The Ion-Electron Method for Balancing Equations

We have seen how analytical calculations in titrimetric analysis involve stoichiometry (Sections 4.5 and 4.6). We know that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, balancing equations by inspection can be quite challenging, if not impossible. Thus, several special schemes have been derived for balancing redox equations. The ion-electron method for balancing redox equations takes into account the electrons that are transferred, since these must also be balanced. That is, the electrons given up must be equal to the electrons taken on. A review of the ion-electron method of balancing equations will therefore present a simple means of balancing redox equations. 

The method makes use of only those species, dissolved or otherwise, that actually take part in the reaction. So-called spectator ions, or ions that are present but play no role in the chemistry, are not included in the balancing procedure. Solubility rules are involved here, since spectator ions result only when an ionic compound dissolves and ionizes. Also, the scheme is slightly different for acid and base conditions. Our purpose, however, is to discuss the basic procedure thus spectator ions will be absent from all examples from the start, and acidic conditions will be the only conditions considered. The stepwise procedure we will follow is below  

Step 1 Look at the equation to be balanced and determine what is oxidized and what is reduced. This involves checking the oxidation numbers and discovering which have changed. 

Step 2 Write a half-reaction for both the oxidation and reduction processes and label oxidation and reduction. These half-reactions show only the species being oxidized (or the species being reduced) on the left side, with only the product of the oxidation (or reduction) on the right side. 

Step 3 If oxygen appears in any formula on either side in either equation, it is balanced by writing H20 on the opposite side. This is possible since the reaction mixture is a water solution. The hydrogen in the water is then balanced on the other side by writing H1, since we are dealing with acid solutions. Now balance both half-reactions for all elements by inspection. 

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To balance equations that are ionic, charge must also be balanced (in addition to atoms and ions) The ion-electron method for balancing equations is ... 

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