REACTIONS BETWEEN ELEMENTS AND COMPOUNDS SUBSTITUTION BY NEGATIVE IONS

REACTIONS BETWEEN ELEMENTS AND COMPOUNDS SUBSTITUTION BY NEGATIVE IONS 

We have already seen that the heat of formation of a halide is greater, the smaller the negative ion in the compound. It is obvious that if the heat of formation of a fluoride is greater than that of a chloride then between fluorine and potassium chloride the following reaction will occur 

The fluorine can be said to displace the chlorine from the compound, in the same way that chlorine displaces bromine or iodine 

We have therefore deduced from the formula for the heat of formation of ionic compounds the important rule that the halogens replace one another in the order F2, Cl2, Br2, I2. There are ho known exceptions to this rule. Fluorine displaces chlorine, bromine and iodine from all chlorides, bromides and iodides, while chlorine and bromine displace iodine from all iodides. It is to be expected that the same substitution reactions can take place with the chal-cogens 02, S, Se and Te, and, since the heat of formation of the oxides is, as a rule, greater than that of the sulphides, the reactions of the type 

In all reactions between sulphides and oxygen occurring at high temperatures, the sulphur produced is oxidized by an excess of oxygen, so that actually a different reaction is observed in the oxidation of zinc sulphide 

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