Edexcel Jun 2013 Paper 3 Q3 (with explained solutions) |

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Edexcel Jun 2013 Paper 3 Q3

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3 Chloroalkanes and bromoalkanes can be made from alcohols by reaction of the alcohol with sodium chloride or bromide, in the presence of 50% aqueous sulfuric acid. Iodoalkanes cannot be made from sodium iodide and sulfuric acid; red phosphorus and iodine can be used instead as the halogenating agent. (a) (i) What would you see if concentrated sulfuric acid was added to solid sodium iodide? Give two observations. (2) 1.2.(ii) Explain why sodium iodide and sulfuric acid cannot be used to make iodoalkanes from alcohols. (2)(b) Give the equation for the reaction between phosphorus and iodine to form phosphorus(III) iodide. State symbols are not required. (1) *P41652A0612*<br />
(c) A preparation of 1-iodobutane is given in outline below. Procedure 1. Suitable quantities of red phosphorus and butan-1-ol are placed in a round-bottomed flask fitted with a reflux condenser. 2. The mixture is heated until it boils gently and then the heat source is removed. 3. A suitable quantity of powdered iodine is added in small portions down the condenser at a rate which just maintains gentle boiling. The reaction should be allowed to subside after each addition. 4. After the addition of iodine is complete, the mixture is heated under reflux for 3060 minutes, until little or no iodine is visible. 5. The apparatus is allowed to cool and the condenser rearranged for distillation. 6. The crude 1-iodobutane is distilled off until the residue in the distilling flask is about one-fifth of its original volume. Double its volume of water is added and the distillation continued until no more oily drops condense into the receiver. 7. The crude 1-iodobutane is separated and washed with dilute sodium thiosulfate solution and then with dilute sodium carbonate solution. 8. The organic layer is separated and allowed to stand over anhydrous calcium chloride. (i) What does the manner in which the iodine is added in step 3 suggest about the nature of the reaction? (1)(ii) Completion of step 4 requires that little or no iodine is visible. State what you would look for in this step to ensure that this is true. (1)*P41652A0712* Turn over<br />
(iii) Draw the apparatus that is used in step 6 for distillation. (iv) Suggest why the first washing of the product in step 7 is with dilute sodium thiosulfate solution rather than with water alone. (3) (1)(v) State why calcium chloride used in step 8 must be anhydrous. (1)(vi) To complete the preparation, after decanting the mixture from the calcium chloride, there should be a step 9. What is this step? (1)*P41652A0812*<br />
(d) Chloroalkanes can be made from an alcohol and phosphorus(V) chloride, PCl5. The equation for the reaction of butan-1-ol with PCl5 is CH3CH2CH2CH2OH + PCl5 o CH3CH2CH2CH2Cl + HCl + POCl3 This reaction is not suitable for the manufacture of 1-chlorobutane on a large scale. (i) In a laboratory preparation of 1-chlorobutane, 95.0 g of butan-1-ol was used. Calculate the maximum mass of 1-chlorobutane that could be obtained. (Assume the molar masses are, in g mol1, butan-1-ol = 74.0, 1-chlorobutane = 92.5) (2) (ii) In practice, 95.3 g of 1-chlorobutane was obtained. Calculate the percentage yield. (iii) Give one reason why the actual yield is lower than the maximum possible yield. (1) (1)(iv) Give two reasons why this reaction would not be used industrially to make 1-chlorobutane. (2)(Total for Question 3 = 19 marks) TOTAL FOR PAPER = 50 MARKS *P41652A0912*<br />

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$Question Number 3(a)(i) Question Number 3(a)(ii) Question Number 3(b) Reject White fumes Steamy white Smoke Yellow fumes Yellow liquid Acceptable Answers Any two from Misty /steamy fumes (1) Purple / violet fumes (1) ALLOW purple gas / vapour Brown or black solution/ liquid /solid OR grey / grey-black solid (1) Yellow solid / deposit ALLOW yellow precipitate (1) IGNORE effervescence, bubbles, colour change, coloured fumes, solid disappears, description of smells, identification of products even if incorrect, follow-on tests e.g. effect on potassium dichromate paper Acceptable Answers Reject There is little / no HI formed (which is the reagent needed) (1) Because HI is oxidized (to iodine)/ because iodide ions are oxidized (to iodine)/sulfuric acid is oxidizing / HI reduces sulfuric acid / iodide ions reduce sulfuric acid (1) Must mention oxidation or reduction correctly for second mark IGNORE an elimination reaction would occur ALLOW HI is oxidized to iodine for both marks Iodide ions react with sulfuric acid sulfuric acid oxidizes iodine HI is reduced to iodine Acceptable Answers Reject 2P + 3I22PI3 OR P + 3 / 2 I2PI3 OR P4 + 6I24PI3 ALLOW reversible sign IGNORE state symbols even if incorrect Equations with ions I for I2 Mark Mark Mark<br /> Question Number 3(c)(i) Question Number 3(c)(ii) Acceptable Answers Exothermic ALLOW fast / vigorous / violent Acceptable Answers (very) pale purple / yellow / straw coloured OR colourless mixture / is decolourised OR no purple colour Question Number 3(c)(iii) Diagram to show: Acceptable Answers Reject Dangerous Reactive (In)flammable Volatile Reject clear for colourless no (grey) solid remains add starch Reject Conical flask Mark Mark Mark Distillation flask and still-head and heat (1) (no need for a thermometer) ALLOW appropriate tubing as alternative to still head ALLOW heating with electrical, water bath, Bunsen or just arrow IGNORE thermometer and position, tap funnel in still head, absence of reagents in flask Condenser sloping downwards (1) With water entering at the bottom and suitable receiver (e.g. flask or beaker) (1) Question Number 3(c)(iv) Acceptable Answers Reject This removes / reacts with (any residual) iodine OR Removes excess iodine / I3 Removes acid Removes impurities Removes iodide Removes ions (other than I3 Just reduces iodine to iodide ) Mark<br /> Question Number 3(c)(v) Acceptable Answers Reject Anhydrous calcium chloride / it is drying agent OR anhydrous salt needed to remove water/hydrated salt will not remove water Just calcium chloride is a drying agent Allow moisture for water and absorb for remove Acceptable Answers Question Number 3(c)(vi) Distillation / re-distillation (over a narrow range of temperature) (either side of the boiling temperature of 1-iodobutane) ALLOW fractional distillation IGNORE filtering before distillation and any temperatures given Reject Recrystallization Just purification Reject Acceptable Answers (95.074.0) x 92.5 g (1) = 118.75 / 118.8/ 119 g (1) ALLOW 118.77 (from use of 1.284) 3, 4 or 5 sf in final answer Correct final answer scores 2 marks OR Rounding errors by dividing 95.074.0 as a first step e.g. (95.074.0)=1.28, followed by 1.28 x 92.5 = 118.4 /118 (1) e.g. (95.074.0)=1.3 followed by 1.3 x 92.5 =120.25 /120.3/120 (1) Question Number 3(d)(i) Mark Mark Mark<br /> Question Number 3d(ii) Question Number 3d(iii) Question Number 3d(iv) Mark Mark Mark Acceptable Answers 95.33(d)(i) (95.3118.75) x 100 = 80.2563 = 80.25/80.3 % Many candidates give the answer to 3 d(i) to 3sf e.g. 119 but keep the full answer in their calculator resulting in a answer of 80.25 which is correct and should be allowed TE from 3(d)(i) Reject 80.2 Acceptable Answers Reject One of: Handling / transfer losses competing reactions / (unwanted) side reaction / byproducts form incomplete reaction Just losses/spillage Impure reagents Loss by evaporation Other products form Not enough PCl5 to react Acceptable Answers Reject Two of: low atom economy Ignore low percentage / 80% yieldphosphorus(V) chloride expensivedisposal of unwanted materials expensive or difficultno (large scale) use for POCl3difficult/expensive to separate required product No credit for Slow / time consuming Exothermic Not efficient High energy use Competing reactions Non renewable reactants HCl toxic / acidic Unwanted products Just Atom economy not 100% Just It would be expensive Anything to do with environmental friendliness or the ozone layer or the end of life on Earth! Total for Question 3 = 19 Marks Total for paper = 50 Marks<br />$

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