Cell Potential and the Nernst Equation Electrochemistry Cell Potential and the Nernst Equation

The Electrochemical Cell Lead-Acid (car) Battery Producing Copper Lab Source: www.battterybot.com Source: http://www.reuk.co.uk Alkaline Battery Lead-Acid (car) Battery

The Electrochemical Cell The species being oxidized LOSES electrons Occurs at the anode (both start with vowels!) is the reducing agent is the more active metal Will go from metal to ion, thus putting out electrons From the producing copper lab… 2Cl- (aq) → Cl2 (g) + 2e-

The Electrochemical Cell The species being reduced GAINS electrons Occurs at the cathode (both start with consonants!) is the oxidizing agent is the less active metal Will go from ion to metal, thus taking in electrons From the producing copper lab… Cu+2 (aq) + 2e- → Cu (s)

The Electrochemical Cell Cu2+ (aq) + Zn (s) → Zn2+ (aq) + Cu (s) Copper Zinc CuSO4 ZnSO4 Write the oxidation half-reaction Write the reduction half-reaction What is the oxidizing agent? What is the reducing agent? Where is the cathode? Where is the anode? Image: http://commons.wikimedia.org/wiki/File:Galvanic_cell_unlabeled.svg

The Electrochemical Cell Cu2+ (aq) + Zn (s) → Zn2+ (aq) + Cu (s) Copper Zinc CuSO4 ZnSO4 How can we determine the voltage (the cell potential) created by an electrochemical cell? Electrons flow from anode to cathode. We can measure this. Image: http://commons.wikimedia.org/wiki/File:Galvanic_cell_unlabeled.svg

The Electrochemical Cell Cu2+ (aq) + Zn (s) → Zn2+ (aq) + Cu (s)

Determining Cell Potentials Ecell = Eoreduction + Eooxidation Write the oxidation and reduction half-reactions Use the standard reduction table to determine: Ecell = Eoreduction + Eooxidation *Remember to flip the sign if you flip the half-reaction! Determine if spontaneous or not: + Ecell = spontaneous - Ecell = not spontaneous

Cell Potential Example Determine the cell potential and say if the reaction is spontaneous as written: Mn2+(aq) + 4H2O(l) + Sn2+(aq) → MnO4-(aq) + 8H+(aq) + Sn(s)

The Nernst Equation Until now, we have been calculating cell potentials at standard conditions (1 M, 25oC, and 1 atm). How do we calculate cell potentials under NON standard conditions? At standard temp, this simplifies to:

Nernst Equation Example Find the cell potential for the following reaction at 25°C if [Cd2+] = 0.020 M and [Pb2+] = 0.200 M. Cd2+ (aq) + Pb (s) → Pb2+ (aq) + Cd (s)