In this lesson, we focus on displacement reactions of Group 17 elements, also known as the halogens.
Halogens are very reactive non-metals. Their reactivity decreases down the group.
Fluorine is the most reactive non-metal in the Periodic Table. Chlorine is more reactive than bromine, and bromine is more reactive than iodine.
Remember this rule:
A more reactive halogen can displace a less reactive halogen from its halide solution.
We will go through the six possible combinations using chlorine, bromine and iodine added to sodium chloride, sodium bromide and sodium iodide.
Important exam note
The colour of a halogen solution depends on its concentration. Use the wording your exam board uses.
| Solution | O-Level / SEC | IGCSE |
|---|---|---|
| Chlorine water, Cl₂(aq) | light yellow | pale green (may look colourless if very dilute) |
| Bromine water, Br₂(aq) | reddish-brown | orange or yellow |
| Iodine solution, I₂(aq) | brown | brown |
| Solution | More concentrated | More diluted |
|---|---|---|
| Chlorine water, Cl₂(aq) | light yellow / yellow-green | pale green / almost colourless |
| Bromine water, Br₂(aq) | reddish-brown | orange / yellow |
| Iodine solution, I₂(aq) | brown | pale brown / yellow-brown |
Halide solutions such as NaCl(aq), NaBr(aq) and NaI(aq) are colourless.
Scenario 1: Aqueous chlorine added to sodium bromide
Chlorine is more reactive than bromine. Chlorine displaces bromine from bromide solution, forming bromine and chloride solution.
The more reactive chlorine displaces less reactive bromine from aqueous sodium bromide, forming sodium chloride and bromine.
Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq)
Observation
- O-Level / SEC: light yellow aqueous chlorine is added to colourless sodium bromide. A reddish-brown solution of aqueous bromine is formed.
- IGCSE: pale green aqueous chlorine is added to colourless sodium bromide. An orange solution of aqueous bromine is formed.
Scenario 2: Aqueous chlorine added to sodium iodide
Chlorine is more reactive than iodine, so it displaces iodine from iodide solution.
The more reactive chlorine displaces less reactive iodine from aqueous sodium iodide, forming sodium chloride and iodine.
Cl₂(aq) + 2NaI(aq) → 2NaCl(aq) + I₂(aq)
Observation
- O-Level / SEC: light yellow aqueous chlorine is added to colourless sodium iodide. A brown solution of aqueous iodine is formed.
- IGCSE: pale green aqueous chlorine is added to colourless sodium iodide. A brown solution of aqueous iodine is formed.
Scenario 3: Aqueous bromine added to sodium chloride
Bromine is less reactive than chlorine. Bromine cannot displace chlorine from chloride solution.
No reaction.
- O-Level / SEC: remains reddish-brown
- IGCSE: remains orange
Scenario 4: Aqueous bromine added to sodium iodide
Bromine is more reactive than iodine. Bromine displaces iodine from iodide solution.
The more reactive bromine displaces less reactive iodine from aqueous sodium iodide, forming sodium bromide and iodine.
Br₂(aq) + 2NaI(aq) → 2NaBr(aq) + I₂(aq)
Observation
- O-Level / SEC: reddish-brown aqueous bromine is added to colourless sodium iodide. A brown solution of aqueous iodine is formed.
- IGCSE: orange aqueous bromine is added to colourless sodium iodide. A brown solution of aqueous iodine is formed.
Scenario 5: Aqueous iodine added to sodium chloride
Iodine is less reactive than chlorine.
No reaction. The solution remains brown.
Scenario 6: Aqueous iodine added to sodium bromide
Iodine is less reactive than bromine.
No reaction. The solution remains brown.
Summary
Halogens are reactive non-metals. Reactivity decreases down Group 17 / VII.
A more reactive halogen can displace a less reactive halogen from its halide solution.
Reactivity order, decreasing down the group:
fluorine > chlorine > bromine > iodine
| Experiment | Observation | Reaction? | Equation |
|---|---|---|---|
| Cl₂ + NaBr | O-Level / SEC: light yellow → reddish-brown IGCSE: pale green → orange | Yes | Cl₂(aq) + 2NaBr(aq) → 2NaCl(aq) + Br₂(aq) |
| Cl₂ + NaI | O-Level / SEC: light yellow → brown IGCSE: pale green → brown | Yes | Cl₂(aq) + 2NaI(aq) → 2NaCl(aq) + I₂(aq) |
| Br₂ + NaCl | O-Level / SEC: remains reddish-brown IGCSE: remains orange | No | — |
| Br₂ + NaI | O-Level / SEC: reddish-brown → brown IGCSE: orange → brown | Yes | Br₂(aq) + 2NaI(aq) → 2NaBr(aq) + I₂(aq) |
| I₂ + NaCl | remains brown | No | — |
| I₂ + NaBr | remains brown | No | — |
Exam tip
For Br₂ + NaI, both solutions can look brownish. In the exam, still write that the solution turns brown because iodine is formed.
Watch the lesson
Watch the video, then revise the Group 17 / VII halogen properties lesson before this one.
