1. CHM 112 Summer 2007 M. Prushan Chapter 18 Electrochemistry

2. CHM 112 Summer 2007 M. Prushan Oxidation–Reduction Redox involves the transfer of electrons

3. CHM 112 Summer 2007 M. Prushan Half-Reactions In any oxidation–reduction reaction, there are two half-reactions Oxidation takes place when a species loses electrons to another species Cu(s)  Cu 2+ (aq) + e – EOS Reduction takes place when a species gains electrons from another species Zn 2+ (aq) + e –  Zn(s)

4. CHM 112 Summer 2007 M. Prushan Voltaic Cells A half-cell consists of an electrode immersed in a solution of ions

5. CHM 112 Summer 2007 M. Prushan An Illustration …

6. CHM 112 Summer 2007 M. Prushan The solutions in the two half-cells are joined by a salt bridge Voltaic Cells

7. CHM 112 Summer 2007 M. Prushan Metal wires connect the electrodes to the terminals of an electric meter called a voltmeter Voltaic Cells The salt bridge keeps the two half-cells in contact with one another so that there can be a flow of electrons

8. CHM 112 Summer 2007 M. Prushan Important Electrochemical Terms An electrochemical cell is a device that combines two half-cells with the appropriate connections between electrodes and solutions A voltaic cell is an electrochemical cell in which electric current is generated from a spontaneous redox reaction The anode is the electrode at which oxidation occurs and the cathode is the electrode at which reduction occurs The cell potential (E cell ) is the potential difference that propels electrons from the anode to the cathode

9. CHM 112 Summer 2007 M. Prushan Cell Diagrams – Conventions Place the anode on the left side of the diagram Place the cathode on the right side of the diagram Use a single vertical line ( | ) to represent the boundary between different phases, such as between an electrode and a solution Use a double vertical line ( || ) to represent a salt bridge or porous barrier separating two half-cells

10. CHM 112 Summer 2007 M. Prushan An Example Cell Diagram

11. CHM 112 Summer 2007 M. Prushan Criteria for Spontaneous Change If E cell is positive, the reaction in the forward direction (from left to right) is spontaneous If E cell is negative, the reaction is nonspontaneous If E cell = 0, the system is at equilibrium When a cell reaction is reversed, E cell and  G change signs

12. CHM 112 Summer 2007 M. Prushan  G o = –RTlnK eq = –n × F × E o cell E o cell = standard cell potential R is the gas constant (8.3145 J mol –1 K –1 ) T is the Kelvin temperature n is the number of moles of electrons involved in the reaction F is the faraday constant Equilibrium Constants for Redox Reactions

13. CHM 112 Summer 2007 M. Prushan General Values and Meanings Reaction behavior can be predicted using the following information …