1. 1 REVERSIBLE ELECTROCHEMISTRY 1

2. Voltaic Or Galvanic Cells Voltaic or Galvanic cells are electrochemical cells in which spontaneous oxidation- reduction reactions produce electrical energy. 1.The two halves of the redox reaction are separated, 2.Requiring electron Transfer to occur through an external circuit, in this way useful electrical energy is obtained.

3. 6/24/2016 Every one is familiar with some voltaic cells : The batteries commonly used in flashlights, portable radios,photographic equipments & many toys are voltaic cells. Automobile batteries consist of voltaic cells connected in series so that their voltage added.

4. First - The Zinc –Copper Cell Consider a standard cell made up of two half –cells,one a strip of Metallic Cu immersed in 1M copper sulphate solution and the other a strip of Zn immersed in 1M zinc sulphate solution.This cell Is called the Daniell cell. 1- The initial voltage is 1.100 volts 2- The mass of the zinc electrode decreases.The concentration of Zn 2+ increases in the solution around the zinc electrode as the cell operates. 3-The mass of the copper electrode increases The concentration of Cu 2+ decreases in the Solution around this electrode as the cell Operates The Zn electrode loses mass because some Zn metal Is oxidized to Zn 2+ ions, which go into solution.Thus the Zn electrode is the anode.At the cathode,Cu 2+ Ions are reduced to Cu metal

5. The reaction at the anode (Zn electrode ) : Zn Zn 2 + +2e- The reaction at the cathode ( Cu electrode ) : Cu 2+ +2e Cu The overall reaction : Zn (s) +Cu 2+ (aq ) Zn 2+ (aq) +Cu (s) The standard potential of this cell is 1.10 volt The Zn –Cu cell can be represented as Zn Zn 2+ (1M) Cu 2+ (1M) Cu

6. Voltaic Cells In spontaneous oxidation- reduction (redox) reactions, electrons are transferred and energy is released.

7. Voltaic Cells We can use that energy to do work if we make the electrons flow through an external device. We call such a setup a voltaic cell.

8. Galvanic Cell Galvanic cell (also called voltaic cell) uses chemical reaction to produce electrical energy (flow of electrons). W hen zinc metal placed in CuSO 4 solution, following reaction take place: Zn (s) + CuSO 4(aq) ZnSO 4(aq) + Cu (s) Oxidation: Zn(s) Zn +2 + 2e -1 Reduction: Cu +2 + 2e - 1 Cu Overall: Zn (s) + Cu +2 Zn +2 + Cu (s)

9. Shorthand Notation for: Zn° + Cu +2 Z n +2 + Cu ° Example Write shorthand notation for: Fe (s) + 2Fe +3 (aq) 3Fe +2 (aq) Fe ° Fe +2 Fe +3 Fe 0 Write shorthand notation for: 2Ag+1 (aq) + Ni (s) 2Ag (s) + Ni +2 (aq) Ni ° Ni +2 Ag +1 Ag °

10. Second : The Copper – Silver Cell Consider a similar standard Voltaic cell consisting of a strip of Cu immersed in 1M CuSO4 solution and a strip of Ag immersed in 1M AgNO 3 solution. A wire And a salt bridge complete the circuit. The following observation have been made: 1- The initial voltage of the cell is 0.462 volt. 2-The mass of the copper electrode decreases. The Cu ion concentration increases in the solution around the silver electrode. 3- The mass of the silver electrode increases.The Ag+ ion concentration decreases In the solution around the silver electrode. Reaction at the anode : Cu Cu 2+ +2e- (oxidation ) Reaction at the cathode :2Ag + +2e- 2Ag (reduction ) Overall reaction : Cu+ 2Ag + Cu 2+ +2Ag The standard potential of this cell is 0.462 volt The standard cell can be represented as : Cu Cu 2+ (1M) Ag+ (1M) Ag