Question

Using the reaction and the E∘ given below

2Co3+(aq)+2Cl−(aq)→2Co2+(aq)+Cl2(g)      E∘=0.46 V

what is the cell potential at 25 ∘C if the concentrations are [Co3+]= 3.10×10⁻²M , [Co2+]= 0.732 M , and [Cl−]= 0.544 M and the pressure of Cl2 is PCl2= 9.80 atm ?

2)At 57.0 ∘C , what is the maximum value of Q needed to produce a non-negative E value for the reaction

SO4^2−(aq)+4H+(aq)+2Br−(aq)⇌Br2(aq)+SO2(g)+2H2O(l)

In other words, what is Q when E=0 at this temperature?

Answer 1

To solve this for cell potential, we will use the Nernst equation:

E_cell= E^o_cell - RT/nF lnQ

where E_cell represents the cell potential, R= gas constant= 0.0821 Latm/K/mol, T= temperature, n= number of electrons involved in the redox reaction, Q= activity coefficient

To know "n", let us break down the redox equation into separate equations at the anode and cathode.

Anode reaction: 2Cl^- = Cl₂ + 2e^-

Cathode reaction: (Co³⁺ + e^- =Co²⁺) multiply this equation by two and add it to anode equation to arrive at the final equation.

From the above reactions, we see that n=2

Now, the Nernst equation can be rewritten as:

E_cell= E^o_cell- 0.0591/n log Q (On substituting the value of R=0.0821, T=298.15K, F=96500C, ln= 2.303log)

E_cell= 0.46- 0.0591/2 log [Co²⁺]² P_Cl2/ [Co³⁺]² [Cl-]²

E_cell= 0.46 - 0.0591/2 log 0.732 X 0.732 X 9.8 / .031 X .031 X .544 X .544

= 0.334 V.