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Chemistry part 31, Julia Burdge,2e (2009)

Tham khảo sách 'chemistry part 31, julia burdge,2e (2009)', khoa học tự nhiên, vật lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 760 CHAPTER 19 Electrochemistry J __ Multimedia Electrochemistry oxidation-reduction reactions. Balancing Redox Reactions Now is a good time to review how oxidation numbers are assigned ection4.4j. In Chapter 4 we briefly discussed oxidation-reduction or redox reactions those in which electrons are transferred from one species to another. In this section we review how to identify a reaction as a redox reaction and look more closely at how such reactions are balanced. A redox reaction is one in which there are changes in oxidation states which we identify using the rules introduced in Chapter 4. The following are examples of redox reactions 2KClO3 5 2KCl s 3O2 g 1 5-2 1-1 0 CH4 g 2O2 g CO2 g T 2H O 0 Sn v Cu2 dt Cu C Sn2 Ag 0 2 0 2 Equations for redox reactions such as those shown here can be balanced by inspection the method of balancing introduced in Chapter 3 1 Section 3.3 but remember that redox equations must be balanced for mass number of atoms and for charge number of electrons 144 Section 4.4 . In this section we introduce the half-reaction method to balance equations that cannot be balanced simply by inspection. Consider the aqueous reaction of the iron II ion Fe2- with the dichromate ion Ci O2- Fe2 Cr2O ------- Fe3 Cr3 Because there is no species containing oxygen on the product side of the equation it would not be possible to balance this equation simply by adjusting the coefficients of reactants and products. However there are two things about the reaction that make it possible to add species to the equation to balance it without changing the chemical reaction it represents The reaction takes place in aqueous solution so we can add H2O as needed to balance the equation. This particular reaction takes place in acidic solution so we can add H 1 as needed to balance the equation. Some reactions take place in basic solution enabling us to add OH- as needed for balancing. We will learn more about this shortly. After writing the unbalanced equation we balance