On the balancing of equations for redox reactions

Equations for redox reactions in solution are best balanced via the appropriate half equations. These are combined in such away as to eliminate the electrons in them. For example, for the dissolution of aluminium in acid, the half equations are 

Half equations can be routinely balanced by one of the following two procedures. Whichever procedure is chosen, the result can be checked by applying the other one. 

As an example of the application of these procedures, consider the reduction of dichromate to chromium(III) in acid solution. Procedure A gives  

Procedure B begins similarly, but after (2), the oxidation number of the chromium in Cr207 is determined (+6), and from this, the number of electrons required in the equation  

Note that both procedures lead to equations containing fT ions. For reactions in alkaline solutions, it is as well to eliminate these by adding nOW ions to each side of the equation, where n is the number of iT ions. You can then replace lT + OFT on one side of the equation by nH20, leaving nOH on the other side. 

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