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

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

3. Oxidation takes place when a species loses electrons to another species Cu(s)  Cu2+(aq) + e– Reduction takes place when a species gains electrons from another species Zn2+(aq) + e– Zn(s) EOS Half-Reactions In any oxidation–reduction reaction, there are two half-reactions CHM 112 Summer 2007 M. Prushan

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

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

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

7. 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 CHM 112 Summer 2007 M. Prushan

8. 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 (Ecell) is the potential difference that propels electrons from the anode to the cathode CHM 112 Summer 2007 M. Prushan

9. 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 CHM 112 Summer 2007 M. Prushan

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

11. Criteria for SpontaneousChange If Ecell is positive, the reaction in the forwarddirection (from left to right) is spontaneous If Ecell is negative, the reaction is nonspontaneous If Ecell= 0, the system is at equilibrium When a cell reaction is reversed, Ecell and DGchange signs CHM 112 Summer 2007 M. Prushan

12. Equilibrium Constantsfor Redox Reactions DGo = –RTlnKeq = –n × F × Eocell Eocell = 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 CHM 112 Summer 2007 M. Prushan

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