20080201 Electrochem 3

20080201 Electrochem 3 - Intro Chemistry II 030.102 -...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Intro Chemistry II 030.102 - Chapter 12 Spring, 2008 1 12.3. Concentration Effects and the Nernst Equation Apply thermodynamic principles to understand how concentration and pressure affect cell voltage. First, relate free energy change to reaction quotient Q (chapter 9): G = G + RT ln Q Refresher: For general gas-phase reaction aA ( g ) + bB ( g ) cC ( g ) + dD ( g ) Reaction coefficient is Q = ( P C ) c ( P D ) d ( P A ) a ( P B ) b Pressures in atm (standard state) Combine free energy equation above with G = n F E and G = n F E gives n F E = n F E + RT ln Q Nernst Equation From n F E = n F E + RT ln Q we obtain the Nernst equation E = E ln Q Rewrite in terms of common (base-10) logarithms: ln Q = 2.303 log 10 Q At 25 C (298.15 K), we have RT n F 2.303 RT F = (2.303) (8.315 J K 1 mol 1 )(298.15 K) 96485 C mol 1 = 0.0592 J C 1 = 0.0592 V Nernst equation becomes ! E = ! E 0.0592 V n log 10 Q (at 25 C) Exercise: Compute cell voltage with Nernst equation Intro Chemistry II 030.102 - Chapter 12 Spring, 2008 2 Problem 12.27 A galvanic cell is constructed that carries out the reaction Pb 2+ ( aq ) + 2 Cr 2+ ( aq ) Pb( s ) + 2 Cr 3+ ( aq ) If the initial concentration of Pb 2+ ( aq ) is 0.15 M, that of Cr 2+ ( aq ) is 0.20 M, and that of Cr 3+ ( aq ) is 0.0030 M, calculate the initial voltage generated by the cell at 25 C. From Appendix E: Pb 2+ ( aq ) + 2 e Pb( s ) E = 0.1263 V Cr 3+ ( aq ) + e Cr 2+ ( aq ) E = 0.424 V Note: Standard voltage for (Cr 3+ ,Cr 2+ ) is slightly different in the student solutions manual. Measuring Equilibrium Constants For overall cell reaction, G = n F E Standard free energy change related to equilibrium constant: G = RT ln K Hence, ln K = E or log 10 K = E (at 25 C) Exercises: (1) Calculate equilibrium constant from standard cell voltage.(1) Calculate equilibrium constant from standard cell voltage....
View Full Document

This note was uploaded on 04/14/2008 for the course CHEMISTRY 102 taught by Professor Dagdigian during the Spring '08 term at Johns Hopkins.

Page1 / 8

20080201 Electrochem 3 - Intro Chemistry II 030.102 -...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online