(Halogens : restricted to chlorine, bromine and iodine)
Valid deductions may be expected about other elements in the group.

Physical properties of halogens, limited to colour and physical state at room temperature.
The halogens are a group of reactive non-metals, which are essentially similar to each other with only gradual changes as the atomic number increases

They are all p-block elements with a simple molecular structure consisting of covalently bonded diatomic molecules, X2.

o o o o
o o o
o X o X o X X
o o o o
There are only weak Van der Waals forces between the molecules. The strength of the forces increases as the number of electrons (Mr) in the molecule increases.
F2< Cl2< Br2< I2

In the case of iodine, the forces are sufficiently strong to bind the iodine molecules together in a 3-D crystal lattice. The X-X bond strength decreases down the group
Cl2> Br2 > I2 as the atoms get larger and the attraction of the nucleus for the shared electrons decreases (electronegativity decreases).
There is a slight tendency to metallic character with increasing atomic number. The halogens complete their octet by gaining one electron forming a halide ion, X- (see electron affinity values) or by sharing one electron. Fluorine is restricted to an oxidation state of -1 but the remaining elements have empty d orbitals and can promote electrons to give oxidation states of +1, +3, +5 and +7.
They are all oxidising agents and combine readily with metals and hydrogen.

Chlorine is a greenish-yellow gas.
Bromine is a red-brown volatile liquid.
Iodine is a black shiny solid, which sublimes on heating to produce a purple vapour.



Reactions of chlorine gas with elements (see Section 7.5.1), water, alkalis (under different conditions to form ClO- and ClO3-), other halides in aqueous solution, iron(II) ions in solution, hydrocarbons (see Sections 7.3.2, 7.3.3 and 7.3.4).

Reaction of halides with elements
Metals
The halogens combine readily with most metals forming the metal halides.
The vigour of the reaction decreases from chlorine to iodine.
Group I and II halides are ionic.

2Na (s) + Cl2 (g) 2Na+Cl- (s)
Mg (s) + Cl2 (g) Mg2+2Cl- (s)

The halides of Group III are predominantly covalent.

2Al (s) + 3Cl2 (g) 2AlCl3 (s)

Non-metals
The elements react directly with many non-metals the oxidising power decreasing from chlorine to iodine.
The elements combine directly with phosphorus, the oxidation state of the product depending on the oxidising power of the halogen.

2P (s) + 5Cl2 (g) 2PCl5 (s)
2P (s) + 3Br2 (l) 2PBr3 (l)
Solubility of the elements
All three elements are only slightly soluble in water because of the relatively strong hydrogen-bonding between the water molecules, which does not exist between the halogen molecules
i.e. solvent-solvent attractions > solute-solvent attractions > solute-solute
attractions.
Cl2> Br2>I2
solubility decreasing

They are soluble in non-polar organic solvents such as toluene and TCE.
(Why?)


Chlorine reacts slowly with water forming hydrochloric acid and chloric(I) acid. This reaction involves disproportionation:- a change in which one particular molecule, atom or ion is simultaneously both oxidised and reduced.

reduction

Cl2 (g) + H2O (l) HCl (aq) + HClO (aq) (chlorine water)
o.n. 0 -1 +1
oxidation

Exercise 1


Write balanced equations for each of the above reactions.
(Use the symbol X for a general halogen reaction)
The identification of halide ions in solution by the use of silver ions and aqueous ammonia; the [Ag(NH3)2]+ ion

Reaction of the halide ions in solution, X-(aq)
Most metal halides are soluble except lead and silver halide. Therefore solutions of lead and silver ions are used to test for the presence of halide ions in solution.







Bromine and iodine disproportionate in a similar way but to a lesser extent.

Reaction of chlorine with aqueous sodium hydroxide.
Chlorine reacts faster with dilute sodium hydroxide than with water.
When chlorine is added to cold dilute alkali it disproportionates to chloride and chlorate(l).
(i)
reduction

Cl2 (g) + 2NaOH (aq) NaCl (aq) + NaOCl (aq) + H2O
o.n. 0 -1 +1
oxidation

( 2OH- + Cl2 Cl- + OCl- + H2O )

(ii) In hot concentrated alkali, if the solution is warmed to 70oC, the chlorate(I) disproportionates further to chlorate(V).

reduction

3NaOCl (aq) 2NaCl (aq) + NaClO3 (aq)
o.n. +1 -1 +5
oxidation

If chlorine is bubbled directly into hot conc. alkali then

(iii) reduction

3Cl2 (g) + 6NaOH(aq) 5NaCl (aq) + NaClO3 (aq)
o.n. 0 -1 +5
oxidation

( 6OH- + 3Cl2 5Cl- + ClO3- + 3H2O )

For bromine, both reactions (i) and (ii) are fast at 15oC.
For iodine, decomposition of IO- occurs rapidly at 0oC so it is difficult to prepare NaIO free from NaIO3.
NaClO is a mild antiseptic (Milton).
NaClO3 powerful weed killer.


Relative oxidising ability of the halogens linked to redox potentials.

Displacement reactions of the halogens
Since they are very electronegative, all the halogens are oxidising agents. Their standard electrode potentials, Eq, become less positive on descending the group, showing that their oxidising power decreases.
X2 + 2e- 2X-
s.e.p. Eq /volts
Cl2 (g) /2Cl- + 1.36
Br2 (g) / 2Br- + 1.09
I2 (g) / 2I- + 0.54

Therefore chlorine oxidises bromide ions to bromine and iodide ions to iodine.
These are displacement reactions.

Cl2 (g) + 2Br- (aq) ----- Br2 (l)+ 2Cl- (aq)
(colourless) (yellow/orange)

Cl2 (g) + 2I- (aq) -- I2 (s) + 2Cl- (aq)
(colourless) (red/brown)

Bromine oxidises iodide to iodine

Br2 (g) + 2I- (aq) ---- I2 (s) + 2Br- (aq)

Iodine does not oxidise any of the others.


Exercise 2
Write an equation for the reaction of sodium chloride solution with
(a) (a) lead nitrate solution and


(b) silver nitrate solution followed by the addition of ammonia.



The reaction of solid halides with concentrated sulphuric acid to illustrate the relative reducing ability of halide ions and the hydrogen halides. Presence of halide ions in sea water.

Reaction of the metal halides with conc. sulphuric acid H2SO4

When concentrated sulphuric acid is added to a sodium halide the first product is fumes of the hydrogen halide HX, because each of these compounds is more volatile than sulphuric acid.
NaCl (s) + H2SO4 (1) HCl (g) + NaHSO4 (s)

NaBr (s) + H2SO4 (1) HBr(g) + NaHSO4 (s)

However conc. sulphuric acid is also a strong oxidising agent and will oxidise
HBr Br2 and HI I2, but not HF and HCl.

oxidised

2HBr(g) + H2SO4 Br2 + SO2 (g) + 2H2O

reduced


Similarly 2HI(g) + H2SO4 I2 + SO2(g) + 2H2O

Therefore conc. sulphuric acid cannot be used for the preparation of
HBr(g) and HI(g).
However conc. phosphoric(V) acid, H3PO4, can be used for the preparation since it is relatively non volatile and a poor oxidising agent.

NaBr (s) + H3PO4 (1) HBr (g) + NaH2PO4
NaI (s) + H3PO4 (1) HI (g) + NaH2PO4

Exercise 3
1. 1. Research: Look up the presence of halide ions in sea water.
2. 2. Make notes on the uses of the halogens and their compounds.