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)
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)
1 mark