11589 16 The bromate(V) ion, BrO− 3, is an oxidising agent produced by the reaction of bromine with a hot concentrated solution of sodium hydroxide. (a) (i) Write the ionic equation for the reaction of bromine with sodium hydroxide. [2] (ii) State the colour change observed for the above reaction. [1] (b) Acidified bromate(V) ions will oxidise iodide ions to iodine. (i) Write a half-equation for the reduction of bromate(V) ions to bromide. [1] (ii) Write a half-equation for the oxidation of iodide ions to iodine. [1] (iii) Write the overall equation for this reaction. [1] 11589 [Turn over (c) The iodine produced can then be reduced by thiosulfate ions. Titrations of the liberated iodine with sodium thiosulfate solution can be used to determine the concentration of bromate(V) ions. A 20.0 cm3 solution containing acidified bromate(V) ions was added to a solution containing excess iodide ions and the resulting mixture made up to 1.0 dm3. A 25.0 cm3 aliquot was titrated against 0.10 M sodium thiosulfate, adding starch indicator just before the end point. The titre was found to be 23.8 cm3. (i) Explain why it is necessary to add the starch indicator just before the end point. [1] (ii) Calculate, to two significant figures, the concentration of the original bromate(V) solution. [4]