Electrolytic Cell Problem - Can't calculate req'd voltage

[mburt]

Homework Statement

What voltage must be applied to decrease the mass of Ag(s) in the electrolytic cell
reaction below?

2 Ag⁺(aq) + Cd(s) → 2 Ag(s) + Cd²⁺(aq)

Homework Equations

Standard reduction potential of Ag⁺: Eo = +0.80 V
Standard reduction potential of Cd²⁺: Eo = -0.40 V

The Attempt at a Solution

Since the question says that Ag(s) will lose mass, I assume it must be oxidized so I wrote the equation:

Ag(s) → Ag⁺ + e^- where Eo = -0.80 V (Change sign,cause oxidation)

and therefore the reduction reaction is:

Cd²⁺ + 2e- → Cd(s) where Eo = -0.40 V (keep sign)

So the net voltage is : -1.20 V... so the req'd voltage to make this non-spontaneous reaction occur is +1.20 V.

But the answer in the answer key says the req'd voltage is +1.30 V. Note that this is a multiple choice question and the answer of +1.20 V was an option as well.

Thanks!

 

[Borek]

If you apply 1.2V that's not enough yet to force reaction, for that you need a little bit more.

 

[mburt]

Okay thanks, so the calculation just liberates electrons from the metal, but doesn't proceed with the reaction...so to removed actual mass from the object you need more voltage.

Thanks!