Chapter 18 notes

Chapter 18 notes - Chapter 18 Objectives(Electrochemistry...

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Unformatted text preview: Chapter 18 Objectives (Electrochemistry) Spring 2084 1. Introduction. a) For reactions in which ΔG° is negative, spontaneity means the reaction can perform work. b) In this unit, we describe how this work can be derived electrically and how electrical energy can be used to drive non-spontaneous reactions in a desired direction. c) It is useful to note that in addition to the movement of electrons in a wire, electrical conductivity also involves movement of ions in solution. d) In this unit, we deal specifically with oxidation-reduction reactions and we will be working with oxidation half-reactions and reduction half-reactions. We assign numerical voltage values to these half-reactions based on 2 + ) aq ( H + 2 e¯ → H 2(g) having 0.00 voltage at 1 M, 1 atm, and 25° C. 2. Voltaic Cells. a) These produce electrical work from spontaneous redox reactions. b) Consider the spontaneous process Zn (s) + + 2 ) aq ( Cu → + 2 ) aq ( Zn + Cu (s) (a single replacement reaction). c) Oxidation: Zn (s) → + 2 ) aq ( Zn + 2 e¯ Reduction: + 2 ) aq ( Cu + 2 e¯ → Cu (s) Overall: Zn (s) + + 2 ) aq ( Cu → + 2 ) aq ( Zn + Cu (s) R G ∆ = + + ∆- ∆ 2 Cu 2 Zn F F G G = -147.1 – 65.5 = -212.6 kJ d) If Zn (s) is kept from direct contact with the + 2 ) aq ( Cu , electrons can be made to flow through a wire if there is a complete electrical circuit, established by use of a salt bridge. e) Oxidation always occurs at the anode; reduction occurs at the cathode. f) If both + 2 ) aq ( Zn and + 2 ) aq ( Cu are initially 1 M, the voltmeter reads 1.10 volts. g) As the cell functions, [Zn 2+ ] increases and [Cu 2+ ] decreases and the voltage gradually diminishes to zero volts and the system is at equilibrium. Chapter 18 Objectives (Electrochemistry) Spring 2084 h) Because absolute voltages of the half-cells cannot be assigned, a table of standard reduction potentials based on 2 + ) aq ( H + 2 e¯ → H 2(g) having ε° = 0.00 V has been created. Note that standard conditions are 1 M ionic concentration and 1 atm of pressure for all gases. atm of pressure for all gases....
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This note was uploaded on 04/07/2008 for the course CHEM 0970 taught by Professor Kogut during the Spring '08 term at Pittsburgh.

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Chapter 18 notes - Chapter 18 Objectives(Electrochemistry...

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