The sulfuric acid acts as an acid and donates a proton to the halide. Misty fumes of HI are observed. NaI + H2SO4 → NaHSO4 + HI
A redox reaction also occurs: The iodide ions are strong enough reducing agents to reduce the concentrated sulfuric acid to sulfur dioxide, sulfur and hydrogen sulfide gas. The iodide ions are oxidised to iodine. Overall: 2HI + H2SO4 → 2H2O + SO2 + I26HI + H2SO4 → 4H2O + S + 3I28HI + H2SO4 → 4H2O + H2S + 4I2 You get a mixture of four gases (HI, SO2, H2S and iodine vapour), including purple fumes of iodine gas, plus yellow solid sulfur. Sulfur dioxide is colourless.
A redox reaction also occurs: The iodide ions are strong enough reducing agents to reduce the concentrated sulfuric acid to sulfur dioxide, sulfur and hydrogen sulfide gas. The iodide ions are oxidised to iodine. Overall: 2HI + H2SO4 → 2H2O + SO2 + I26HI + H2SO4 → 4H2O + S + 3I28HI + H2SO4 → 4H2O + H2S + 4I2 You get a mixture of four gases (HI, SO2, H2S and iodine vapour), including purple fumes of iodine gas, plus yellow solid sulfur. Sulfur dioxide is colourless.