Solid Halides with Concentrated Sulfuric Acid

This video demonstrates the reactions between concentrated sulfuric acid and three potassium halides: potassium chloride (KCl), potassium bromide (KBr) and potassium iodide (KI).
Concentrated sulfuric acid is an oxidising agent, and sulfur is in the +6 oxidation state.
When added to potassium chloride (1:52) misty white fumes of hydrogen chloride (HCl) are evolved. Chlorine is near the top of group 7. The chloride ion keeps its electrons. This is not a redox reaction: it is a simple acid-base reaction.
When added to potassium bromide (3:45) sulfur dioxide gas is produced. Bromine is further down group 7 compared to chlorine. Therefore, the bromide ion can lose a moderate number of electrons in this reaction. This is a redox reaction and the sulfur is reduced: its oxidation state decreases from +6 (H2SO4) to +4 (SO2). Sulfur dioxide can be tested using potassium dichromate paper, which turns from orange to green.
When added to potassium iodide (6:59) a complicated mixture of sulfur containing products is produced: sulfur dioxide, hydrogen sulfide and elemental sulfur. Iodine is even further down group 7 compared to chlorine and bromine. The iodide ion loses electrons very easily in this reaction. This is a redox reaction and the sulfur is been reduced: its oxidation state decreases from +6 (H2SO4) to +4 (SO2), 0 (S) and -2 (H2S). Sulfur dioxide can be tested using potassium dichromate paper, which turns from orange to green. Hydrogen sulfide can be tested using lead acetate paper, which turns from white to black. A ‘bad egg’ smell is also produced.