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Halogens are the highly reactive non-metals of group 7.
You have to remember the colours of the Halogens in the table below, it is very useful in your AS/A-level Chemistry exams! 👩🏫
- The boiling and melting points of the halogens increase down the group. This is due to the increasing strength of the Van der Waals forces as the size and relative mass of the atoms increases.
- This trend is shown in the changes of thee physical state from chlorine gas to solid iodine.
- A substance is said to be volatile if it has a low boiling point, so volatility decreases down the group.
- Halogens get less reactive down the group.
- Their atoms react by gaining an electron in their outer shell. This means they’re reduced, they oxidise another substance- so they’re oxidising agents.
- As you go down the group, the atoms become larger so the outer electrons are further away from the nucleus. The outer electrons are also shielded more from the attraction of the positive nucleus, because there are more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion, so larger atoms are less reactive. Another way of saying that the halogens get less reactive down the group is to say that they become less oxidising.
- Halogens displace less reactive halide ions from solution. The halogens’ relative oxidising strengths can be seen in their displacement reactions with halide ions. For example, if you mix bromine water with potassium iodide solution, the bromine displaces the iodide ions (it oxidises them), giving iodine and potassium bromide solution. When these displacement reactions happen, there are colour changes- you can see what happens by following them. Iodine water, is brown and bromine water is orange.
- You can make the changes easier to see by shaking the reaction mixture with an organic solvent like cyclohexane. The halogen that is present will dissolve readily in the organic solvent, which settles out as a distinct layer above the aqueous solution. A pink colour shows in the presence of iodine. An orange colour shows bromine and a very pale yellow shows chlorine
Displacement Reactions
- Displacement reactions can help to identify solutions. A halogen will displace a halide from solution if the halide is below it in the periodic table.
- Silver nitrate solution is used to test for halides.
- First you add dilute nitric acid to remove any ions that might interfere with the test.
- Then you just add silver nitrate solution. A precipitate of the silver halide will form. The colour of the precipitate identifies the halide.
- Then to be extra sure you can add ammonia solution, as each silver halide has different solubility in ammonia:
Reference:
https://getrevising.co.uk/files/documents/134_group_7/1.3.4_Group_7%5B1%5D.pdf
This is the end of the topic!
Drafted by Cherry (Chemistry)