The reaction of halide salt with concentrated sulphuric acid

• Fluoride and chloride

  F- and Cl- ions are not strong enough reducing agents to reduce the S in H2SO4. No redox reactions occur. Only acid base reactions

  NaF (s) + H2SO4 (l) --> NaHSO4 (s) + HF (g)

  • Observations is white steamy fumes of HF forming

  NaCl (s) + H2SO4 (l) --> NaHSO4 (s) + HCl (g)

  • Observations is white steamy fumes of HCl forming
• Bromide

  Br- ions are stronger reducing agents then Cl- and F- and after initial acid base reactions, it reduces the sulphur from +6 to +4.

  Acid base reaction

  • NaBr (s) + H2SO4 (l) --> NaHSO4 (s) + HBr (g)
  • Observations is white steamy fumes of HBr are evolved

  Redox step

  • 2HBr + H2SO4 --> Br2 + SO2 + 2H2O
  • Observations is red fumes of bromine are evolved as well as a colourless and acidic gas sulphur dioxide
• Bromide pt2 half equations

  Oxidation

  • 2Br- → Br2 + 2e-

  Reduction

  • H2SO4 + 2H+ + 2e- → SO2 + 2H2O
• Iodide pt1

  I- ions are the strongest halide reducing agents

  NaI (s) + H2SO4 (L) --> NaHSO4 (s) + HI (g)

  2HI + H2SO4 --> I2 (s) + SO2 (g) + 2H2O ( L)

  6HI + H2SO4 --> 3I2 + S (s) + 4H2O (L)

  8HI + H2SO4 --> 4I2 + H2S (g) + 4H2O (L)
• Iodide pt2

  They can reduce sulphur in H2SO4 from +6 to +4 in SO2 , to 0 in S and to -2 in H2S

  Observations:

  • White steamy fumes of HI are evolved
  • Black solid and purple fumes of iodine are also evolved
  • A colourless, acidic SO2
  • A yellow solid of sulphur is produced
  • H2S a gas with a bad egg smell