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

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

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 0.320 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 = 0.320 J/C) How do we figure out what n is? Ans. -61.8 kJ Do Pract Exer 13 & 14 p. 938

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

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

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

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 = - 0.25 V Cr3+ + 3e-  Cr Eo = - 0.74 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. +0.44 V

The rxn of tin metal with acid can be written as Example 20.10 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. +0.02 V Ans. -0.16V

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