[dnbwise]

The problem is:

A voltaic cell consists of Ni/Ni2+ and Co/Co+ half-cells with concentrations of [Ni2+] = .80 M and [Co2+] = .20 M. Calculate the value of Ecell initial.

I made the following calculations to come to my answer:

(I got the E values from a table of half-cell reactions)
Ni2+(aq) + 2e- -> Ni(s) (E = -0.25)
Co(s)              -> Co2+(aq) + 2e- (E = -0.28)
___________________________________________
Ni2+(aq) + Co(s) -> Ni(s) + Co2+(aq)
from this rxn I calculate Ecell = E(reduction) - E(oxidation) = -0.25 - (-0.28) = 0.03

My book has the answer as 0.05, so I am not sure what I did wrong. Any help would be much appreciated.

 

[Donaldson Tan]

you use the standard potential values to solve, that's why you are wrong. t

the given reaction conditions are non-standard. you must use Nerst's Equation to find the non-standard electrode potential values.

                    R T    
      E = E° - ----- ln K
                     n F    
where K is the equilibrium constant, E° is the standard electrode potential, F is Faraday's Constant