Trends in the Electronegativity and Boiling Point
- Electronegativity decreases down the group.
- Electronegativity is the tendancy of an atom to attract a bonding pair of electrons.
- The halogens are all highly electronegative elements.
- But larger atoms attract shared electrons less than smaller ones.
- So going down the group, as the atoms become larger, the electronegativity decreases.
- The boiling 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 increase.
- This trend is shown in the changes of physical state from flourine(gas) to iodine(solid).
Halogens gain an electron when they react, they are oxidising agents.
They get less reactive down the group due to decreasing electronegativity. So you can say that halogens become less oxidising down the group.
Their relative oxidising strengths is shown by their displacements with halide ions.
A halogen will displace a halide from solution if the halide is below it on the periodic table - e.g. chlorine will displace bromine but will itself be displaced by flourine.
If you mix chlorine gas with dilute sodium hydroxide at room temperature, you get sodium chlorate(I) solution, NaClO(aq), which just happens to be common household bleach.
In this reaction some of the chlorine is oxidised and some is reduced. This is called disproportionation.
- 2NaOH(aq) + Cl2(aq) >>> NaClO(aq) + NaCl(aq) + H2O(aq)
- Oxidation state: 0 +1 -1
The sodium chlorate(I) solution (bleach) has loads of uses - it's used in water treatment, to bleach paper and textiles.
Chlorine and Water
When you mix chlorine with water it undergoes disproportionation. You end up with a mixture of Hydrochloric acid and chloric(I) acid:
- Cl2(g) + H2O(l) <<<>>> HCl(aq) + HClO(aq)
0 -1 +1
Chloric(I) acid ionises to make chlorate(I) ions:
- HClO(aq) + H2O(l) <<<>>> ClO-(aq) + H3O+(aq)
In bright sunlight, chlorine and water undergo an additional reaction:
- 2Cl2 + 2H2O >>>(U.V.)>>> 4HCl + O2
Chlorine is an important part of water treatment. It kills disease-causing bacteria, and some chlorine persists in the water and prevents reinfection further down the line.
Chlorine prevents the growth of algae, eliminating bad tastes and smells, and removes discolouration caused by organic compounds.
However, there are risks from using chlorine to treat water. Chlorine gas is very harmful if breathed in - it irritates the respiratory system. Liquid chlorine on the skin or eyes causes severe chemical burns.
Chlorine also reacts with the variety of organic compounds that water contains to form chlorinated hydrocarbons, of which many are carcenogenic,
However, this increased cancer risk is small compared to the risks from untreated water - a cholera epidemic could kill thousands of people.
There are ethical considerations too. We don't get a choice about having our water chlorinated - some people object to this as forced 'mass medication'.