Half-reaction method, for balancing redox

Half-Reaction Method for Balancing Redox Reactions... 

Review of Oxidation-Reduction Concepts Half-Reaction Method for Balancing Redox Reactions Electrochemical Cells... 

The half-reaction method for balancing redox reactions divides the overall redox reaction into oxidation and reduction half-reactions. Each half-reaction is balanced for atoms and charge. Then, one or both are multiplied by some integer to make electrons gained equal electrons lost, and the half-reactions are recombined to give the balanced redox equation. The half-reaction method is commonly used for studying electrochemistry because... 

What is a half-reaction Why must the number of electrons lost in the oxidation half-reaction equal the number of electrons gained in the reduction half-reaction Summarize briefly the steps in the half-reaction method for balancing redox reactions. What two items must be balanced in a redox reaction (or any reaction) ... 

Follow the half-reaction method for balancing redox reactions. Begin by assigning oxidation states. 

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. 

Separating a redox reaction into its half-reactions is useful if you need to balance the reaction. One method for balancing redox reactions is reviewed in Appendix 4. 

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... 

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. 

Use the half-reaction method to balance these equations for redox reactions. Add water molecules and hydrogen ions (in acid solutions) or hydroxide ions (in basic solutions) as needed. 

The redox reaction between dichromate ion and iodide ion in acid solution is shown in Figure 19.15. Use the half-reaction method to balance the equation for this redox reaction. 

In Chapter 7, we learned how to balance chemical equations by inspection. Some redox reactions can be balanced in this way. However, redox reactions occurring in aqueous solutions are usually difficult to balance by inspection and require a special procedure called the half-reaction method of balancing. In this procedure, the overall equation is broken down into two half-reactions one for oxidation and one for reduction. The half-reactions are balanced individually and then added together. For example, consider tiie redox reaction ... 

The half-reaction method, or ion-electron method, for balancing redox equations consists of seven steps. Oxidation numbers are assigned to all atoms and polyatomic ions to determine which species are part of the redox process. The oxidation and reduction equations are balanced separately for mass and charge. They are then added together to produce a complete balanced equation. These seven steps are applied to balance the reaction of hydrogen sulfide and nitric acid. Sulfuric acid, nitrogen dioxide, and water are the products of the reaction. 

Equations for redox reactions, such as those shown here, can be balanced by inspection, the method of balancing introduced in Chapter 3 [Ml Section 3.3], but remember that redox equations must be balanced for mass (number of atoms) and for charge (number of electrons) [M4 Section 4.4]. In this section we introduce the half-reaction method to balance equations that cannot be balanced simply by inspection. 

Half-Reaction Method of Balancing Aqueous Redox Equations In Acidic Solution (18.2) Examples 18.1, 18.2 For Practice 18.1, 18.2 Exercises 37-40... 

Revised method for balancing redox half-reactions... 

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. 

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. 

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

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). 

To balance a redox reaction which occurs in a basic solution is a very similar to balancing a redox reaction which occurs in acidic conditions. First, balance the reaction as you would for an acidic solution and then adjust for the basic solution. Here is an example using the half-reaction method ... 

This equivalence provides the basis for balancing redox equations. Although there is no single best method for balancing all redox equations, two methods are particularly useful (1) the half-reaction method, which is used extensively in electrochemistry (Chapter 21)... 

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