Electrochemistry Red Ox Part Deux Electrochemical cells Voltaic

Electrochemistry Red. Ox: Part Deux

Electrochemical cells Voltaic Converts chemical E from a Favorable Red. Ox reaction into electrical E Exothermic reaction a. k. a. battery Electrolytic Uses electrical E to force an UNFAVORABLE Red. Ox 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)

Flow of e- Flow of e. Cu cathode Zn anode Zn 0 Zn+2 + 2 e- Cu+2 + 2 e- salt bridge half cell Cu 0 half cell Zn+2 anions Zn. SO 4 (aq) cations Cu+2 Cu. SO 4 (aq)

Things to remember. . . e- flow 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 E =E 0 0 red + E 0 ox

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 E 0 cell is positive, the reaction is favorable, Negative is UNfavorable Zero is at equilibrium (the battery is dead)

Practice problems • Calculate E 0 cell for the following reaction • Fe(s) + Pb 2+(aq) --> Fe 2+(aq) + Pb(s) E 0 cell = +0. 32 V • Based on E 0 cell is this reaction favorable? yes

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

Electrolytic Cells UNfavorable Red. Ox 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- Cu 2+ + 2 e 2 e- + Cu 2+ cathode Cu 0

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

The End