Oxidation-reduction reactions half-reaction method

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

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

When using the half-reaction method, keep in mind that, in a redox reaction, the number of electrons lost through oxidation must equal the number of electrons gained through reduction. Figure 10.11 provides another example. 

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. 

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... 

The half-reaction method is a way of balancing oxidation-reductions reactions by the recognition of oxidation and reduction with separate reactions. Included in the reactions are the number of electrons that move and the nature of movement (gain or loss). The steps for this technique are ... 

Master the half-reaction method before studying electrochemistry. The studies of electrochemistry rely on the recognition of oxidation and reduction, understanding the half-reactions, and possessing the ability to balance oxidation-reduction reactions. Because of these factors, the half-reaction method will be stressed in this book. 

This method of balancing oxidation-reduction reactions is somewhat similar to the half-reaction method. 

Using the half-reaction method, we separate the oxidation and reduction half-reactions and then balance. We use H2O (reaction in solution) and H+ (acid medium due to the presence of HC1). 

Oxidation-reduction reactions, even complex ones, can be balanced using either the half-reaction method or the oxidation number method. The half-reaction method will be discussed first, using the reaction of iron with chlorine to produce iron chloride. 

One of the main purposes for using oxidation numbers is to follow the movement of electrons during an oxidation-reduction reaction. Doing so helps to predict the products and determine the outcomes of such reactions. There are a few different ways to analyze redox reactions, but we will focus on only one the ion-electron method (also called the half-reaction method). The procedure requires that you know the reactants and products of the reaction, but, by going through the process, you will gain a better understanding of the mechanisms by which these reactions proceed. 

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. 

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. 

Balance the following oxidation-reduction reactions, which occur in acidic solution, using the half-reaction method. 

In the half-reaction method we separate and completely balance equations describing oxidation and reduction half-reactions. Then we equalize the numbers of electrons gained and lost in each. Finally, we add the resulting half-reactions to give the overall balanced equation. The general procedure follows. 

Skill 11.2 Determining oxidation numbers and balancing oxidation-reduction reactions (e.g., half-reaction method)... 

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... 

B. Balancing Oxidation-Reduction Reactions by the Half-Reaction Method... 

The Half-Reaction Method for Balancing Equations for Oxidation-Reduction Reactions Occurring in Acidic Solution... 

Iodide ion, I , is one of fhe mosf easily oxidized species. Balance each of fhe following oxidation-reduction reactions, which take place in acidic solution, by using the half-reaction method. 

A salt bridge completes the electrical circuit in a cell. It allows ion flow from one half of the cell to another without allowing bulk mixing of the solutions. It is typically a U-shaped tube filled with an electrolyte that is not involved in the oxidation-reduction reaction. Any method that allows ion flow without large-scale mixing of solutions (such as a porous cup or frit) can be used in place of a salt bridge. 

It is important to be able to balance oxidation-reduction reactions. One method involves the use of oxidation states (discussed in this section), and the other method (normally used for more complex reactions) involves separating the reaction into two half-reactions. We ll discuss the second method for balancing oxidation-reduction reactions in Chapter 18. 

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