Ion-electron method for balancing equations

We know from our Chapter 8 discussion that a balanced chemical equation is needed for basic stoichiometry. With redox reactions, we must not only pay attention to the element balance but also to electron balance. The number of electrons given up must equal the number of electrons taken on. Our Chapter 8 technique of balancing by inspection must therefore be modified. Several special schemes have been derived for balancing redox equations. The ion-electron method directly and clearly takes into account the electrons that are transferred, so that is the method that we have chosen here. 

This 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 acidic and basic conditions. Our purpose, however, is to discuss the procedure at a fundamental level, and thus spectator ions will be absent from all examples from the start, and acidic conditions will be the only conditions considered. The step-wise procedure we will follow is given here and an example follows. 

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 process and the reduction process and label as oxidation and reduction. These half-reactions show only the species being oxidized (or the species being reduced) on the left side and 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 H2O 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 H+, since we are dealing with acid solutions. Now balance both half-reactions for all elements by inspection. 

---

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 ... 

The half-reaction method, or ion-electron method, for balancing redox equations consists of seven steps. Oxidation numbers are assigned to all atoms and polyatomic ions to determine which species are part of the redox process. The oxidation and reduction equations are balanced separately for mass and charge. They are then added together to produce a complete balanced equation. These seven steps are applied to balance the reaction of hydrogen sulfide and nitric acid. Sulfuric acid, nitrogen dioxide, and water are the products of the reaction. 

In the ion-electron method of balancing redox equations, an equation for the oxidation half-reaction and one for the reduction half-reaction are written and balanced separately. Only when each of these is complete and balanced are the two combined into one complete equation for the reaction as a whole. It is worthwhile to balance the half-reactions separately since the two half-reactions can be carried out in separate vessels if they are suitably connected electrically. (See Chap. 14.) In general, net ionic equations are used in this process certainly some ions are required in each half-reaction. In the equations for the two half-reactions, electrons appear explicitly in the equation for the complete reaction—the combination of the two half-reactions—no electrons are included. 

Practice Exercise Use the ion-electron method to balance the following equation for the reaction in an acidic medium ... 

The principles of oxidation-reduction provide the basis of two simple systematic methods for balancing these equations. If all the products of reaction are known, the balancing may be done either by the ion-electron method or by the oxidation-state method. (The two methods are compared following Problem 11.8.) After students have acquired more experience, they will be able to predict some or all of the principal products if they keep in mind such facts as the following ... 

Here s an overview of the ion-electron method The unbalanced redox equation is converted to the ionic equation and then broken down into two halfreactions — oxidation and reduction. Each of these half-reactions is balanced separately and then combined to give the balanced ionic equation. Finally, the spectator ions are put into the balanced ionic equation, converting the reaction back to the molecular form. (Buzzword-o-rama, eh For a discussion of molecular, ionic, and net-ionic equations, see Chapter 8.) It s important to follow the steps precisely and in the order listed. Otherwise, you may not be successful in balancing redox equations. 

Several methods are used to balance ionic redox equations, including, with slight modification, the oxidation-number method just shown for molecular equations. But the most popular method is probably the ion-electron method. 

Suppose, for example, that we are asked to balance the eqnation for the oxidation of Fe ions to Fe ions by dichromate ions (Cr207 ) in an acidic medinm. hi this reaction, the CraO ions are rednced to Cr ions. The following steps wiU help us balance the equation by the ion-electron method. 

Strategy Follow the procedure for balancing redox equations by the ion-electron method. The reaction takes place in a basic medium, so any ions that appear in the two half-reactions must be neutralized by adding an equal nttmber of OH ions to both sides of the equation. 

Balancing equations. Balance each of the following equations by the ion-electron method. Show the balanced partial equations for oxidation and reduction, and the ionic equation for the overall reaction ... 

Redox equations. Use the ion-electron method to write a balanced ionic equation for the oxidation of CH3CHO to CH3COOH by acidified MnO " which is reduced to Mn +-... 

Ion-Electron Half-Reaction Method for Balancing Organic Oxidation-Reduction Equations... 

Using the ion-electron half-reaction method, write balanced equations for the following oxidation reactions. 

The general method of balancing electron-transfer equations requires that halfreaction equations be available. Short lists of common half-reactions, similar to Table 17-1, are given in most textbooks, and chemistry handbooks have extensive lists. However, no list can provide all possible half-reactions, and it is not practical to carry lists in your pocket for instant reference. The practical alternative is to learn to make your own half-reaction equations. There is only one prerequisite for this approach you must know the oxidation states of the oxidized and reduced forms of the substances involved in the electron-transfer reaction. In Chapter 8 you learned the charges on the ions of the most common elements now we review the method of determining the charge (the oxidation state) of an element when it is combined in a radical. 

The first step in any method of balancing oxidation-reduction equations is to identify the element that is oxidized and the one that is reduced. Because the change in oxidation number is equal to a change in the number of electrons controlled, and the electrons must be controlled by some atom, the total gain in oxidation number is equal to the total loss in oxidation number. The oxidation half of a reaction may be written in one equation, and the reduction half in another. Neither half-reaction can be carried out without the other, but they can be done in different locations if they are connected in such a way that a complete electrical circuit is made (Chapter 17). The half-reaction method is illustrated by balancing the equation for the reaction of zinc metal with dilute nitric acid to produce ammonium ion, zinc ion, and water ... 

In Chapter 4 (Section 4.6) we introduced the half-reaction method of balancing simple oxidation-reduction reactions. We now extend this method to reactions that occur in acidic or basic solution. The steps used to balance these equations successfully are built upon those presented in Chapter 4. Keep in mind that oxidation-reduction reactions involve a transfer of electrons from one species to another. For example, in the reaction described in the chapter opener, zinc metal becomes zincfll) ion each zinc atom loses two electrons, and copper(II) ion becomes copper metal (each copper ion gains two electrons). 

Equations for oxidation-reduction (redox) reactions are often difiBcult to balance. The method to be described in this section is especially appropriate for reactions involving ions in solution. The method resolves the reaction into two parts, or partial reactions a loss of electrons (oxidation) and a gain of electrons (REDUCTION). In reality, most oxidation-reduction reactions probably do not proceed in this way. The method is essentially a bookkeeping device enabling us to deal with the oxidation and with ihe reduction separately, and finally to combine the results into a balanced equation. This artificial recipe, while giving the correct answer for the overall reaction, should not be interpreted as an explanation of the reaction. 

Redox equations are often so complex that the inspection method (the fiddling-with-coefficients method) of balancing chemical equations doesn t work well with them. (See Chapter 7 for a discussion of this balancing method.) So chemists developed other methods of balancing redox equations, such as the ion electron (half-reaction) method. 

---