Balancing Simple Oxidation-Reduction Equations

Oxidation-reduction reactions can often be quite difficult to balance. Some are so complex in fact that chemists have written computer programs to accomphsh Ihe task. In this section, we will develop a method for balancing simple oxidation— 

At first glance, the equation representing the reaction of zinc metal with silver(l) ions in solution might appear to be balanced. 

However, because a balanced chemical equation must have a charge balance as well as a mass balance, this equation is not balanced it has a total charge of +1 for the reactants and +2 for the products. Let us apply the half-reaction method for balancing this equation. 

Half-Reaction Method Applied to Simple Oxidation-Reduction Equations 

The half-reaction method consists of first separating the equation into two halfreactions, one for oxidation, the other for reduction. You balance each half-reaction, then combine them to obtain a balanced oxidation—reduction reaction. Here is an illustration of the process. First we identify the species being oxidized and reduced and assign the appropriate oxidation states. 

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We have already balanced a number of simple oxidation-reduction equations, starting in Chapter 8. Most combination and decomposition reactions and all single substitution and combustion reactions are oxidation-reduction reactions. However, many oxidation-reduction reactions are much more complicated than the ones we have already considered, and we must use a systematic method for balancing equations for them. Unfortunately, many different systematic methods are used, and each chemistry instructor seems to have his or her own favorite method. Most instructors will accept any valid method that a student understands, however. The method outlined here is a standard method that should be acceptable. 

If you know the reactants and products of a chemical reaction, you should be able to write an equation for the reaction and balance it. In writing the equation, ryt write the correct formulas for all reactants and products. After they are written, only then start to balance the equation. Do not balance the equation by changing the formulas of the substances involved. For simple equations, you should balance the equation by inspection. (Balancing of oxidation-reduction equation will be presented in Chap. 14.) The following rules will help you to balance simple equations. 

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). 

Because it is very challenging to deal with all three of these factors simultaneously, only a small proportion of oxidation-reduction equations can be balanced by simple inspection. To make this point clear, consider the following reactions, each of which appears to be balanced. 

Our first major task of this chapter is to learn to balance equations for chemical reactions. Balancing simple equations will be covered in this chapter equations for more complicated oxidation-reduction reactions will be considered in Chapter 16. 

Oxidation-reduction reactions are often complicated, which means that it can be difficult to balance their equations by simple inspection. Two methods for balancing redox reactions will be considered here (1) the oxidation states method and (2) the half-reaction method. 

The assignment of electrons is somewhat arbitrary, but the procedure described below is useful because it permits a simple statement to be made about the valences of the elements in a compound without considering its electronic structure in detail and because it can be made the basis of a simple method of balancing equations for oxidation-reduction reactions. 

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 ... 

There is a simple relationship between the amount of electricity passed through an electrolytic cell and the amounts of substances produced by oxidation or reduction at the electrodes. From the balanced half-equations... 

We conventionally cite the oxidized form first within each symbol, which is why the general form is o,r> so pb4+ Pb + is correct, but 2+ 4+ is not. Some people experience difficulty in deciding which redox state is oxidized and which is the reduced. A simple way to differentiate between them is to write the balanced redox reaction as a reduction. For example, consider the oxidation reaction in Equation (7.1). On rewriting this as a reduction, i.e. Al3+(aq) + 3e = A Em, the oxidized redox form will automatically precede the reduced form as we read the equation from left to right, i.e. are written in the correct order. For example, o,r for the couple in Equation (7.1) is Ai3+,ai-... 

The experimental design is simple. A given sample of nitrogenase equipped with an ATP-generating system, Mg2+, and reductant is allowed to turn over without substrate (case I), and the dihydrogen production is monitored. The amount of dihydrogen produced is found to be equal (within experimental error) to the dithionite oxidized (50). Therefore, the electron balance equation is ... 

Equations for simple redox reactions can be balanced by inspection. Most redox equations, however, require more systematic methods. The equation-balancing process requires the use of oxidation numbers. In a balanced equation, both charge and mass are conserved. Although oxidation and reduction half-reactions occur together, their reaction equations are balanced separately and then combined to give the baianced redox-reaction equation. 

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