Reactions of group 7
 

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Reactions of group 7 (7A, 17, the Halogens)

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1. The qualitative analysis of halide ions.

The test solution is made acidic with nitric acid, which decomposes hydroxides, carbonates or sulphites that would interfere with the test. Silver nitrate solution is then added:

Ag+(aq) + Cl- (aq)  "   AgCl(s)    white precipitate
Ag+(aq) + Br- (aq)  "   AgBr(s)   cream precipitate
Ag+(aq) + I- (aq)   "   AgI(s)    yellow precipitate

The precipitates are then treated with ammonia solution. The chloride dissolves in dilute ammonia to give a colourless solution:

AgCl(s) + 2NH3(aq)   "   [Ag(NH3)2]+ (aq)  +  Cl- (aq)

The bromide is less water-soluble, and dissolves only in concentrated ammonia to give a colourless solution:

AgBr(s) + 2NH3(aq)  "   [Ag(NH3)2]+ (aq)  +  Br - (aq)

The iodide is even less soluble in water, and does not dissolve in ammonia at all. It may become a paler yellow, however. The water solubility of the halide is significant because the overall equilibrium constant for the formation of the ammine complex is the product of the equilibrium constant for the halide solubility (the solubility product) and the equilibrium constant for the formation of the ammine. If the first value is very small, as in the case of the iodide, the overall constant is also very small and the iodide won't dissolve in ammonia.

Inorganic catalogue contents

Analysis of halides
Halides with concentrated sulphuric acid
Disproportionation reactions
Displacement reactions

2. The reaction of halide salts with concentrated sulphuric acid.

In the reaction with chlorides, H2SO4 acts as an acid displacing the more volatile HCl; with bromides and iodides, the halide ions are more easily reducible and the halogen is produced as well as other products. The reactions of the sodium salts are typical.

NaCl(s)  +  H2SO4(l)  "   NaHSO4(s)  +  HCl(g)

NaBr(s)  +  H2SO4(l)  "   NaHSO4(s)  +  HBr(g)
           2HBr(g) + H2SO4(l)  "   Br2(g)  +  SO2(g)  +  2H2O(l)

NaI(s)  +  H2SO4(l)  "  NaHSO4(s)  +  HI(g)
           2HI(g) + H2SO4(l)  "   I2(s)  +  SO2(g)  +  2H2O(l)
        6HI(g) + H2SO4(l)   "  3I2(s)  +  S(s)  +  4H2O(l)
             8HI(g) + H2SO4(l)  "   4I2(s)  +  H2S(s)  +  4H2O(l)

  

3. The disproportionation reactions of chlorine and chlorate(I).

Chlorine is both oxidised and reduced when it reacts with water or with sodium hydroxide solution – disproportionation reactions.

With water: Cl2(aq)  +  H2O(l)   "  HOCl(aq)  +  HCl (aq)

With cold dilute NaOH solution: Cl2(aq)  +  2NaOH(aq)   "  NaCl (aq)  +  NaOCl (aq)  +  H2O(l)

With hot concentrated NaOH solution: 3Cl2(aq) +  6NaOH(aq)  "   5NaCl(aq)  +  NaClO3(aq)  +  3H2O(l)

Chlorate(I) ions on heating in solution: 3 OCl- (aq)  "   ClO3 - (aq)  +  2Cl – (aq)

Bromine and iodine do the same thing, but the higher positive oxidation states are formed more easily the larger the halogen.

  

4. The displacement reactions of halide ions by halogens.

Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions.

Cl2(aq) +  2Br – (aq)  "   2Cl – (aq)  +  Br2(aq)

The reaction above is used to manufacture bromine from seawater.

Cl2(aq) +  2I – (aq)  "   2Cl – (aq)  +  I2(aq)

Br2(aq) +  2I – (aq)  "   2Br – (aq)  +  I2(aq)

  

© JRG Beavon 2007