1 / 7

The Nernst Equation

The Nernst Equation. Thermodynamics of the relationship of D E and D G - electrical work - relating D G and D E: D G = - n F D E - finding the equilibrium constant from measured cell potentials.

lilah-buckner
Télécharger la présentation

The Nernst Equation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Nernst Equation Thermodynamics of the relationship of DE and DG - electrical work - relating DG and DE: DG = - n F DE - finding the equilibrium constant from measured cell potentials. Dependence of the cell potential on activity - the Nernst Equation - the calomel electrode The calomel electrode The pH meter - the glass electrode

  2. Relationship of Cell Potential and DG DG = DH - TDS (const T) = DE + pDV - TDS (const P) = (qrev + w) + pDV - qrev (reversible, const T) w = - pDV + wel,rev expansion work electrical work wel = - Q DE work = charge * (potential difference) = - n F DE Joules Volts Coulombs DG = wel,rev = - n F DE (const T, P, reversible)

  3. Concentration Effects and The Nernst Equation DG° is the standard DG - all solute concentrations must be 1 M and partial pressures must be 1 atm. DG = DG° + RT ln Q - this allows you to calculate DG at non-standard concentrations and partial pressures. DG = - n F DEcell - this is always true; the cell potential is a very fundamental quantity, just like DG DEcell = DE°cell - (RT/nF) lne Q- the Nernst equation. It allows you to calculate DG at non-standard conditions

  4. DE° = (RT/nF) lne K - For reactions in solution, K and can be measured from a standard cell potential. DG°, Measuring Equilibrium Constants DG° = - RT lne K - K can be calculated from DG°, which for many compounds can be obtained from thermodynamic tables. DG = - n F DEcell - this is always true; the cell potential is a very fundamental quantity, just like DG DEcell = DE°cell - (RT/nF) lne Q- The Nernst equation relates DEcell to DE°cell = DE°cell - 2.303 (RT/nF) log10 Q = DE°cell - (0.0152/n) log10 Q (at T=25°C)

  5. PbO2(s) + SO42-(aq) + 4 H3O+(aq) + 2 e- PbSO4(s) + 6 H2O(l) E°=+1.685 V 1 E = E° - (.0592/2) log10 [H3O+]4 [SO42-] The Nernst Equation DEcell = DE°cell - (0.0592/n) log10 Q at T = 25°C Pt(s) The cell potential changes by 0.118 V for each pH unit or by 0.0296 V for each factor of 10 change in [SO42-]: SO42-(aq) slurry of PbO2(s) + PbSO4(s) pH = 0 E = +1.685 V pH = 4 E = +1.211 V H3O+(aq)

  6. pH measurement Standard Hydrogen Electrode: 2 H3O+(aq, 1M) + 2 e- H2(g) + 2 H2O(l) E = +0.241 V - (.0592/2) log10 [H3O+]2 = +0.241 + .0592 * pH Calomel Electrode: Hg2Cl2(s) + 2 e- 2Hg(l) + 2 Cl-(sat’d) Pt(s) Pt(s) H2(g) salt bridge sat’d KCl(aq) paste of Hg(l) and Hg2Cl2(s) H3O+(aq) KCl(s) standard hydrogen electrode calomel reference electrode E° = +0.000 V E° = +0.241 V

  7. 0.0592 V [H+]out2 Ecell = E°cell - log10 2 [H+]in2 The Glass Electrode and the pH Meter The glass electrode is non-metallic electrode composed of a special glass that is porous to H3O+. A diffusion potential develops across the membrane in response to a pH gradient. The potential varies by 59.6 mV for each unit change in pH. Notice that Ecell is uniformly sensitive to pH over 14 orders of magnitude!! Ecell = constant + (0.0592 v) * pH

More Related