Halogen Displacement
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- The reactivity of halogens is also shown by their displacement reactions with other halide ions in solutions
- A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive halogen
- Eg. The addition of chlorine water to a solution of bromine water:
Cl2 (aq) + 2NaBr (aq) → 2NaCl (aq) + Br2 (aq)
- The chlorine has displaced the bromine from solution as it is more reactive which can be summarised in the following ionic equation by removing the sodium spectator ions:
Cl2 (aq) + 2Br- (aq) → 2Cl- (aq) + Br2 (aq)
- Chlorine can also displace iodine from a solution of iodide ions:
Cl2 (aq) + 2I- (aq) → 2Cl- (aq) + I2 (aq)
- We can see these are both redox reactions by taking a look at changes in the oxidation number of each element in the reaction
- Br and I both change from = -1 → 0 so the bromine and iodine have been oxidised in their respective reactions
- Cl = 0 → -1 so the chlorine has been reduced in both reactions
- No change in oxidation number for the sodium
Chlorine with Bromides & Iodides
- If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:
- The solution becomes yellow-orange as bromine is formed
- The solution becomes brown as iodine is formed
- If an organic solvent is added, such as cyclohexane, the following observations are seen:
- The organic layer will appear yellow-orange as bromine is formed
- The organic layer will appear purple as iodine is formed
- The organic solvent is useful as the the halogens are more soluble in this layer which helps observe the colour changes more easily
- Chlorine is above bromine and iodine in Group 7 so it is more reactive
- Chlorine will displace bromine or iodine from an aqueous solution of the metal halide:
Cl2 (aq) + 2KBr (aq) → 2KCl (aq) + Br2 (aq)
chlorine + potassium bromide → potassium chloride + bromine
Cl2 (aq) + 2KI (aq) → 2KCl (aq) + I2 (aq)
chlorine + potassium iodide → potassium chloride + iodine
Bromine with Iodides
- Bromine is above iodine in Group 7 so it is more reactive
- Bromine will displace iodine from an aqueous solution of the metal iodide
bromine + potassium iodide → potassium bromide + iodine
Br2 (aq) + 2KI (aq) → 2KBr (aq) + I2 (aq)
- The reaction will turn brown as iodine is formed
- If an organic solvent is added the organic layer will appear purple as iodine is formed
- We can show that this is a redox reaction by looking at the changes in oxidation numbers:
- I = -1 → 0 so the iodine has been oxidised
- Br = 0 → -1 so the bromine has been reduced
- No change in oxidation number for the potassium
- Rather than writing the full equation, we can also write the ionic equation by removing the potassium spectator ion
Br2 (aq) + 2I- (aq) → 2Br- (aq) + I2 (aq)