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# Ch21_2 - Some redox reactions have equations that must be...

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Unformatted text preview: Some redox reactions have equations that must be balanced by special techniques. Balancing Equations Balancing Equations for Redox Reactions for Redox Reactions 1 2 Balancing Equations Balancing Equations Consider the reduction of Ag+ ions with ions copper metal. Balancing Equations Balancing Equations Step 1: Divide the reaction into halfreactions, one for oxidation and the other for reduction. Ox Cu ---> Cu2+ Red Ag+ ---> Ag ---> Step 2: Balance each for mass. Already done in this case. Step 3: Balance each half-reaction for charge by adding electrons. Ox Cu ---> Cu2+ + 2e2eRed Ag+ + e- ---> Ag e- 3 Mn = +7 MnO4- + 5 Fe2+ + 8 H+ Fe 2+ ---> Fe3+ + H2 O Fe = +2 ---> Mn + 5 Fe Mn = +2 Fe = +3 Cu + Ag+ --give--> Cu2+ + Ag --give--> Ag Balancing Equations Balancing Equations Step 4: Multiply each half-reaction by a factor that means the reducing agent supplies as many electrons as the oxidizing agent requires. Reducing agent Cu ---> Cu2+ + 2e2eOxidizing agent 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add half-reactions to give the overall equation. Cu + 2 Ag+ ---> Cu 2+ + 2Ag ---> 2Ag 4 Balancing Equations Balancing Equations Balance the following in acid solution— VO2+ + Zn ---> VO 2+ + Zn2+ VO Zn Zn Step 1: Write the half-reactions Ox Zn ---> Zn 2+ Red VO2+ ---> VO 2+ ---> Step 2: Balance each half-reaction for mass. Ox Zn ---> Zn 2+ Red 2 H+ + VO2+ ---> VO2+ + H2O ---> Add H2O on O-deficient side and add H + on other side for H-balance. 5 Balancing Equations Balancing Equations Step 3: Ox Red Step 4: Ox Red 2eBalance half-reactions for charge. Zn ---> Zn 2+ + 2e2ee- + 2 H+ + VO2+ ---> VO2+ + H2 O VO ---> Multiply by an appropriate factor. Zn ---> Zn 2+ + 2e2e+ 4 H+ + 2 VO2+ ---> 2 VO2+ + 2 H2O ---> Step 5: Add half-reactions Zn + 4 H+ + 2 VO2 + VO ---> Zn 2+ + 2 VO2+ + 2 H2O ---> VO 6 The equation is now balanced for both charge and mass. Page 1 7 Tips on Balancing Equations • Never add O 2, O atoms, or O2- to balance oxygen. to • Never add H2 or H atoms to or balance hydrogen. • Be sure to write the correct charges on all the ions. • Check your work at the end to make sure mass and charge are balanced. Page 2 ...
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