Question

Consider a galvanic cell based on the following two half reactions at 298 K: SRP Fe3+ + e-  Fe2+ 0.77 V Cu2+ + 2 e-  Cu 0.34 V How many of the following changes will increase the potential of the cell? Why?

1. Increasing the concentration of Fe+3 ions

2. Increasing the concentration of Cu2+ ions

3. Removing equal volumes of water in both half reactions through evaporation

4. Increasing the concentration of Fe2+ ions

5. Adding a NO3 - solution to the anode portion of the cell

Answer 1

When the cell is NOT under standard conditions, i.e. 1M of each reactants at T = 25°C and P = 1 atm; then we must use Nernst Equation.

The equation relates E°cell, number of electrons transferred, charge of 1 mol of electron to Faraday and finally, the Quotient retio between products/reactants

The Nernst Equation:

Ecell = E0cell - (RT/nF) x lnQ

In which:

Ecell = non-standard value

E° or E0cell or E°cell or EMF = Standard EMF: standard cell potential
R is the gas constant (8.3145 J/mol-K)
T is the absolute temperature = 298 K
n is the number of moles of electrons transferred by the cell's reaction
F is Faraday's constant = 96485.337 C/mol or typically 96500 C/mol
Q is the reaction quotient, where

Q = [C]^c * [D]^d / [A]^a*[B]^b

pure solids and pure liquids are not included. Also note that if we use partial pressure (for gases)

Q = P-A^a / (P-B)^b

substitute in Nernst Equation:

Ecell = E° - (RT/nF) x lnQ

then

Cu+2 + 2e- = Cu

Fe+3 + e- = Fe;

most l ikely to occur is

2Fe+3 + 2e- = 2Fe+2

Cu = Cu+2 + 2e-

Q = [Fe+2]^2 / ([Cu2+][Fe+3]^2)

therefore

1.

increasing Fe+3, reactants, will favour more EMF since there are more reagents

2.

increase in Cu2+, products, will decrease MF since there is no more to react

3.

removing volume will decrease concentrations, this effect

changes:

Q = [Fe+2]^2 / ([Cu2+][Fe+3]^2)

Q = 1/V^2 / (1/V * 1/V^2) = 1/V, then decrease in V, increases, Q

increase in Q decreases EMF

Q4

increasing Fe2+, th eproducts, will not favour reaction, decreases EMF

5.

addition of NO3- will have no effect ( if we ignore activity) since it is inert