Question

Using a table of standard reduction potentials, determine the best answer to the questions.
a) Which of the following reagents would oxidize Cr to Cr^3+, but not Ag to Ag^+?
     Br^-
     Co
     Ca^2+
     Ca
     Br2
     Co^2+
b) Which of the following reagents would oxidize Ag to Ag^+, but not Cl^- to Cl2?
     Ca^2+
     Co
     Br^-
     Ca
     Co^2+
     Br2

Answer 1

1) Cr³⁺(aq) + 3e^- ----------> Cr(s) ; E⁰_red = -0.41 V

Ag⁺(aq) + e^- ------------> Ag(s) ; E⁰_red = 0.8 V

Now, the agent which can oxidize Cr to Cr³⁺ but not Ag to Ag³⁺ must have E⁰_red > -0.41 but less than 0.8 V

Hence the correct option is :- Co²⁺

Co²⁺(aq) + 2e^- ----------> Co(s) ; E⁰_red = -0.28 V

2) Ag⁺(aq) + e^- ------------> Ag(s) ; E⁰_red = 0.8 V

Cl₂ + 2e^- ---------> 2Cl^- ; E⁰_red = 1.358 V

Now, the agent which can oxidize Ag to Ag⁺ but not Cl^- to  Cl₂ must have E⁰_red > 0.8 but less than 1.358 V

Hence the correct option is :- Br₂

Br₂ + 2e^- ---------> Br^- ; E⁰_red = 1.066 V