Ion-electron method for balancing

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. 

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 ... 

There are two essentially different methods to balance redox reactions—the oxidation number change method and the ion-electron method. The first of these is perhaps easier, and the second is somewhat more useful, especially for electrochemical reactions (Chap. 14). 

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... 

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 ... 

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