GROUP 7

To finish off with F321, comes the lovely group 7. After doing F321 for a year I have realised how easy it becomes (you know - with further knowledge of the F322 unit it really does help).

Keep in mind that F321 is your chance to pull your grades to the highest notch to ready yourself for year 13. So remember... Memorise those things I have mentioned. They really do help.

First of we start with a pretty picture to easy you into the toxic topic that group 7 really is. Uhhh, Quite literally. For they are toxic so remember about the fumes cupboard.

Yeah, that's what I thought. To your notes, now.

There many things they could ask you about group 7, all very straight forward if you know how to play the game.

You need to be able to explain the trend in boiling points of group 7. You know the drill, don't you? What are the intermolecular forces here??? Well there's no hydrogen bonding or any permanent dipole-dipole interactions. So that's out. What's left? That's right Van der waals's forces. That old chestnut.

As you go down group 7 there are more electrons, which means that the Van der Waals's forces will be much stronger. When melting or boiling something you break the intermolecular forces first and then you break any intramolecular forces if you have to (like covalent, ionic bonding - these have much higher boiling points as they are usually a giant lattice). Stronger forces mean more energy so higher boiling point as you go down the group.

This can be seen in the physical states of the elements. Flourine and chlorine are gases. Bromine is a liquid, whilst iodine is a solid (all at room temperature).

Up next: Identifying group 7. If you wish to test a substance to see if it contained any halide ions (a halogen ion bonded to a positive ion) you should add some SILVER NITRATE AgNO3 to it. 

If Chloride present: White precipitate forms.
Ag+(aq) +Cl-(aq) ---> AgCl(s)

If Bromide present: Cream precipitate forms.
Ag+(aq) +Br-(aq) ---> AgBr(s)

If iodine present: Yellow precipitate forms.
Ag+(aq) +I-(aq) ---> AgI(s)

Now, what if you have two of these substances in at once? Well obviously one of them is more reactive so will displace the less reactive one and you'll only see one precipitate form. Now different halide precipitates have different solubility's in aqueous ammonia - this will enable you to confirm the halide present.

Chloride: Soluble in dilute ammonia.
Bromide: Soluble in concentrated ammonia.
Iodide: Insoluble in concentrated ammonia.

Another trend that you may be asked to look at is the reactivity of group 7. It is pretty much the opposite of group 1 elements.

As you go down group 7 the reactivity decreases, this is because:

• The atomic radius increases down the group due to the extra shell that is added each time. . This means the nuclear attraction on the outer electrons is less so it is harder to attract an electron (to reach a full outer shell).
• The electron shielding increases as there is an extra shell of electrons each time you go down. This means the nuclear attraction on the outer electrons is weaker. So harder to attract electrons.
• The nuclear charge increases as there are more protons as you go down the group, which mean the nuclear attraction on outer electrons is stronger. However the other two factors outweighs this and overall the attraction on outer electrons is weaker so more difficult to attract electrons so reactivity goes down.

We can show the decrease in reactivity by a REDOX reaction. How you ask?? Well, A halide is already present in our solution and a halogen is introduced. You will get a displacement reaction - the more reactive element taking over and becoming the halide. Halogens form solutions of different colours, so any change in colour will indicate a displacement reaction. An organic solvent is added, usually CYCLOHEXANE. This is very useful, plus it gives very pretty colours.

Cl2 : Pale green in water and in cyclohexane.
Br2: Orange in water and in cyclohexane.
I2: Brown in water and purple in cyclohexane.

This leads on nicely to the DISPROPORTIONATION reaction. It is very simply put: a reaction is which the same element is oxidised and reduced.

We have two reactions that need to be understood and preferable memorised.

First off, is Chlorine in water:

Chlorine added to water will kill bacteria and make the water safe to drink. Chlorine will react with water to produce two acidic products: Hydrochloric acid and Chloric acid.

Cl2 (aq) + H2O (l) ---> HCl (aq) + HClO (aq)
0                        -1                 Chlorine reduced.
0                                   +1      Chlorine oxidised.

Second, is Chlorine in aqeuous Sodium hydroxide.

Remember that Chlorine is only slightly coluble in water and has a mild bleaching action. The house stuff that we use is formed when dilute NaOH and CL2 react together.This is another disproportination reaction.

Cl2 (aq) + 2NaOH (aq) ---> NaCl (aq) + NaClO (aq) + H2O (l)
0                            -1               Chlorine is reduced.
0                                        +1   Chlorine is oxidised.

You may also want to keep in mind that it is the ClO- ion which is responsible for all that bleaching magic.

To finish off with, you are all fully aware that Chlorine is used in water treatment. Now there are good point and bad point to that.

It kills bacteria and enables water to be drinkable. However, chlorine gas is toxic and there are also some risks of chlorinated carbons to form, which are responsible for the destruction of the ozone layer. Which is bad - trust me