14 5 This question concerns iodine monochloride, ICl, a red‑brown solid which melts at 27 °C to form a red‑brown liquid. Iodine monochloride is used in measuring unsaturation in organic compounds. (a) Iodine monochloride gas can be produced by the reaction between iodine vapour and chlorine gas. The reaction is exothermic. I2(g)  +  Cl2(g)  →  2ICl(g)    ΔrH = −30 kJ mol−1 The table shows bond energy values for the bonds in iodine and chlorine. Calculate the value of the bond energy of the I Cl bond using these data and the equation. Bond Energy / kJ mol−1 I I 151 Cl Cl 243 (2) 15 Turn over (b) Iodine monochloride is a polar molecule which adds rapidly to double bonds in a similar way to hydrogen chloride. This reaction can be used to determine the degree of unsaturation in oils. (i) Add the dipole to a molecule of iodine monochloride. (1) I Cl (ii) Draw the mechanism for the addition of iodine monochloride to propene. You should include all curly arrows and relevant lone pairs and dipoles. (3) 16 (c) (i) To determine the extent of unsaturation of an oil, 0.250 g of the oil was treated with 25.00 cm3 of a 0.100 mol dm−3 ICl solution. Unreacted ICl reacted with excess potassium iodide solution, forming iodine according to the equation: ICl  +  KI  →  I2  +  KCl The amount of iodine produced was measured by reacting the mixture with a solution of sodium thiosulfate, Na2S2O3. The iodine released reacted with 32.65 cm3 of 0.100 mol dm−3 sodium thiosulfate solution in the mole ratio of 1 mol I2 : 2 mol Na2S2O3. Calculate the number of moles of iodine monochloride which reacted with 0.250 g of the oil. (3) 17 Turn over (ii) Unsaturation in oils is measured using a scale called ‘Iodine number’. This is the mass of iodine which will react with 100 g of the oil. Because iodine adds very slowly to double bonds, the reaction of iodine monochloride is used instead. Given that 1 mol of I2 is equivalent to 1 mol of ICl, use your answer in (c)(i) to calculate the mass of iodine that would react with 100 g of oil and hence identify the unsaturated oil from the list of possible oils and their iodine numbers. Oil Iodine number cocoa butter 35–40 coconut oil 7–10 cod liver oil 145–180 palm oil 44–51 peanut oil 84–106 (2) (iii) Give a reason why the reaction of iodine monochloride is significantly faster than the reaction of iodine. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 5 = 12 marks) 18 BLANK PAGE