Edexcel Jun 2016 (IAL) Paper 2 Q

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 16 Sunflower oil is an important edible oil. It can be used as an alternative to butter in cooking. A useful method of comparing fats and oils is by measuring their iodine values. An iodine value is the amount of iodine in grams that reacts with 100 g of a fat or oil. The iodine value is a measure of the degree of unsaturation of the fat or oil. The iodine value of sunflower oil can be determined in the following way. Add 0.200 g of sunflower oil to a 250 cm3 conical flask. Add 10 cm3 of solvent to dissolve the oil. Add 10.0 cm3 of a solution of iodine monochloride, called Wijs solution. Stopper the flask and allow to stand in the dark for half an hour. Add 15 cm3 (an excess) of 10% potassium iodide solution and 100 cm3 of water, and shake the mixture. Titrate the liberated iodine with 0.100 mol dm3 sodium thiosulfate solution. This gives the sample titre. Carry out a blank titration with the same sodium thiosulfate solution, using 10 cm3 of solvent, 10.0 cm3 of Wijs solution, 15 cm3 of 10% potassium iodide solution and 100 cm3 of water. (a) Trichloromethane and 1,1,1-trichloroethane are two possible solvents for this reaction. (i) Give the skeletal formulae for trichloromethane and 1,1,1-trichloroethane . (2) Trichloromethane 1,1,1-trichloroethane 18 *P46659A01824*<br /> (ii) Explain why 1,1,1-trichloroethane has a higher boiling temperature than trichloromethane. (2)(iii) Suggest why solvents such as trichloromethane and 1,1,1-trichloroethane are no longer used. (1)(b) (i) Complete the formula of the product when iodine monochloride reacts with linoleic acid, CH3(CH2)4CH CHCH2CH CH(CH2)7COOH, the most abundant unsaturated compound in sunflower oil. CH3(CH2)4 C C CH2 C C (CH2)7COOH (ii) Iodine monochloride solution is preferred to iodine solution for this reaction because it is more reactive. Explain why this is so. (1) (1)(iii) Suggest why Wijs solution is stored in a brown bottle. (1)*P46659A01924* 19 Turn over<br /> (iv) The equation for the reaction between iodide ions and iodine monochloride is given below. Show that this is a redox reaction by giving all the oxidation numbers and identifying the oxidizing agent. ICl + II2 + Cl Oxidation numbers(2) Oxidizing agent(c) (i) Starch solution is usually added as an indicator towards the end of the titration. Describe how the colour of the mixture would change during the titration, before starch is added. (1)(ii) Explain why starch solution is not added at the start of the titration. (1)(d) In the blank titration, 40.0cm3 of 0.100 mol dm3 sodium thiosulfate solution reacted with 10.0 cm3 of Wijs solution. (i) Calculate the number of moles of 0.100 mol dm3 sodium thiosulfate that reacted in the blank titre. (ii) Calculate the number of moles of iodine, I2, which reacted with the thiosulfate solution in the blank titration. 2S2O3 2(aq) + I2(aq)S4O6 2(aq) + 2I(aq) (1) (1) 20 *P46659A02024*<br /> (iii) Using your answer to (d)(ii), and the equation in (b)(iv), deduce the corresponding number of moles of iodine monochloride solution in 10.0 cm3 of Wijs solution. (iv) The number of moles of iodine monochloride left after reacting the Wijs solution with the sample of the sunflower oil, calculated from the titre, is 1.10103 mol. Use this, and your answer to (d)(iii), to calculate the number of moles of iodine monochloride that reacted with the sample. (v) Your answer to (d)(iv) is equal to the number of moles of iodine that would have reacted with 0.2 g of sunflower oil. Calculate the number of moles of iodine that would have reacted with 100 g of sunflower oil. (vi) Calculate the mass of iodine, I2, which would have reacted with 100 g of sunflower oil, which is the iodine value for the sunflower oil. (1) (1) (1) (1) *P46659A02124* 21 Turn over<br /> (e) Butter contains a smaller percentage of unsaturated molecules than sunflower oil. Would the titre value and iodine value for butter be higher, lower or about the same as the values for sunflower oil? (1) Titre valueIodine value(Total for Question 16 = 19 marks) TOTAL FOR SECTION C = 19 MARKS TOTAL FOR PAPER = 80 MARKS 22 *P46659A02224*<br />

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