CH145L_09_08

CH145L_09_08 - CH 145- Experiment 8- Fall 2009 Experiment 8...

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CH 145- Experiment 8- Fall 2009 1 Experiment 8 Electrochemistry 1 Pre-laboratory reading assignment: Textbook chapter 4:4 and chapter 20 This laboratory handout There will be a pre-lab quiz! Background on Electrochemical Theory An electrochemical (or galvanic ) cell, also known as a battery, is a device that produces an electric current as the result of an electron transfer reaction. Such electron transfer reactions are also known as oxidation-reduction, or redox, reactions. Electron transfer occurs as one substance is oxidized, or loses electrons, while another substance is reduced, or gains electrons. For example, if a piece of zinc metal were immersed in a solution containing copper(II) ions, the zinc would spontaneously lose electrons while the Cu(II) would spontaneously gain electrons. This process can be expressed as two half-reactions that sum to yield the overall reaction: Zn (s) --> Zn +2 (aq) + 2 e - Cu +2 (aq) + 2 e - --> Cu (s) NET: Zn (s) + Cu +2 (aq) --> Cu (s) + Zn +2 (aq) Any spontaneous redox reaction can be harnessed to produce electrical energy under the right conditions. The problem with simply dropping a piece of zinc metal into a solution of Cu(II) is that the electrons provided by the zinc move directly to the aqueous Cu(II) ions without doing any work. In order to create a useful battery, the two half reactions must be physically separated so that the electrons will flow through an external circuit as shown in Figure 1. A salt bridge is necessary for charge balance: in this case sulfate ions flow from the copper to the zinc compartment. The electrochemical cell shown in Figure 1 can be represented by the following shorthand: Zn (s) Zn +2 (aq)  Cu +2 (aq) Cu (s) In this type of “line notation”, the components at the site of oxidation (the anode) are listed on the left; at the site of reduction (the cathode), on the right; and the salt bridge is represented by central double vertical lines. A phase difference is indicated by a single vertical line. 1 Adapted from Chemistry The Central Science, Laboratory Experiments , 6 th Edition, by J.H. Nelson and K.C. Kemp and Laboratory Inquiry in Chemistry by R.C. Bauer, J.P. Birk, and D.J. Sawyer.
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CH 145- Experiment 8- Fall 2009 2 Figure 1. Functional galvanic cell based on oxidation of Zn and reduction of Cu +2 . Electrons that are generated at the anode of an electrochemical cell are driven toward the cathode by a thermodynamic tendency called the electromotive force (emf), measured in volts. The emf is also called the
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This note was uploaded on 02/09/2010 for the course CH CH242 taught by Professor Katz during the Spring '10 term at Colby.

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CH145L_09_08 - CH 145- Experiment 8- Fall 2009 Experiment 8...

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