Question

Balance the following redox equations and show work:

a)         H₂S + Hg ₂²⁺------> Hg + S + H⁺

b)         Zn + BrO₄^-+ OH^-+ H₂O ------> [Zn(OH)₄]^2- + Br ^-

c)         MnO₄^- + S ^2-+ H₂O -------> MnO₂+ S + OH ^-

d)         Cu + H ⁺+ NO₃^-----------> Cu ⁺²+ NO₂ + H₂O

e)         Cu + H ⁺+ NO₃^-----------> Cu ⁺²+ NO + H₂O

f)          MnO₄^-+ NO₂^-+ H₂O -----> MnO₂+ NO₃^-+ OH ^-

  

Answer 1

a)         H₂S + Hg ₂²⁺ ------> 2Hg + S + 2H⁺

b)         Zn + BrO₄^- + OH^- + H₂O ------> [Zn(OH)₄]^2- + Br ^-

           first half of the above reaction

         Zn ........................> [Zn(OH)₄]^2-

     add 4 OH- molecules to balance the O atoms

       Zn   + 4OH- ........................> [Zn(OH)₄]^2-

balance charge by adding 2e- on the left hand side of the above reaction

   Zn   + 4OH- ........................> [Zn(OH)₄]^2-   + 2e- .......................1

second half reaction

BrO₄^- ...............> Br-

to balance the O atoms add 4 H2O molecules on the left hand side and double the no. of OH- ions on the right hand side

BrO₄^- +4H2O + 7e-...............> Br- + 8OH- ...............2

do 7*eq1 + 2*eq2

2BrO₄^- +8H2O + 12OH- + 7Zn ...............> 2Br- + 7[Zn(OH)₄]2-

c)         MnO₄^- + S ^2- + H₂O -------> MnO₂ + S + OH ^-

               first half reaction

                 MnO4^-   ...................> MnO2

                 MnO4^-   + 2H2O..................> MnO2 + 4OH-

               MnO4^-   + 2H2O + 3e-..................> MnO2 + 4OH- ......................1

second half reaction

                 S^-2 .......................> S

                 S^-2 .......................> S + 2e- ...........................................2

do 2*eq.1 + 3*eq.2

           3 S^-2 + 2 MnO4^-   + 4H2O ..................> 2MnO2 + 8OH- + 3S

d)         Cu + H ⁺ + NO₃^- ----------> Cu ⁺² + NO₂ + H₂O

                       first half reaction

                   Cu ..................> Cu^+2

                     Cu   ..................> Cu^+2 + 2e- ...................1

second half reaction

     NO3^- .............................> NO2

   NO3^- .............................> NO2 + H2O

     NO3^-   + 2H+ + 1e-.............................> NO2 + H2O ...............2

   do eq1 + 2*eq2

   2NO3^-   + 4H+ + Cu............................> 2NO2 + 2H2O + Cu^+2

e)         Cu + H ⁺ + NO₃^- ----------> Cu ⁺² + NO + H₂O

                     first half reaction
                  Cu ..................> Cu^+2

                     Cu   ..................> Cu^+2 + 2e- ...................1

second half reaction

   NO3^- ................> NO

   NO3^- .............................> NO+ 2H2O

     NO3^-   + 4H+ + 3e-.............................> NO + 2H2O ...............2

   do 3* eq1 + 2*eq2

             2 NO3^-   + 8H+ + 3Cu ............................>2 NO + 4H2O + 3Cu^+2

f)          MnO₄^- + NO₂^- + H₂O -----> MnO₂ + NO₃^- + OH ^-

    first half reaction

                MnO4^-   ...................> MnO2

                 MnO4^-   + 2H2O..................> MnO2 + 4OH-

               MnO4^-   + 2H2O + 3e-..................> MnO2 + 4OH- ......................1

second half reaction

      NO2^- ...................> NO3^-

        NO2^- + 2OH-..................> NO3^- + H2O +2e-.................................2

do 2*eq.1 + 3*eq.2

   2MnO4^-   + H2O +3NO2^- ................> 2MnO2 + 2OH- + 3NO3^-