Edexcel Jan 2014 (IAL) Paper 3 Q4 (with explained solutions) |

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Edexcel Jan 2014 (IAL) Paper 3 Q4

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4 Cyclohexene, C6H10, can be prepared from cyclohexanol, C6H11OH, using the procedure below. Step 1 Place 0.100 mol of cyclohexanol in a flask and add about 4 cm3 of concentrated phosphoric(V) acid, drop by drop, while shaking the flask. Step 2 Assemble the flask for distillation, and collect the liquid which distils over between 70 C and 90 C. Step 3 Add the distillate to an equal volume of a saturated solution of sodium chloride. Shake the mixture, allow the layers to separate, and discard the aqueous (sodium chloride) layer. Step 4 Transfer the layer containing cyclohexene into a small flask. Add a few pieces of a solid drying agent to the crude cyclohexene, stopper the flask and shake it for a few minutes. Step 5 Decant the crude liquid alkene and carry out a final purification in order to obtain pure cyclohexene. (a) (i) Use the formulae of the reactant and product to deduce the role of phosphoric(V) acid in this reaction. (1)(ii) Suggest the main hazard when using concentrated phosphoric(V) acid in this preparation. Give one precaution which should be taken when using it, other than the use of safety goggles and a laboratory coat. (2) HazardPrecaution12 *P42974RA01216*<br />
(b) A student suggested using the apparatus shown in the diagram below to carry out Step 2. Describe two ways in which this apparatus must be modified for safe and efficient use in Step 2. Assume the apparatus is suitably clamped. (2) water out water in electrical heating device anti-bumping granules 12*P42974RA01316* 13 Turn over<br />
(c) (i) Cyclohexene can be separated from other products in Step 3 because it is insoluble in aqueous solutions. Explain this lack of solubility. (2)(ii) Draw a diagram of the apparatus which should be used in Step 3. Label the cyclohexene layer. Data Substance Density / g cm3 Cyclohexene Saturated sodium chloride solution 0.81 1.20 (2) 14 *P42974RA01416*<br />
(d) (i) Suggest a suitable solid drying agent to use in Step 4. (1)(ii) What change would you see in the appearance of the organic liquid when it is dried in Step 4? (1)(e) Suggest a method for the final purification of the crude cyclohexene in Step 5. (1)(f) (i) Calculate the volume of 0.100 mol of cyclohexanol, C6H11OH. The density of cyclohexanol is 0.962 g cm3. (ii) After final purification, the yield of cyclohexene was 5.50 g. Calculate the percentage yield in this reaction. Each mole of cyclohexanol can give a maximum yield of one mole of cyclohexene. (2) (2) (Total for Question 4 = 16 marks) TOTAL FOR PAPER = 50 MARKS *P42974RA01516* 15<br />

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Acceptable answers Reject Mark Dehydrating agent / dehydration/ removes (elements of) water / removes H2O / eliminates water / eliminates H and OH IGNORE reference to catalyst Drying agent Just elimination Acceptable answers Reject Mark Corrosive / burns skin (1) Wear gloves (1) Second mark depends on first being corrosive or harms skin or irritant Just harms skin Toxic Use tongs Avoid spillage Use fume cupboard Acceptable answers Reject Mark First mark Apparatus should not be completely sealed / put vent in apparatus / leave gap between condenser and receiving flask / insert gas outlet / use receiving flask with opening (1) ALLOW Open end of apparatus for pressure release Second mark Move (bulb of) thermometer to opposite opening to condenser (1) These points may be shown on diagram. Just Move thermometer up/ position in neck of flask / position in mouth of flask Question Number 4(a)(i) Question Number 4(a)(ii) Question Number 4(b)<br />
Acceptable answers Reject Mark Just cyclohexene is non-polar Cyclohexene forms permanent dipole-dipole forces Just there are hydrogen bonds in water EITHER Cyclohexene only forms London forces / cyclohexene only forms van der Waals forces / cyclohexene can only form weak forces / cyclohexene is non-polar AND water is polar (1) Hydrogen bonds would be broken if cyclohexene mixed with water / cyclohexene cannot form hydrogen bonds with water / cyclohexene cannot replace hydrogen bonds with a strong bond / cyclohexene cannot form bonds with water of comparable strength (to original ones) (1) OR (alternative approach) Hydrogen bonds would be broken if cyclohexene mixes with water (1) Only weaker London forces would replace them (1) IGNORE comments on ionic bonding in sodium chloride Acceptable answers Reject Mark Separating funnel with tap (and stopper) ALLOW Any shaped tube with opening at top which can be stoppered and tap at bottom (1) Cyclohexene in upper layer (1) Dont penalise if labelled cyclohexane, not ene. Mark independently Filter funnel Buchner funnel Very large opening at the top of the funnel. 3 layers Question Number 4(c)(i) Question Number 4(c)(ii)<br />
Acceptable answers Reject Mark (anhydrous) calcium chloride / CaCl2 / magnesium sulfate / MgSO4 / sodium sulphate / Na2SO4 ALLOW silica gel Other compounds, even if anhydrous Incorrect formulae (concentrated) sulfuric acid Acceptable answers Reject Mark (cloudy) liquid would go clear/ liquid becomes less cloudy Volume decreases Water layer disappears Viscosity changes Acceptable answers Reject Mark (re)distillation (collecting liquid close to its boiling point) ALLOW Simple distillation Fractional distillation Correct description of process collecting liquid more than 5 from its boiling point) Filtering Acceptable answers Reject Mark Mass cyclohexanol = (0.100 x 100) = 10.0/ 10 g (1) Volume = 10.0 0.962 = 10.395 / 10.40/ 10.4 (cm3) (1) Second mark TE from mass of cyclohexanol calculated 10/ 10.39(cm3) Question Number 4(d)(i) Question Number 4(d)(ii) Question Number 4(e) Question Number 4(f)(i)<br />
Acceptable answers Reject Mark 0 overall if yield greater than 100% EITHER Max yield = (0.100 x 82) = 8.20/ 8.2g (1) % yield = (5.50 x 100 ) = 8.20 67.073/ 67.1/ 67% (1) Second mark TE from mass of cyclohexene, but NOT if max yield = 10.4 or 10 (ie from volume of cyclohexanol or molar mass of cyclohexanol). OR Mol cyclohexene = 5.5 82 = 0.067073 (1) Ignore sf except 1 sf % yield = (0.067073 x 100 ) = 0.1 67.073/ 67.1/ 67% (1) Correct answer with no working scores 2 Use of 84 as molar mass cyclohexene scores max 1 Ignore SF except 1 Total for Question 4 = 16 marks Question Number 4(f)(ii)<br />

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