8.2.3The Periodic Table
Group VII Displacement Reactions — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 8 Unit quiz
Section 1 of 7Reactivity of the halogens
Trend in reactivity
- Trend in reactivity Reactivity of the Group VII elements decreases down the group: chlorine is more reactive than bromine,
- which is more reactive than iodine.
- Smaller atoms attract an extra electron more strongly because the outer shell is closer to the nucleus.
What displacement means
- What displacement means A more reactive halogen takes the place of a less reactive halogen in a solution of its halide salt.
- The less reactive halogen is released as the free element.
Colours in water
- Colours in water Chlorine solution is colourless or very pale green, bromine solution is orange (yellow when dilute) and iodine solution is brown.
Section 2 of 7Carrying out the test
1
Pour about 2 cm3 of a halide solution (KCl, KBr or KI) into each of three test tubes.
2
Add a few drops of one halogen solution to each tube and shake gently.
3
Look for a colour change and record it. A change shows that displacement occurred; no change means no reaction.
4
Repeat with the other halogen solutions, wearing eye protection and working in a ventilated area because chlorine and bromine are toxic.
Section 3 of 7Results grid
| Chlorine | Bromine | Iodine | |
|---|---|---|---|
| Potassium chloride | No reaction | No reaction | No reaction |
| Potassium bromide | Orange: bromine made | No reaction | No reaction |
| Potassium iodide | Brown: iodine made | Brown: iodine made | No reaction |
Section 4 of 7Equations
Chlorine and potassium bromide
- Chlorine and potassium bromide 2KBr(aq) + Cl2(aq) → 2KCl(aq) + Br2(aq)
- The solution changes from colourless to orange.
Bromine and magnesium iodide
- Bromine and magnesium iodide MgI2(aq) + Br2(aq) → MgBr2(aq) + I2(aq)
- The solution becomes brown.
Ionic form (HT only)
- Ionic form (HT only) 2Br-(aq) + Cl2(aq) → 2Cl-(aq) + Br2(aq).
- The potassium ions are spectators and do not appear.
Section 5 of 7Why displacement happens (HT only)
1Stronger electron attraction
- Stronger electron attraction The more reactive halogen has a greater tendency to gain electrons.
2Electrons transferred
- Electrons transferred Halide ions of the less reactive halogen lose electrons to it.
3Oxidation and reduction
- Oxidation and reduction The halide ion is oxidised to the element;
- the reacting halogen is reduced to its halide ion.
4New halogen appears
- New halogen appears The element formed is seen by its colour, confirming the order of reactivity.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Displacement reaction | A reaction in which a more reactive element replaces a less reactive one from a compound. |
| Halide | A compound or ion containing a halogen, such as bromide Br-. |
| Oxidation | Loss of electrons. |
| Reduction | Gain of electrons. |
| Aqueous | Dissolved in water. |
Section 7 of 7Exam technique
1
Saying a halogen displaces a more reactive halide. A halogen only displaces halides of halogens below it in the group.
2
Writing that chlorine displaces chloride. There is no reaction because it is the same halogen.
3
Mixing up the coloursbromine is orange or yellow, iodine is brown.
4
Forgetting to balance the equation, such as writing KBr + Cl2 → KCl + Br2 without the 2s.
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