Electrochemistry RedOx: Part Deux

Electrochemical cells Voltaic Converts chemical E from a Favorable RedOx reaction into electrical E Exothermic reaction a.k.a. battery Electrolytic Uses electrical E to force an UNFAVORABLE RedOx reaction to take place Endothermic reaction a.k.a. electroplating

Parts of a voltaic (galvanic) cell anode more active metal on activity series ½ cell electrode that gets oxidized; decreases in mass cathode ½ cell electrode where reduction takes place; increases in mass salt bridge allows for conduction of IONS (cations to the cathode & anions to the anode) Wire allows for conduction of ELECTRONS (anode to cathode)

half cell half cell Flow of e- Flow of e- Cu Zn anode cathode salt bridge half cell Zn0 Zn+2 + 2e- Cu+2 + 2e- Cu0 cations Zn+2 anions ZnSO4 (aq) CuSO4 (aq) Cu+2

Things to remember . . . e- flow Big Reduction Cathode Anode  Cathode through wire more active metal  less active metal Big Reduction Cathode Anode active Oxidation loses mass Ions flow across the bridge

Half-Cells & Cell Potentials Electrical potential is a measure of the cell’s ability to produce an electric current. This results from competition for electrons between two half-cells E0 = E0red + E0ox

Standard Electrode Potentials Standard Reduction potentials The more positive the value, the easier the reduction half reaction takes place. measured in volts

How do they know?

When using the formula . …. . switch the sign for oxidation & ADD If E0cell is positive, the reaction is favorable, Negative is UNfavorable Zero is at equilibrium (the battery is dead)

Fe(s) + Pb2+(aq) --> Fe2+(aq) + Pb(s) Practice problems Calculate E0cell for the following reaction Fe(s) + Pb2+(aq) --> Fe2+(aq) + Pb(s) Based on E0cell is this reaction favorable? E0 cell = +0.32V yes

will be the reduction rxn, for the oxidation and ADD. The more positive will be the reduction rxn, THEN SWITCH the sign for the oxidation and ADD. What is the maximum cell voltage? Ag+ + e- --> Ag Zn+2 + 2e- --> Zn Is this redox reaction spontaneous? Ni, Ni+2 // Fe+2, Fe+3

UNfavorable RedOx reaction Electrolytic Cells UNfavorable RedOx reaction needs an outside power source to force e- to flow from anode to cathode Used for electroplating coating an inexpensive metal with an expensive metal a.k.a. Electrolysis

flow of e- Cu0 Cu2+ + 2e- 2e- + Cu2+ Cu0 cathode

Similarities & Differences Both use RedOx reactions Anode is site of oxidation Cathode is site of reduction e- flow through the wire from anode to cathode Voltaic Cell RedOx is favorable Electrolytic RedOx is unfavorable power source necessary No salt bridge

The End