Oxidation number change method

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). 

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

Balance the equation for the reduction of HNO3 to NH4NO3 by Mn by the oxidation number change method. Add other compounds as needed. 

One important use of oxidation numbers is in balancing redox equations. There are essentially two methods to balance redox reactions the oxidation number change method and the ion-electron method. In the former method, the changes in oxidation number are used to balance the species in which the elements that are oxidized and reduced appear. The numbers of atoms of each of these elements is used to give 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. Then, balance by inspection, as was done in Chapter 7. 

Section 16.1 introduces the concept of oxidation number and how to calculate the oxidation number of an element from the formula of the compound or ion of which it is a part. Section 16.2 describes how to use the oxidation numbers to name compounds, formalizing and extending the rules given in Chapter 6. Section 16.3 shows how to predict possible oxidation numbers from the position of the element in the periodic table and how to use these oxidation numbers to write probable formulas for covalent compounds. Section 16.4 presents a systematic method for balancing equations in which oxidation numbers change. 

A more general and fundamental view is obtained by a consideration of (a) the number of electrons involved in the partial ionic equation representing the reaction, and (b) the change in the oxidation number of a significant element in the oxidant or reductant. Both methods will be considered in some detail. 

The other procedure which is of value in the calculation of the equivalents of substances is the oxidation number method. This is a development of the view that oxidation and reduction are attended by changes in electronic charge and was originally developed from an examination of the formulae of the initial and final compounds in a reaction. The oxidation number (this will be abbreviated to O.N.) of an element is a number which, applied to that element in a particular compound, indicates the amount of oxidation or reduction which is required to convert one atom of the element from the free state to that in the compound. If oxidation is necessary to effect the change, the oxidation number is positive, and if reduction is necessary, the oxidation number is negative. 

Rather than quantifying a complex total petroleum hydrocarbon mixture as a single number, petroleum hydrocarbon fraction methods break the mixture into discrete hydrocarbon fractions, thus providing data that can be used in a risk assessment and in characterizing product type and compositional changes such as may occur during weathering (oxidation). The fractionation methods can be used to measure both volatile and extractable hydrocarbons. 

In section 10.2, you learned that a redox reaction involves changes in oxidation numbers. If an element undergoes oxidation, its oxidation number increases. If an element undergoes reduction, its oxidation number decreases. When balancing equations by the half-reaction method in section 10.3, you sometimes used oxidation numbers to determine the reactant(s) and product(s) in each half-reaction. 

Simple redox reactions can be balanced by the trial-and-error method described in Section 3.1, but other reactions are so complex that a more systematic approach is needed. There are two such systematic approaches often used for balancing redox reactions the oxidation-number method and the half-reaction method. Different people prefer different methods, so we ll discuss both. The oxidation-number method is useful because it makes you focus on the chemical changes involved the halfreaction method (discussed in the next section) is useful because it makes you focus on the transfer of electrons, a subject of particular interest when discussing batteries and other aspects of electrochemistry (Chapter 18). 

The key to the oxidation-number method of balancing redox equations is to realize that the net change in the total of all oxidation numbers must be zero. That is, any increase in oxidation number for the oxidized atoms must be matched by a corresponding decrease in oxidation number for the reduced atoms. Take the reaction of potassium permanganate (KMn04) with sodium bromide in aqueous acid, for example. An aqueous acidic solution of the purple permanganate anion (Mn04 ) is reduced by Br- to yield the nearly colorless Mn2+ ion, while Br- is oxidized to Br2. The unbalanced net ionic equation for the process is... 

Oxidation-reduction reactions can be written leaving out the spectator ions (ions that do not change oxidation number during a reaction). An alternate method for writing oxidation-reduction reactions is to include all ions and compounds involved and not pay any attention to eliminating spectators and those items that do not change in oxidation number. This technique identifies ionic compounds within the reaction and uses a series of conventions. 

Among the electrochemical syntheses related to the change of metal oxidation number, we emphasize obtaining acetylacetonates of divalent iron, cobalt, and nickel [551,623]. The method of alternating-current electrochemical synthesis was applied to isolate Ji-complexes of monovalent copper with allylamines, allylimines, and ally-lurea from the salts of divalent copper [624-628], We note that the same method was used for preparation of analogous ji-complexes with copper(II) halides (X = Cl, Br) [629a]. Other electrochemical syntheses with participation of metal salts and complexes are described in monographs [201,202] and literature cited therein. 

The change of metal oxidation number in quinone complexes can also be reached by electrochemical methods. For example, the electrochemical oxidation [(5.20), (5.21)] of [MIV(DBCat)3]2 (M = Mn, Tc, Re) yields products having different oxidation state of the central atom [171,172] ... 

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 ... 

I Be sure to start the half-reaction method by balancing the number of atoms of the element oxidized or reduced (before proceeding to step 2). The other elements involved cannot change oxidation number after the element oxidized or reduced has been balanced (in step 1). For example, do not add H2 to balance hydrogen atoms in step 4. 

Most redox equations can be balanced and (2) the change-in-oxidation-number method. Many redox equations can be balanced... 

In problems like the above, involving unusual oxidation states, the student may ask, What will happen if I choose the wrong oxidation numbers The answer is that the same result will be obtained so long as the method is followed consistently. If one chooses to assign +4 to the Fe and -2 to the S in the above example the total oxidation state increase is still 11. An advantage of the ion-electron method is that such decisions are avoided, since the oxidation state is not used to determine the electron change. 

The Half-Reaction Method 11-6 Adding H+, OH, or H2O to Balance Oxygen or Hydrogen 11-7 Change-in-Oxidation-Number Method... 

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