Electrochemistry

Question

Predict whether the following reaction would proceed spontaneously as written 298K.

Co + Fe2+ -> CO2+ + Fe

Given that [Co2+]=0.15M and [Fe2+]=0.68M

Co2+ + 2e- ->Co E°=-0.28V

Fe2+ + 2e- -> Fe E°=-0.44

Answer 1

For an electrochemical reaction to be spontaneous, the Ecell must be POSITIVE. We will use standard reduction potentials to calculate the Eº and the Nernst equation to find the Ecell since the conditions are not standard.

Standard reduction potentials:

Co²⁺ + 2e- ==> Co Eº = -0.28 V

Fe²⁺ + 2e- ==> Fe Eº = -0.44 V

According to these potentials, Co2+ would be reduced and Fe would be oxidized. Since the equation as written, Co + Fe²⁺ -> Co²⁺ + Fe has the opposite reactions taking place, this will NOT be spontaneous.

To confirm this, we will calculate the Ecell and see if it is positive.

As written, cathode is Fe and anode is Co. Eº = cathode - anode = -0.44 - (-0.28) = -0.16 V

Using the Nernst equation @ 298K, we have...

Ecell = Eº - 0.0592 / n log Q

n = 2

Q = [Co²⁺] / [Fe²⁺] = 0.15 / 0.68 = 0.22

Ecell = -0.16 - 0.0296 log 0.22

Ecell = -0.16 - (-0.019)

Ecell = -0.14 V (still a negative value)

Reaction is NOT SPONTANEOUS