REACTIONS BETWEEN ELEMENTS AND COMPOUNDS SUBSTITUTION BY POSITIVE IONS

REACTIONS BETWEEN ELEMENTS AND COMPOUNDS SUBSTITUTION BY POSITIVE IONS 

In the previous section substitution by negative ions was considered, but similar phenomena also occur with positive ions. By the action of sodium on iron chloride, sodium chloride and iron are obtained and the following is a reaction of the same type 

These reactions are two examples of a process used for obtaining metals from oxides, in which the metal is said to be reduced9. Since technical processes are concerned only with the production of the metal they are called reduction processes, overlooking the fact that hydrogen is oxidized during the reaction. Actually, these reactions consist of the substitution of one positive ion in an oxide by another. From the stability rules it is possible to predict which metal will 

The compounds of elements of the first three columns of the periodic system are not reduced by hydrogen, and those in the subsequent columns only partly so. At high temperatures TiCl4 is easily reduced to TiCl3, but the reaction proceeds no farther because the charge of the titanum ion is reduced. Further complications can occur in the reaction of a metal with a compound if the two metals can, themselves, combine to give a compound. In such instances the reaction will proceed in quite a different way from that expected from the stability rules. Because of the different heats of formation of halides and chalcides it can happen that a given element, for example Na, can reduce a chloride but not an oxide. 

It is surprising that a non-metallic element can sometimes act as a reducing agent iodine has a relatively large affinity for fluorine and chlorine. When iodine reacts with a chloride that can give up a part of its chlorine easily, for example VC14, a chloride of lower valency, I Cl, is formed, as follows 

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