Group 7 reactions

Water

These are reversible reactions 

Cl2 + H2O = (H+) + (Cl-) + HOCl        This is a disproportionation reaction of chlorine 

Br2 + H2O= (H+) + (Br-) + HOBr         Bromine reacts in a similar way but the position of  equilibrium lies slightly more to the left side 

                                                              Iodine does not react with water

Chlorine, bromine and iodine all react in the same way with cold aqueous sodium hydroxide by disproportionation reactions making a mixture of halide and halate(I) salts. 

eg. Cl2 + 2NaOH = NaCl + NaOCl + H2O

        0                        -1           +1              <----------Oxidation states of chlorine

       Cl2 + 2(OH-) = (Cl-) + (OCl-) + H20       <-----------ionic equation

if this solution containing chlorate(I) ions is heated, further disproportionation takes place and chlorate(V) ions are formed.

eg. 3(OCl-) = 2(Cl-) + (ClO3)- 

           +1          -1     +5 

This is also a disproportionation reaction. However the the halogen is oxidised to the +5 state in the chlorate(V) ion straight away.

3Cl2 + 6(OH-) = 5(Cl-) + 3H2O + (ClO3)-           <----- ionic equation

    0                       -1                    +5                <----- oxidation states

Bromine and iodine react similarly:

3I2 + 6NaOH = 5NaI + NaIO3 + 3H2O            The atom economy for forming sodium                                                                                    iodate(V) is poor being just 16.7 %. 

In a limited supply of chlorine -   Phosphorus trichloride is formed

2P + 3Cl2 = 2PCl3                This can then react with more chlorine to from phosphorus                                             pentachloride    

PCl3 + Cl2 = PCl5

Phosporous pentachloride is formed straight away in excess chlorine

2P + 5Cl2 = PCl5 

Iodine reacts with the red allotrope of phosphorus to form a phosphorus triiodide 

2P + 3I2 = 2PI3 

However if the mixture is damp this then reacts with water forming hydrogen iodide gas

PI3 + 3H2O = 3HI + H3PO3

A halogen will displace a less reactive halogen from one of its salts

• When chlorine is bubbled into a solution of pottasium bromide, it oxidises the bromide ions. A brown colour of liberated bromine is then observed.

Cl2 + 2(Br-) = Br2 + 2(Cl-)

• Chlorine displaces iodine from iodides forming a dark grey precipatate of iodine

Cl2 + 2(I-) = I2 + 2(Cl-)

• Because bromine is more reactive that iodine it also displaces iodine from iodides

Br2 + 2(I-) = I2 + 2(Br-) 

On heating metals react with halogens forming halides in which the halogen is in the -1 state. For example chlorine oxidises iron to iron(III) chloride:

2Fe + 3Cl2 = 2FeCl3

Iodine is a less powerful oxidising agent than chlorine so iodine oxidises iron to iron(II) iodide:

Fe + I2 = FeI2