Half-reaction method of balancing

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 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. 

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. 

Oxidation-reduction (redox) reactions Involve the movement of electrons. The half-reaction method of balancing a redox reaction separates the overall reaction into two half-reactions. This reflects the actual separation of the two half-cells in an electrochemical cell... 

Whether an electrochemical process releases or absorbs free energy, it always involves the movement of electrons from one chemical species to another in an oxidation-reduction (redox) reaction. In this section, we review the redox process and describe the half-reaction method of balancing redox reactions. Then we see how such reactions are used in electrochemical cells. 

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 ... 

Oxidation-reduction (redox) reactions involve the movement of electrons from one reactant to another. The half-reaction method of balancing redox reactions separates the overall reaction into two half-reactions, which mimics the actual separation of an electrochemical cell into two half-cells. Two types of electrochemical cells are distinguished by whether they generate electrical energy (voltaic) or use it (electrolytic). In both types of cell, electrodes dip into an electrolyte solution, the oxidation half-reaction occurs at the anode, and the reduction half-reaction occurs at the cathode. (Section 21.1)... 

The half-reaction method of balancing divides the overall reaction into halfreactions that are balanced separately and then recombined. 

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). 

This is known as the half-reaction method of balancing redox equations. We will use this method extensively when we examine more complex redox reactions in Chapter 19. 

Half-Reaction Method of Balancing Aqueous Redox Equations in Acidic Solution... 

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... 

The term displaces means that aluminum goes into solution as Al (aq), forcing Zn (aq) out of solution as zinc metal. A1 is oxidized to Al +, and Zn + is reduced to Zn. In combining the half-cell equations to produce the overall equation, we must take care to ensure that the number of electrons involved in reduction equals the number involved in oxidation. (This is the half-reaction method of balancing redox equations discussed in Section 5. ) The cell diagram is written with the reduction half-cell equations as the right-hand electrode. 

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