4   3 This question is about compounds containing elements from Group 7. (a) Which change occurs when concentrated sulfuric acid is added to potassium bromide? (1) A bromide ions oxidise sulfuric acid forming sulfur B bromide ions oxidise sulfuric acid forming sulfur dioxide C bromide ions reduce sulfuric acid forming sulfur D bromide ions reduce sulfuric acid forming sulfur dioxide (b) Chemists can test for the presence of bromide ions in solution by adding a small amount of acidified silver nitrate solution. The solubility of the precipitate in aqueous ammonia is then tested. (i) Which statement is correct for bromide ions? (1) A a white precipitate forms that dissolves in concentrated ammonia only B a white precipitate forms that dissolves in both dilute and concentrated ammonia C a cream precipitate forms that dissolves in concentrated ammonia only D a cream precipitate forms that dissolves in both dilute and concentrated ammonia (ii) Give a reason why the silver nitrate must be acidified. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iii) Explain which acid needs to be used to acidify the silver nitrate solution and why other acids are unsuitable. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 5   (c) Iodine trichloride forms a dimer, I2Cl6 , in the solid state. When molten, it is suggested that it breaks down as shown. I2Cl6  ICl2 + + ICl4 – (i) Draw a labelled diagram of a simple experiment to confirm this dissociation has occurred, stating the positive result. (2) Result ................................................................................................................................................................................................................................................................... (ii) What is the shape of the ICl4 – ion? (1) A octahedral B square planar C tetrahedral D trigonal bipyramidal (iii) The equilibrium position for the dissociation of molten I2Cl6 lies to the left. I2Cl6  ICl2 + + ICl4 – What is the most likely numerical value of Kc for this equilibrium? (1) A 1.0 × 106 B 5.0 × 103 C 1.0 D 5.0 × 10–3 (Total for Question 3 = 9 marks)