Reactivity of the HalogensOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is halogen displacement?
A more reactive halogen can displace a less reactive halogen from an aqueous solution of one of its salts. This means the more reactive halogen takes the place of the less reactive one in the compound, releasing the less reactive halogen as a free element.
General pattern: more reactive halogen + metal halide (of a less reactive halogen) → new metal halide + less reactive halogen
Chlorine + potassium bromide → potassium chloride + bromine (chlorine, being more reactive, displaces bromine)
Chlorine + potassium iodide → potassium chloride + iodine, and bromine + potassium iodide → potassium bromide + iodine, since both chlorine and bromine are more reactive than iodine
Section 2
How can displacement reactions be observed?
Displacement reactions can be observed as colour changes in solution, since each halogen has a distinctive colour when dissolved:
- Chlorine water is pale green/colourless
- Bromine water is orange
- Iodine solution is brown
For example, adding chlorine water to colourless potassium bromide solution produces an orange colour, showing that bromine has been displaced.
If asked to predict the outcome of a halogen displacement reaction, always check the relative positions of the halogens in Group 7 first — no reaction occurs if the halogen added is less reactive than the one already in the compound.
Section 3
Why does reactivity change down Group 7?
The trend in reactivity down Group 7 (decreasing) is explained by the energy level of the outer electrons:
- Halogens react by gaining one electron to complete their outer shell
- Further down the group, the outer shell is at a higher energy level, further from the nucleus, and shielded by more inner shells
- This makes it harder for the atom to attract and gain an extra electron
- So reactivity decreases down the group
Section 4
How does this compare with the Group 1 explanation?
Group 1 metals react by losing an outer electron. Further down Group 1, that outer electron is at a higher energy level, further from the nucleus, and more weakly held — so it is lost more easily, and reactivity increases down the group. The two groups show opposite trends because one group loses electrons to react, and the other gains electrons to react.
Do not just state 'reactivity decreases down Group 7' without explaining why — full marks require the energy level/electron gain reasoning.
Must Know
- A more reactive halogen displaces a less reactive halogen from a solution of its salt
- Displacement can be seen by colour changes (chlorine pale green, bromine orange, iodine brown)
- Halogens react by gaining an electron; reactivity decreases down the group
- Further down the group, the outer shell is at a higher energy level, further from the nucleus, so it is harder to gain an electron
- Group 1 shows the opposite trend (reactivity increases down the group) because those atoms lose electrons rather than gain them
- Fluorine and chlorine are more reactive than bromine and iodine
That's the notes covered.
Carry on to the next subtopic.