1. Electrochemistry Part IV: Spontaneity & Nernst Equation
Jespersen Chap. 20 Sec 4 & 5
Skipping Sec 6 & 8
Dr. C. Yau
Fall 2014 1 1

2. Spontaneity of Reaction
We know to have a spontaneous rxn…
E > 0
ΔG < 0
How are these two related?
ΔG = - n FEcell where n = moles of e-
F = Faraday's constant
9.65x104 C/mol e-
(remember 1 V = 1J/C)
ΔGo = - n FEocell
(under standard conditions) 2

3. NiO2 (s) + 2Cl(aq) + 4H+ (aq)  Cl2 (g) + Ni2+ (aq) + 2H2O (l)
Example 20.7 p. 937
Calculate ΔGo for the reaction, given that its standard cell potential is V at 25oC.
NiO2 (s) + 2Cl(aq) + 4H+ (aq)
 Cl2 (g) + Ni2+ (aq) + 2H2O (l)
ΔGo = - n FEocell F = 9.65x104 C/mol e-
(1 V = 1J/C, so Eo = J/C)
How do we figure out what n is?
Ans kJ
Do Pract Exer 13 & 14 p. 938

4. Calculating K from Cell Potential
We know ΔGo = - n FEocell
We also know ΔGo = - RT ln K (Chap. 19)
so n FEo = - RT ln K
Example 20.8 p. 789
Calculate K for the reaction in Example 20.8.
NiO2 (s) + 2Cl(aq) + 4H+ (aq)
 2Cl2 (g) + Ni2+ (aq) + 2H2O (l)
Collect all the constants we need.
Do Pract Exer 15 & 16 p. 939

5. Derivation of the Nernst Eqn
What happens when it is not under standard conditions? Divide both sides of eqn by (-nF) we get...
ΔGo = - n FEocell
ΔG = - n FEcell
Nernst Equation

6. Common simplified version of the Nernst Equation for 25.0 oC:
This version of Nernst Eqn will be given also, but remember it’s only for 25.0oC

7. Nernst Equation
Eo is the cell potential under standard conditions (for aqueous soln, 1M)
What if it is not 1 M?
Example 20.9 p. 940
Suppose a galvanic cell employs the following:
Ni2+ + 2e-  Ni Eo = V
Cr3+ + 3e-  Cr Eo = V
Calculate the cell potential when [Ni2+] = 4.87x10-4M
and [Cr3+] = 2.48x10-3 M
This type of quest will be on your final exam.
Ans V

8. The rxn of tin metal with acid can be written as
Example p. 941
The rxn of tin metal with acid can be written as
Sn (s) + 2H+ (aq)  Sn2+ (aq) + H2 (g)
Calculate the cell potential
(a) when the system is at standard state.
(b) when the pH = 2.00
(c) when the pH is 5.00.
Assume that [Sn2+] = 1.00 M and the partial pressure of H2 is also 1.00 atm.
Do Pract Exer 17, 18, 20 p. 942
Ans V
Ans V

9. What we are skipping in Chap. 20:
Concentration from E Measurements
Sec 20.6 Electricity
Batteries: Lead Storage Batteries
Zinc-Manganese Dioxide Cells (LeClanche cell)
Nickel-Cadmium Rechargeable Batteries
Nickel-Metal Hydride Batteries
Lithium Batteries
Lithium Ion Cells
Fuel Cells
Photovoltaic Cells