Balancing half-reaction method

Balancing a redox reaction is often more challenging than balancing other types of reactions, because we must balance both electrons and elements. Perhaps the simplest way to balance a redox reaction is by the half-reaction method, which consists of the following steps. 

Just as before, the first step in balancing a reaction must be to decide the products. Again, experiment provides the answer. Let us reconsider one of the same examples we balanced previously by the half-reaction method. For these we already know the products. 

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

There are many methods of balancing redox equations by the half-reaction method. One such method is presented here. You should do steps 1 through 5 for one half-reaction and then those same steps for the other half-reaction before proceeding to the rest of the steps. 

EXAMPLE 13.19. Balance the following equation by the ion-electron, half-reaction method ... 

Although this equation can be balanced by the half-reaction method, it also can be balanced by inspection. First notice that all of the N is present in N,04(g) in the products. This implies that there are two HN03(aq) 2HN03(l)- N204(g) + H20(l) + 02(g)... 

The balancing of redox reactions is beginning to appear on the AP exam, so we have included the half-reaction method of balancing redox reactions in the Appendix, just in case you are having trouble with the technique in your chemistry class. 

The following steps may be used to balance oxidation—reduction (redox) equations by the ion-electron (half-reaction) method. While other methods may be successful, none is as consistently successful as is this particular method. The half-reactions used in this process will also be necessary when considering other electrochemical phenomena, thus the usefulness of half-reactions goes beyond balancing redox equations. 

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 half-reaction method. 

Half-Reaction Method for Balancing Redox Reactions... 

You could balance the chemical equation for the reaction of magnesium with aluminum nitrate by inspection, instead of writing half-reactions. However, many redox equations are difficult to balance by the inspection method. In general, you can balance the net ionic equation for a redox reaction by a process known as the half-reaction method. The preceding example of the reaction of magnesium with aluminum nitrate illustrates this method. Specific steps for following the half-reaction method are given below. 

The half-reaction method of balancing equations can be more complicated for reactions that take place under acidic or basic conditions. The overall approach, however, is the same. You need to balance the two half-reactions, find the LCM of the numbers of electrons, and then multiply by coefficients to equate the number of electrons lost and gained. Finally, add the halfreactions and simplify to give a balanced net ionic equation for the reaction. The ten steps listed above show this process in more detail. 

The Sample Problem on the next page illustrates the use of these steps for an acidic solution. To balance a net ionic equation for basic conditions by the half-reaction method, balance each half-reaction for acidic conditions, adjust for basic conditions, and then combine the half-reactions to obtain the balanced net ionic equation. The following Concept Organizer summarizes how to use the half-reaction method in both acidic and basic conditions. 

If it is a redox reaction, follow the steps for balancing by the half-reaction method. 

In this section, you learned the half-reaction method for balancing equations for redox reactions. You investigated the redox reactions of metals with acids, and the combustion of two hydrocarbons. After applying the half-reaction method in the following review problems, you will learn a different method in section 10.4. This method will make greater use of oxidation numbers. 

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. 

O ViU Is it possible to use the half-reaction method or the oxidation number method to balance the following equation Explain your answer. 

Q O The combustion of ammonia in oxygen to form nitrogen dioxide and water vapour involves covalent molecules in the gas phase. The oxidation number method for balancing the equation was shown in an example in this section. Devise a half-reaction method for balancing the equation. Describe the assumptions you made in order to balance the equation. Also, describe why these assumptions did not affect the final result. 

Use the half-reaction method to balance each of the following equations. 

The following redox reactions occur in acidic solution. Balance the equations using the half-reaction method. 

Balancing Redox Reactions by the Half-Reaction Method... 

Use the half-reaction method to balance the following equation in a basic medium. Cr02 + CIO- Cr04" -I- Cl ... 

Balance chemical equations for redox reactions by the half-reaction method, Toolbox 12.1 and Examples 12.1 and 12.2. 

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). 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

Balancing Redox Reactions The Half-Reaction Method 139... 

To summarize, balancing a redox reaction in acidic solution by the half-reaction method is a six-step process, followed by a check of the answer (Figure 4.4.)... 

PROBLEM 4.19 Balance the following net ionic equation by the half-reaction method. The reaction takes place in acidic solution. 

Redox reactions can be balanced using either the oxidation-number method or the half-reaction method. The concentration of an oxidizing agent or a reducing agent in solution can be determined by a redox titration. 

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