Electrochemistry Part IV Spontaneity Nernst Equation Jespersen Chap
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Electrochemistry Part IV: Spontaneity & Nernst Equation Jespersen Chap. 20 Sec 4 & 5 Skipping Sec 6 & 8 Dr. C. Yau Fall 2014 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 ΔGo = - n FEocell 4 C/mol e 9. 65 x 10 (under standard (remember 1 V = 1 J/C) conditions) 2
Example 20. 7 p. 937 Calculate ΔGo for the reaction, given that its standard cell potential is 0. 320 V at 25 o. C. Ni. O 2 (s) + 2 Cl (aq) + 4 H+ (aq) Cl 2 (g) + Ni 2+ (aq) + 2 H 2 O (l) ΔGo = - n FEocell F = 9. 65 x 104 C/mol e(1 V = 1 J/C, so Eo = 0. 320 J/C) How do we figure out what n is? Ans. -61. 8 k. J Do Pract Exer 13 & 14 p. 938 3
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. Ni. O 2 (s) + 2 Cl (aq) + 4 H+ (aq) 2 Cl 2 (g) + Ni 2+ (aq) + 2 H 2 O (l) Collect all the constants we need. Do Pract Exer 15 & 16 p. 939 4
Derivation of the Nernst Eqn What happens when it is not under standard conditions? ΔG = - n FEcell ΔGo = - n FEocell Divide both sides of eqn by (-n. F) we get. . . Nernst Equation 5
Common simplified version of the Nernst Equation for 25. 0 o. C: This version of Nernst Eqn will be given also, but remember it’s only for 25. 0 o. C 6
Nernst Equation Eo is the cell potential under standard conditions (for aqueous soln, 1 M) What if it is not 1 M? This type of quest will be on your final exam. Example 20. 9 p. 940 Suppose a galvanic cell employs the following: Ni 2+ + 2 e- Ni Eo = - 0. 25 V Cr 3+ + 3 e- Cr Eo = - 0. 74 V Calculate the cell potential when [Ni 2+] = 4. 87 x 10 -4 M and [Cr 3+] = 2. 48 x 10 -3 M Ans. +0. 44 V 7
Example 20. 10 p. 941 The rxn of tin metal with acid can be written as Sn (s) + 2 H+ (aq) Sn 2+ (aq) + H 2 (g) Calculate the cell potential (a) when the system is at standard state. Ans. +0. 02 V (b) when the p. H = 2. 00 Ans. -0. 16 V (c) when the p. H is 5. 00. Assume that [Sn 2+] = 1. 00 M and the partial pressure of H 2 is also 1. 00 atm. Do Pract Exer 17, 18, 20 p. 942 8
What we are skipping in Chap. 20: pp. 943 -951 Concentration from E Measurements Sec 20. 6 Electricity Batteries: Lead Storage Batteries Zinc-Manganese Dioxide Cells (Le. Clanche cell) Nickel-Cadmium Rechargeable Batteries Nickel-Metal Hydride Batteries Lithium Ion Cells Fuel Cells Photovoltaic Cells 9