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Overview of Chapter 20 Today’s Topics: Electrochemistry • Oxidation/Reduction (Redox) Reactions • Electrochemical cells • Electrochemical Cells • Calculating Voltage • E° and E • E° and K • Voltage at non standard conditions • E° and ∆G Today’s Demos: Cu2+(aq) aq) + Zn(s) ---> Zn2+(aq) aq) + Cu(s) reaction •direct reaction •indirect reaction: Voltaic cell With 1M concentrations of ions With 0.1M concentration of Cu2+ Electrochemical Cells • A redox reaction where electrons are transferred through an external connector. • If a product favored reaction: voltaic or galvanic cell electric current produced • If a reactant favored reaction: electrolytic cell electric current used to cause chemical change Batteries are voltaic cells Electrolysis of H2O Electrochemical Cells Electrochemistry With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.” Electrons are transferred from Zn to Cu2+, but there is no useful electric current. Oxidation: Zn(s) ---> Zn2+(aq) aq) + 2eReduction: Cu2+(aq) aq) + 2e- ---> Cu(s) -------------------------------------------------------Cu2+(aq) aq) + Zn(s) ---> Zn2+(aq) aq) + Cu(s) •To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery. 1. Electrochemical Cells Zn --> Zn2+ + 2e- CELL POTENTIAL, E 1.10 V Cu2+ + 2e- --> Cu Oxidation Anode Negative Reduction Cathode Positive <--Anions Cations--> 1.0 M 1.0 M anode cathode E°cell = E°cathode - E°anode, •Electrons travel through external wire. •Salt bridge allows anions and cations to move between electrode compartments. where E°cathode and E°anode are the standard reduction potentials CELL POTENTIAL, E 1.10 V Zn and Zn2+, anode Cu and Cu2+, cathode 1.0 M 1.0 M E°cell = E°cathode - E°anode Cu2+ (1 M) + 2e- Cu (s) E°cathode = 0.34V Zn (s) + 2e- Zn2+ (1 M) E°anode = -0.76V __________________________________________ Cu2+ + Zn Cu (s) + Zn2+ E°cell = 1.10V 1.10V 2.
