Electrochemistry Review – Answer Key

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					Electrochemistry Review – Answer Key
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 a     c       c        c    b     b      c      d     d    d    b     c     b    b    a    d    d      a    d    c
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 a     c       c        b    a     b      a      d     a    b    b     b     d    b    d    c    a      b    b    b

1.     Balance the following in basic solution.             MnO4- + C2O42- → MnO2 + CO2 (basic)

              ( MnO4-         + 4H+ + 3e- → MnO2 + 2H2O ) x 2
              ( C2O42-        → 2CO2 + 2e-) x 3
       Acid: 2MnO4-            + 3C2O42- + 8H+ → 2MnO2 + 6CO2 + 4H2O
       Water:                            8 H2 O → 8 H+ +      8 OH-
       Basic: 2MnO4-                  2-
                               + 3C2O4 + 4H2O → 2MnO2 + 6CO2 + 8OH-

2.     Consider the electrolysis of 1.0 M H2SO4 using platinum electrodes.

       a)            Write the oxidation half-reaction      H2O → 1/2O2 + 2H+ + 2e-              Eo = -0.82 V

       b)            Write the reduction half-reaction      SO42- + 4H+ + 2e- → H2SO3 + H2O          Eo = +0.17 V

c)     Write the overall reaction and determine the minimum theoretical voltage required.

       SO42- + 4H+ → 1/2O2 + 2H+ + H2SO3                             Eo = -0.65 V          MTV = +0.65 V

3. Consider the following diagram for the electrorefinning of lead.
                                       Power Source



           Cathode                                              Anode
           Pure Pb                                              Impure Pb




a)     On the diagram, label the anode and cathode.

b)     Write the formula for a suitable electrolyte                  Pb(NO3)2

c)     Write the equation for the reduction half-reaction.           Pb2+ + 2e- → Pb(s)

4.     Describe two chemically different methods that can be used to prevent corrosion of iron and
       explain why each method works.

       Method 1:                    Painting
       Explanation:                 Prevents reactants H2O and O2 from contacting the iron

       Method 2:                    Attach some Zn
       Explanation:                 Fe is forced to be the cathode and cannot oxidize
5.   The data below were obtained in a redox titration of a 25.00 mL sample containing Sn2+ ions using
     0.125 M KMnO4 according to the following reaction:
     2MnO4- + 16H+ + 5Sn2+ → 2Mn2+ + 8H2O + 5Sn4+
                                    Volume of KMnO4 used (mL)
                                    Trial 1        Trial 2       Trial 3
     Initial burette reading        2.00           13.80         24.55
     Final burette reading          13.80          24.55         35.32
                                    11.80          10.75         10.77 Average last two to get 10.76 mL
                        2+
     Calculate the [Sn ]
     [Sn2+]           =      0.01076 L      x      0.125 mole x       5 moles Sn2+
                                                   1L                 2 moles MnO4-
                                            0.0250 L
                      =      0.135 M

6.   A student wanted to electroplate a coin with copper.

     a)        Identify a suitable anode            Cu
     b)        Identify an appropriate electrolyte          CuSO4
     c)        To which battery terminal (positive or negative) should the coin be connected?   Negative

7.   Consider the electrolysis of molten magnesium chloride.
     a)    Identify the product at the anode.    Cu2+
     b)    Write the equation for the reduction half-reaction. Mg2+ +             2e-     Mg(s)
     c)    Write the equation for the overall reaction. Mg2+ + 2Cl-             Mg(s)   + Cl2

8.   Consider the following electrochemical cell.

                               voltmeter




                             1.0 M KNO3


          Sn                                      Mn
Cathode                                                Anode




               1M Sn(NO3)2          1M Mn(NO3)2


a)   Write the anode reaction      Sn2+ +         2e-             Sn
b)   Write the cathode reaction Mn         Mn   2+
                                                           +       2e-
c)   Write the overall reaction and determine the initial cell potential (Eo).
     Sn2+ +         Mn                   Mn2+ +           Sn

d)   Explain in words how the electrons move through the cell. From anode Mn to cathode Sn
e)   List all of the ions that migrate toward the Sn electrode. Sn2+       Mn2+         K+
f)   Which electrode loses mass?            Anode Sn
g)   What is the purpose of the salt bridge?
     To allow cations and anions to flow from one half cell to the other
h)   What is the cell potential once equilibrium is achieved?   0.00 v

				
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