Group 7, the halogens

Halide Ions as reducing agents 

Halide ions can act as reducing agents.  Now, in these reactions the halide ions lose (donate/give away) electrons to become more positive, which in turn means they become halogen molecules.  

There is a trend in the group 7's ability to reduce.  It is a trend linked to the atoms size.  The larger the atom, the more easily it loses an electron; the outer shell is further from the nucleus which has the positive charge which attracts the electrons.  

Electronegativity has a very specific definition that needs to be learnt for the exam.  It is as follows - 

Electronegativity is a measure of the ability of an atom to attract electrons, or electron densities towards itself in a covalent bond.  

What does electronegativity depend on?

 - It is depends on the attraction between the nucleus and bonding electrons in the outer shell.  So, it is dependant on the distance of the nucleus to the outer shell.

 - It is also dependant on the amount of shielding the atom has. 

So, fluorine has very little shielding and is a very small atom, leading it to be the most electronegative element.  

As we go down the group, these actually increase.  This is because the intermolecular forces are the van der waals (VDW) forces which increases depending on the size and hence, as the atoms get larger down the group, the melting and boiling points increase.  

The lower the boiling point, the more volatile the element.  Hence, chlorine is more volatile than iodine.  

Oxidising ability 

Halogens, usually, react by gaining electrons to become negative ions with the charge of -1.  (Hence, they are redox reactions)  This is because they are oxidising agents and therefore they get reduced.  

Cl2 + 2e- ---->  2Cl-                 (-----> is the gaining of electrons!)

Displacement reactions

Halogens will react with metal halides in a solution in such as way that the halide in the compound will be displaced by the more reactive halogen.  It is not displaced by a less reactive halogen.  The halogen will act as a oxidising agent.  We don't really refer to them as displacement reactions but as oxidisation; it's A-level.  

(Basically, the element below it on the periodic table will be displaced by it...)

Chlorine is a poisonous gas and was used in the First World War.  Chlorine is soluble in water and in this form has become an essential part of our life because it treats water so it's good for drinking and is used in swimming pools.

The reaction with water 

Chlorine reacts with water in a reversible reaction to form chloric acid (HClO) and hydrochloric acid.  

In this reaction, the oxidation number of one of the chlorine atoms increases from a 0 to a +1.  But, another Cl loses and becomes -1 and during this sort of redox reaction, where the same element decreases and increases, there is a name given to it.  It's called - "disproportionation."

This reaction takes place when chlorine is used to purify water for drinking and swimming pools; it prevents life threatening diseases that are in the water.  It does this because Chloric acid oxidises bacteria, which kills it; it's an oxiding agent!  Chloric acid is also bleach.

Other halogens react similarly, however it is much more slowly as we go down the group.

If chlorine reacts with water in sunlight, then a different reaction will occur.  

Hydrochloric acid is produced along with oxygen.

Chlorine is rapidly lost from a water in sunlight, for example an outdoor swimming pool.  Shallow pools will therefore need chlorine added to them fairly frequently.  

As an alternative, solid sodium chlorate (or calcium) can be added to pools.  This of course dissolves in the water to form chloric acid, HClO, in a reversible reaction:

NaClO + H2O <---> Na+ + OH- + HClO

Reaction with alkali 

Chlorine reacts with cold dilute sodium hydroxide to form sodium chlorate, NaClO.  This is in fact, an oxidising agent and is a ingredient in bleach.  It's a disproportionation reaction, as mentioned earlier.  Other halogens react similarly to this.

The halogens have distinctive colours and at room temperature they are as follows - 

Fluorine - Pale yellow has 

Chlorine - Greenish gas

Bromine - Red-brown liquid

Iodine - Black solid

(There are more but these are focused on in AQA chemistry)

INE at the end is the element

IDE at the end is for an ion.  

Remember that; it's key and you will lose marks if you get it wrong!