Electrochemistry Part IV Spontaneity Nernst Equation Jespersen Chap

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Electrochemistry Part IV: Spontaneity & Nernst Equation Jespersen Chap. 20 Sec 4 & 5

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

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

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

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?

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

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

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

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

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