Balancing oxidation numbers

In earlier sections of this chapter, we showed how to write and balance equations for precipitation reactions (Section 4.2) and acid-base reactions (Section 4.3). In this section we will concentrate on balancing redox equations, given the identity of reactants and products. To do that, it is convenient to introduce a new concept, oxidation number. 

Oxidation-reduction reactions must be balanced if correct predictions are to be made. Just as in selecting a route for a trip from San Francisco to New York, there are several ways to reach the desired goal. Which route is best depends to some extent upon the likes and dislikes of the traveler. We will discuss two ways to balance oxidation-reduction reactions—first, using half-reactions and, next, using the oxidation numbers we have just introduced. 

Use of Oxidation Number in Balancing Oxidation-Reduction Reactions... 

Of course, the oxidation number method gives the same balanced equation as the half-reaction method. 

By use of oxidation numbers, give a balanced equation for each of the following reactions ... 

Use oxidation numbers to balance the reaction between ferrous ion, Fe+2, and permanganate ion, MnOr, in acid solution to produce ferric ion, Fe+3, and manganous ion, Mn+2. 

Show the arbitrariness of oxidation numbers by balancing the reaction discussed in Problem 22 with the assumption that the Oxidation number of manganese in Mn04 is +2. Compare with the result obtained in Problem 22. 

Verify that reaction (23) is an oxidation-reduction reaction and that the oxidation number change of carbon is balanced by the oxidation number change of the sulfur. 

Cupric sulfide, copper(II) sulfide, reacts with hot nitric acid to produce nitric oxide gas, NO, and elemental sulfur. Only the oxidation numbers of S and N change. Write the balanced equation for the reaction. 

Bacteria, 434 Balancing reactions, 42 by half-reactions, 218 by oxidation number, 219 oxidation-reduction reactions, 217, 219... 

H+], calculation of, 192, see also Hydrogen ion Haber, Fritz, 151 Haber process, 140, 150 Hafnium, oxidation number, 414 Haldane, J. B. S., 436 Half-cell potentials effect of concentration, 213 measuring, 210 standard, 210 table of, 211, 452 Half-cell reactions, 201 Half-life, 416 Half-reaction, 201 balancing, 218 potentials, 452 Halides... 

Oxidation-reduction reactions, 202 balancing with half-reactions, 217 balancing with oxidation numbers, 219 Oxides basic, 382... 

Write a balanced chemical equation for (a) the hydrogenation of ethyne (acetylene, C2H2) to ethene (C2H4) by hydrogen (give the oxidation number of the carbon atoms in the reactant and product) (b) the shift reaction (sometimes called the water gas shift reaction, WGSR) (c) the reaction of barium hydride with water. 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

In Sec. 13.2 we will learn to determine oxidation numbers from the formulas of compounds and ions. We will learn how to assign oxidation numbers from electron dot diagrams and more quickly from a short set of rules. We use these oxidation numbers for naming the compounds or ions (Chap. 6 and Sec. 13.4) and to balance equations for oxidation-reduction reactions (Sec. 13.5). In Sec. 13.3 we will learn to predict oxidation numbers for the elements from their positions in the periodic table in order to be able to predict formulas for their compounds and ions. 

The more electronegative element will take the negative oxidation state, (a) The maximum oxidation state of sulfur is +6 the most common negative oxidation number of oxygen is -2. Therefore, it takes three oxygen atoms to balance one sulfur atom, and the formula is SO v (b) The maximum oxidation state of carbon is +4 the only oxidation number of fluorine in its compounds is - 1. Therefore, it takes four fluorine atoms to balance one carbon atom, and the formula is CF4,... 

One of the most important uses of oxidation numbers is in balancing redox (oxidation-reduction) equations. These equations can get very complicated, and a systematic method of balancing them is essential. There are many such methods, however, and each textbook seems to use its own. There are many similarities among the methods, however, and the following discussion will help no matter what method your instructor and your textbook use. 

It is of critical importance in this section to keep in mind the difference between oxidation number and charge. You balance charge one way and changes in oxidation number another way. 

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 total of the oxidation numbers gained in a reaction must equal the total of the oxidation numbers lost, since the numbers of electrons gained and lost must be equal. Therefore, we can balance the species in which the elements that are oxidized and reduced appear by the changes in oxidation numbers. We use the numbers of atoms of each of these elements which will give us equal numbers of electrons gained and lost. If necessary, first balance the number of atoms of the element oxidized and/or the number of atoms of the element reduced. Finally, we balance the rest of the species by inspection, as we did in Chap. 7. 

We sec that Fc and N are the only elements undergoing change in oxidation number. To balance the oxidation numbers lost and gained, we need three FcCI, and three FeCI, for each N atom reduced ... 

Here, the Mn is reduced and the C is oxidized. Before attempting to balance the oxidization numbers gained and lost, we first have to balance the number of carbon atoms. Before the carbon atoms arc balanced, wc cannot say that there is one carbon (as in the C02) or two (as in the H2C204). 

Balance the change in oxidation number by adding electrons to the side with the higher total of oxidation numbers. That is, add electrons on the left for a reduction half-reaction and on the right for an oxidation half-reaction. One way to remember on which side to add the electrons is the following mnemonic ... 

Note that all the added atoms in steps 4 and 5 have the same oxidation number as the atoms already in the equation. The atoms changing oxidation number have already been balanced in steps 1 and 2. 

Balance the following equation by the oxidation number change method ... 

In the balanced equation, the total increase in oxidation number equals the total decease in oxidation number. 

The number of moles of I3"(aq) produced is determined by titrating the iodide-treated sample with sodium thiosulfate. The balanced oxidation reaction that forms the basis for the titration is ... 

Some redox reactions may be simply balanced by inspection. However, many are complex and require the use of a systematic method. There are two methods commonly used to balance redox reactions the oxidation number method and the ion-electron method. 

To balance a redox reaction using the oxidation number method, follow the following rules ... 

The usefulness of determining the oxidation number in analytical chemistry is twofold. First, it will help determine if there was a change in oxidation number of a given element in a reaction. This always signals the occurrence of an oxidation-reduction reaction. Thus, it helps tell us whether a reaction is a redox reaction or some other reaction. Second, it will lead to the determination of the number of electrons involved, which will aid in balancing the equation. These latter points will be discussed in later sections. 

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. 

Oxidation numbers are bookkeeping numbers that allow chemists to do things like balance redox equations. Don t confuse oxidation numbers with the charge on an ion. Oxidation numbers are assigned to elements in their natural state or in compounds using the following rules ... 

In Section 20.2, a redox reaction involving copper and nitric acid is discussed. This reaction is balanced by a method called the oxidation-number method. In this lab, you will carry out this reaction, along with another redox reaction that involves a common household substance. You will practice balancing various redox reactions using both the oxidation-number method (from Section 20.2) and the half-reaction method (from Section 20.3). 

Balance redox reactions using the oxidation-number method. 

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