Half-reaction method balancing equations

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

In the ion-electron method of balancing redox equations, an equation for the oxidation half-reaction and one for the reduction half-reaction are written and balanced separately. Only when each of these is complete and balanced are the two combined into one complete equation for the reaction as a whole. It is worthwhile to balance the half-reactions separately since the two half-reactions can be carried out in separate vessels if they are suitably connected electrically. (See Chap. 14.) In general, net ionic equations are used in this process certainly some ions are required in each half-reaction. In the equations for the two half-reactions, electrons appear explicitly in the equation for the complete reaction—the combination of the two half-reactions—no electrons are included. 

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 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. 

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. 

This is a redox reaction. Use the half-reaction method to balance the equation. 

Balance the following equation 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. 

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

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

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... 

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

When a chemical equation is / V I balanced by the half-reaction method, the number of electrons that occur on both sides of the balanced equation (before canceling) is equal to the value of n. 

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

Balance the following redox reactions, using the half-reaction method. Write your answers as net ionic equations. 

You had a brief introduction to the topic of electrochemistry in Chapter 11 when you reviewed the oxidation-reduction process in which reactions occur by the transfer of electrons. One of the procedures you looked at was the half-reaction method of balancing redox equations. In this chapter, we will be looking at the oxidation and reduction process in even more depth. 

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 ... 

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 general procedure is to balance the equations for the half-reactions separately and then to add them to obtain the overall balanced equation. The half-reaction method for balancing oxidation-reduction equations differs slightly depending on whether the reaction takes place in acidic or basic solution. 

Oxidation-reduction reactions can occur in basic as well as in acidic solutions. The half-reaction method for balancing equations is slightly different in such cases. 

Use the half-reaction method as specified for acidic solutions to obtain the final balanced equation as if H+ ions were present. 

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