Quick Notes Reducing Power of the Halides
- Halide ions can give away an electron to become halogen atoms – they can be oxidised.
- For a halide ion to lose an electron, another species must gain an electron (be reduced).
- Halide ions can act as reducing agents, as they can force other species to be reduced.
- Reducing agents are oxidised!
- The ability of halide ions to act as reducing agents increases down the group.
- The reactions of halide ion salts with concentrated sulfuric acid show the different strengths of the halide ions as reducing agents.
- Chloride ions do not reduce the sulfur in sulfuric acid.
- Bromide ions can reduce the sulfur from 6+ to 4+ oxidation state.
- Iodide ions can reduce the sulfur from 6+ to 4+ to 2- oxidation state.
Full Notes Reducing Power of the Halides
Halogen atoms readily gain one electron to become negatively charged ions; these ions are called halide ions and have a charge of 1-.
Halide ions can lose this extra electron and become halogen atoms again. If this happens, the ions are being oxidised, as they are losing an electron. Another species will be reduced, as it will be gaining the electron. So, halide ions can act as reducing agents.
Reducing agents are oxidised!
As you go down group 7, the strength of the halide ions as reducing agents increases. The number of electron shells increases, resulting in increased shielding and a reduced positive charge on the surface of the ion. This means the halide ions will more readily give up an electron, as it’s held less tightly to the nucleus of the ion.
Concentrated Sulphuric Acid and Halide Ions
Concentrated sufuric acid is a good oxidising agent, so it can be reduced by reducing agents. Halide ions can act as reducing agents, but due to their differing strengths as reducing agents, the products formed when halide ions react with sulfuric acid are different.
We can compare the strength of different halide ions as reducing agents based on the products formed when they react with concentrated sulfuric acid.
At A-level, sodium halide salts are a common and easy source of halide ions.
As you go down the group, the halide ions are more easily oxidised – their ability to act as reducing agents increases.
Chloride ions are not strong enough reducing agents to reduce the sulfur in sulfuric acid.
Bromide ions are stronger reducing agents than chloride ions, they can reduce the sulfur from a +6 oxidation state to a +4 oxidation state.
Iodide ions are stronger reducing agents than chloride, they can reduce sulfur from +6 to +4 to -2.
Sodium chloride with (concentrated) sulphuric acid:
NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
Sodium bromide with (concentrated) sulphuric acid:
NaBr(s) + H2SO4(l) → NaHSO4(s) + HBr(g)
2HBr(g) + H2SO4(l) → SO2(g) + Br2(g) + 2H2O(l)
Sodium iodide with (concentrated) sulphuric acid:
NaI(s) + H2SO4(l) → NaHSO4(s) + HI(g)
2HI(g) + H2SO4(l) → SO2(g) + I2(s) + 2H2O(l)
6HI(g) + SO2(g) → H2S(g) + 3I2(s) + 2H2O(l)