Balancing Oxidation-Reduction Equations

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. 

Determining the net ionic equation by balancing the oxidation-reduction reaction skeletal equation Cu(s) + HN03(ag) - Cu2+(aq) + NO(g)... 

Balance the oxidation-reduction equation for the oxidation of H2S(aq) by HN03(aq) to produce NO(g) and S(s) in aqueous acidic solution (thus H+ and H20 may be involved). 

Balance the oxidation-reduction equation H+NO + H2S - NO + S + H20. Half-Reaction Method... 

Balance the oxidation-reduction equation for this reaction ... 

In general, balancing of oxidation-reduction equations should be made by taking the following steps ... 

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 using the oxidation states method to balance an oxidation-reduction equation, we find the coefficients for the reactants that will make the total increase in oxidation state balance the total decrease. The remainder of the equation is then balanced by inspection. 

The procedures for balancing an oxidation-reduction equation by the oxidation states method are summarized below. 

Balancing an Oxidation-Reduction Equation by the Oxidation States Method... 

When we balance an oxidation-reduction equation, the number of electrons lost by the reducing agent... 

Chemical equations represent chemical changes or reactions. Reactions are classified into types to assist in writing equations and in predicting other reactions. Many chemical reactions fit one or another of the four principal reaction types that we discuss in the following paragraphs. Reactions are also classified as oxidation-reduction. Special methods are used to balance complex oxidation-reduction 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). 

To balance an oxidation—reduction equation in basic solution, you begin by balancing the equation as if it were a reaction in acidic solution. Then, you add the following steps. ... 

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

Redox reactions may be represented by balanced equations. Oxidation numbers are assigned to those substances involved in the oxidation/reduction. The total change in oxidation number is set to zero by adjusting the numbers of reactants and products. In aqueous solution, H2O, H+Caq) or OH (aq) ions are included as necessary to balance the number of hydrogen and oxygen atoms. 

Water. Based on the overall balanced equation for this reaction, a minimum of one mole of water per mole of nitro compound is required for the reduction to take place. In practice, however, 4 to 5 moles of water per mole of nitro compound are used to ensure that enough water is present to convert all of the iron to the intermediate ferrous and ferric hydroxides. In some cases, much larger amounts of water are used to dissolve the amino compound and help separate it from the iron oxide sludge after the reaction is complete. 

Write balanced equations for the reduction of each of the following oxide ores by carbon monoxide ... 

K.18 The industrial production of sodium metal and chlorine gas makes use of the Downs process, in which molten sodium chloride is electrolyzed (Chapter 12). Write a balanced equation for the production of the two elements from molten sodium chloride. Which element is produced by oxidation and which by reduction ... 

The key to writing and balancing equations for redox reactions is to think of the reduction and oxidation processes individually. We saw in Section K that oxidation is the loss of electrons and reduction the gain of electrons. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

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. 

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