Group 7 reactions

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Reactions with 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



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Reactions with alkali at room temperature

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 

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Reactions with hot alkali

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 %. 

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Reactions with Phosphorus

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


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Reactions with solutions of other Halides

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-) 

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Reactions with metals

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

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