Some Reactions of the Halide Ions
Quick Notes
- Halide ions act as reducing agents (lose an electron and reduce something else), with reducing ability increasing down the group.
- Reactions of solid sodium halides with concentrated sulfuric acid produce different products depending on reducing ability:
- Sodium chloride with (concentrated) sulphuric acid:
- NaCl + H2SO4 → NaHSO4 + HCl
- Sodium Bromide with (concentrated) sulphuric acid:
- NaBr + H2SO4 → NaHSO4 + HBr
- 2HBr + H2SO4 → SO2 + Br2 + 2H2O
- Sodium chloride with (concentrated) sulphuric acid:
- NaI + H2SO4 → NaHSO4 + HI
- 2HI + H2SO4 → SO2 + I2 + 2H2O
- 6HI + SO2 → H2S + 3I2 + 2H2O
- Acidified silver nitrate is used to test for halide ions, forming distinct precipitates.
- Solubility of silver halides in ammonia decreases down the group.
| Halide ion | Observation with AgNO3 |
|---|---|
| Cl− | White precipitate (AgCl) |
| Br− | Cream precipitate (AgBr) |
| I− | Yellow precipitate (AgI) |
| Silver halide | Solubility in NH3 |
|---|---|
| AgCl | Dissolves in dilute NH3 |
| AgBr | Insoluble in dilute NH3, dissolves in concentrated NH3 |
| AgI | Insoluble in both dilute and concentrated NH3 |
Full Notes
Reducing Ability of Halide Ions
Halide ions (X−) act as reducing agents by donating electrons.
Trend: Reducing power increases down the group.
Explanation:
- As ionic radius increases, outer electrons are further from the nucleus.
- These electrons are lost more easily, making the halide a stronger reducing agent.
Reactions with Aqueous Silver Ions (AgNO3 Test)
This is a qualitative test to identify halide ions.
Step 1: Add dilute nitric acid (HNO3) to remove carbonate ions (which could interfere).
Step 2: Add silver nitrate (AgNO3). Halide ions form precipitates with Ag+.
| Halide ion | Observation with AgNO3 |
|---|---|
| Cl− | White precipitate (AgCl) |
| Br− | Cream precipitate (AgBr) |
| I− | Yellow precipitate (AgI) |
Step 3: Add aqueous ammonia to confirm:
| Silver halide | Solubility in NH3 |
|---|---|
| AgCl | Dissolves in dilute NH3 |
| AgBr | Insoluble in dilute NH3, dissolves in concentrated NH3 |
| AgI | Insoluble in both dilute and concentrated NH3 |
This helps distinguish the halides even when colour differences are subtle.
Reactions with Concentrated Sulfuric Acid
This tests halide reducing ability. Products vary depending on how strong the halide is as a reducing agent.
Sodium Chloride (NaCl)
Reacts as an acid-base reaction only:
NaCl + H2SO4 → NaHSO4 + HCl
Observations – white misty fumes of HCl gas.
No redox (Cl− is a weak reducing agent).
Sodium Bromide (NaBr)
Initial acid-base reaction, followed by redox:
NaBr + H2SO4 → NaHSO4 + HBr
2HBr + H2SO4 → Br2 + SO2 + 2H2O
Obvserations – brown fumes of Br2 and colourless SO2 gas produced.
Br− is a stronger reducing agent than Cl−.
Sodium Iodide (NaI)
Sulfur gets reduced from +6 to -2, shows Iodide has strongest reducing ability:
NaI + H2SO4 → NaHSO4 + HI
2HI + H2SO4 → I2 + SO2 + 2H2O
6HI + SO2 → 3I2 + H2S + 2H2O
Obsevations – purple iodine vapour and rotten egg smell of H2S.
I− reduces sulfur from +6 to –2 (via +4).
Summary of Sulfur Products
| Halide | Reaction with H2SO4 | Products Observed |
|---|---|---|
| NaCl | Acid–base only | HCl (misty fumes) |
| NaBr | Acid–base + redox | HBr, Br2 (brown fumes), SO2 |
| NaI | Acid–base + extensive redox | HI, I2 (purple vapour), SO2, H2S (rotten egg smell) |
Products vary with halide reducing strength:
- Cl−: no redox, only HCl
- Br−: SO2 formed (S reduced to +4)
- I−: SO2, I2, and H2S (S reduced to –2)
Summary
- Reducing ability of halide ions increases Cl− → I−.
- AgNO3 test: AgCl (white), AgBr (cream), AgI (yellow); solubility in ammonia decreases down the group.
- Reactions with concentrated H2SO4 show increasing redox ability down the group: Cl− (no redox) → Br− (SO2) → I− (SO2, I2, H2S).