Ch21_2 - Balancing Equations Balancing Equations for Redox...

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Unformatted text preview: Balancing Equations Balancing Equations for Redox Reactions for Redox Reactions for Redox 1 2 Balancing Equations Balancing Equations Some redox reactions have equations that must be balanced by special techniques. MnO4- + 5 Fe2+ + 8 H+ 2+ 2+ ---> Fe3+ + H2 O Fe = +2 ---> Mn + 5 Fe Mn = +7 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+ + 2eOxidizing agent 2 Ag+ + 2 e- ---> 2 Ag Step 5: Add half-reactions to give the overall equation. Cu + 2 Ag+ ---> Cu 2+ + 2Ag The equation is now balanced for both charge and mass. Cu + Ag+ 4 --give--> Cu2+ + Ag Balancing Equations Balancing Equations Balance the following in acid solution— 2+ VO2+ + Zn ---> VO 2+ + Zn2+ VO 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. Page 1 3 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 ---> Consider the reduction of Ag+ ions with copper metal. Mn = +2 Fe = +3 Balancing Equations Balancing Equations 5 Balancing Equations Balancing Equations Step 3: Ox Red Step 4: Ox Red 2e- Balance half-reactions for charge. Zn ---> Zn 2+ + 2ee- + 2 H+ + VO2+ ---> VO2+ + H2 O eMultiply 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 + ---> Zn 2+ + 2 VO2+ + 2 H2O 6 7 Tips on Balancing Equations • Never add O 2, O atoms, or O2- to balance oxygen. • Never add H2 or H atoms to 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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