Ch. 20: Electrochemistry Lecture 2: Galvanic Cells

Electrochemical Cells An electrochemical cell converts chemical energy into electrical energy. Alessandro Volta invented the first electric cell but got his inspiration from Luigi Galvani. Galvani’s crucial observation was that two different metals could make the muscles of a frog’s legs twitch. Unfortunately, Galvani thought this was due to some mysterious “animal electricity” or “galvanism.” It was Volta who recognized this experiment’s potential.

Electrochemical Cells Galvani’s nephew Giovanni Aldini was inspired by his uncle’s work, so he attempted to re-animate a corpse. In total, he did fifteen experiments using galvanism on the body of a hanged criminal in These experiments inspired Mary Shelley to write Frankenstein.

Electrochemical Cells Two Types: Galvanic (voltaic) CellElectrolytic Cell * Spontaneous redox reaction produces electricity. Composed of two half-cells. * Non-spontaneous redox reaction forced to occur with external power source.

Galvanic Cells Zn Half CellCu Half Cell Zn(s)  Zn e - Cu e -  Cu (s) ANODE (oxidation)CATHODE (reduction) * Electrons flow from the anode to the cathode. * Salt bridge contains a salt solution to keep charges balanced.

Electrochemistry Terminology #4 Anode  Anode The electrode where oxidation occurs Cathode  Cathode The electrode where reduction occurs Memory device: Reduction at the Cathode Or: Red Cat (reduction at the cathode) An Ox (oxidation at the anode)

Galvanic Cells Salt Bridge: Contains an electrolyte solution (salt) whose ions will not react with other ions or electrodes in the cell. PURPOSE: To complete the circuit and keep charges in each half cell balanced.

Electric Cell Potenials, E cell The potential difference between anode and cathode; the driving force for electron movement. (Measured in Volts) Electromotive force (emf)  Also called the Electromotive force (emf)  For a reaction to be “spontaneous” E cell must be positive. Standard potential, E° cell, at 25°C, 1 atm, 1M solutions. E° cell = E° red (cathode) + E° ox (anode)

Table of Reduction Potentials Measured against the Standard Hydrogen Electrode Always written as reduction rxns

Zn - Cu Galvanic Cell Zn e -  Zn E = -0.76V Cu e -  Cu E = +0.34V From a table of reduction potentials: E° cell = E° red (cathode) + E° ox (anode) Zn is oxidized, so reverse the reaction and change the sign on E.

Zn - Cu Galvanic Cell Cu e -  Cu E = +0.34V The less positive, or more negative reduction potential becomes the oxidation… Zn  Zn e - E = +0.76V Zn + Cu 2+  Zn 2+ + Cu E 0 = V E° cell = E° red (cathode) + E° ox (anode)

Ag - Cd Galvanic Cell The less positive, or more negative reduction potential becomes the oxidation… 2Ag + + 2e -  2Ag(s) E = +0.80V Cd(s)  Cd e - E = V Cd(s) + 2Ag +  Cd Ag(s) E 0 = V E° cell = E° red (cathode) + E° ox (anode) Ag + + e -  Ag(s) E = +0.80V Cd e -  Cd(s) E = -0.40V

REVIEW  What is the purpose for an electrochemical cell? To convert chemical energy to electrical energy  What is a galvanic cell? Two half-cells in which spontaneous redox reaction produces electricity. Cathode?  Which reaction takes place at the Cathode? Reduction (Red-Cat)

REVIEW Anode?  Which reaction takes place at the Anode? Oxidation  What is Electric Potential? The potential difference between anode and cathode; It is the driving force for electron movement.