparatus prevent escape of vapour? (1)*P39305A01316* 13 Turn over<br />
PMT (f) Some water can be removed from the distillate in step 5 by adding a solid drying agent. The solution of propanoic acid can then be decanted leaving the drying agent behind. (i) Suggest a suitable solid drying agent. (1)(ii) Suggest why removing excess solid drying agent by decanting, rather than filtering through filter paper, improves the yield. (1)(g) In a larger scale preparation of propanoic acid, 10.0 g of propan-1-ol was used. (i) Calculate the maximum mass of propanoic acid which could be formed from 10.0 g of propan-1-ol. Propan-1-ol Propanoic acid Molar mass / g mol1 60.1 74.1 (2) 14 *P39305A01416*<br />
PMT (ii) After purification, 6.0 cm3 of dry propanoic acid was obtained. Calculate the percentage yield in the preparation. The density of propanoic acid is 0.99 g cm3. (2) (h) In another experiment, the same reaction mixture (propan-1-ol, X and concentrated sulfuric acid) was heated in the apparatus shown in step 5. Identify the main organic product which would be collected and explain why propanoic acid is not produced. (2) ProductExplanation(Total for Question 5 = 15 marks) TOTAL FOR PAPER = 50 MARKS *P39305A01516* 15<br />

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Question Number 5(a) Question Number 5(b) Question Number 5(c) Question Number 5(d) Correct Answer Reject Mark Potassium dichromate ((VI))/ K2Cr2O7 Sodium dichromate ((VI))/ Na2Cr2O7 IGNORE acidified/solution/aqueous Dichromate ((VI)) ions/Cr2O7 2- Potassium manganate(VII)/ KMnO4 Mark Mark Mark Correct Answer Orange to green/blue/blue-green/ brown Reject yellow Correct Answer Reject Flask fitted with condenser in vertical position (1) Direction of water flow in condenser (1) Heat not required IGNORE contents Stoppered equipment and/ or gaps between flask and condenser loses first mark Correct Answer Reject Any two of Yield would be reduced/reactants and or products would be lost (1) complete oxidation could not occur (1) Vapour is flammable/toxic/ hazardous/harmful/ acidic/irritant (1) To increase (%) yield To prevent boiling dry To allow reaction to go to completion Reactant / product is very volatile 6CH0701 1201<br />
Question Number 5(e) Question Number 5(f)(i) Question Number 5(f)(ii) Reject Mark Correct Answer Any one from Mixture being heated returns to the flask The vapour is (cooled and) condensed The water in the condenser is cold (and flowing) Mark Mark Correct Answer Reject (Anhydrous) calcium chloride/ (Anhydrous) magnesium sulfate/ (Anhydrous) sodium sulfate ALLOW Silica gel ACCEPT formula Calcium oxide Conc sulfuric acid Aluminium chloride Potassium sulfate Copper sulfate Cobalt chloride Calcium carbonate Correct Answer Reject Filter paper absorbs some of the product ALLOW Some product is absorbed BY/ INTO filter paper Answer should be appropriate for collection of a liquid product, not a solid Transfer losses Spillage Product sticks to filter paper Some product is left ON filter paper Decanting is faster 6CH0701 1201<br />
Reject Mark Question Number 5(g)(i) Correct Answer Mol propanol = (10/60.1) = 0.166/ 0.17 = (mol propanoic acid) (1) Mass propanoic acid = (0.166 x 74.1) = 12.32945 = 12.33/12.3 (g) If 0.17 mol then 12.597/12.6 (g) (1) If molar masses are reversed, 8.1066 (g) scores (1) IGNORE sf except 1 sf Correct answer no working (2) marks Question Number 5(g)(ii) Mass propanoic acid = 6 x 0.99 = Correct Answer Reject Mark 5.94 (g) (1) % yield = (5.94/ 12.33)x100 = 48.17% = 48/ 48.2/48.18 % (1) Allow 48.29 (if 12.3 used) 47.14 (if 12.6 used) (1) Allow calculation based on volumes: 12.33g propanoic acid = 12.33/0.99 = 12.455 cm3 (1) % yield = (6.0/ 12.455)x100 = 48.17 % IGNORE sf except 1 sf Correct answer no working (2) marks TE from (i) If molar masses are reversed, 73.24% 6CH0701 1201<br />
Question Number 5(h) Mark Correct Answer Reject (Product) Propanal/CH3CH2CHO/ C2H5CHO (1) ALLOW propan-1-al Product removed as formed/ Incomplete oxidation/only partial oxidation occurs (1) Molecular formulae Formulae written with OH Reaction does not go to completion TOTAL FOR PAPER = 50 MARKS 6CH0701 1201<br />

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