AP Electrochemistry Free Response Questions

[Ur A Nernst]

Homework Statement

(i) The mass of Fe(s) produced when 1 faraday is used to reduce a solution of FeSO4 is 1.5 times the mass of Fe(s) produced when 1 faraday is used to reduce a solution of FeCl3.

Zn + Pb²⁺ (1.0M) --> Zn²⁺ (1.0M) + Pb

(ii) The cell that utilitzed the reaction above has a higher potential when [Zn²⁺] is decreased and [Pb²⁺] held constant, but a lower potential when [Pb²⁺] is decreased and [Zn²⁺] is held constant.

(iii) The cell that utilizes the reaction given in (ii) has the same cell potential as another cell in which [Zn²⁺] and [Pb²⁺] are each .10M.

Homework Equations

E = E naught - (.0592/n)logQ
Where E naught is the cell potential at 1 molar volume for each solution, 25 degrees C, and 1 atm.
n = number of electrons lost or gained
Q = (Products)^[coefficient] / (reactants)^[coefficient]

The Attempt at a Solution

(i) I believe it has something to do with the charge; the charge on Fe is 2+ in FeSO₄ and 3+ on FeCl₃. But I'm just rambling.

(ii) I wasn't sure; according to LeChatlier's principle, as the concentration of reactants increase.. oh wait, I see now. The product is being reduced, so the reactants are higher, so the cell potential goes up.

(iii) I guess in using the Nernst Equation, Q would still be the same as if (.1)^1/(.1)^1 = 1

E = E naught - (.0592/n)logQ
Q = (.1) ^ 1 / (.1) ^ 1
Q = 1
log(1) = 0
E naught - 0 = E
E naught = E

Correct me if I'm wrong (I have no idea about #1, and as I typed I gained knowledge about #2 and 3.)

 

[nate808]

(i) Your reasoning about the charge is correct. The mass of the product is directly proportional to the number of electrons transferred, which is related to the charge of the ions involved in the reaction. In this case, FeSO4 has a charge of 2+ while FeCl3 has a charge of 3+, so the mass of Fe produced would be greater in the FeSO4 solution.

(ii) Your understanding of LeChatlier's principle is correct. When the concentration of reactants is increased, the equilibrium shifts towards the products, resulting in a higher cell potential. In this case, as the concentration of Zn²⁺ decreases, the equilibrium shifts towards the products, resulting in a higher cell potential.

(iii) Your calculation using the Nernst equation is correct. Since the concentrations of Zn²⁺ and Pb²⁺ are equal, the value of Q is 1, resulting in E = E naught. This means that the cell potential is the same as the standard potential of the cell.