Lab manual_6 - UOIT Chem. 1800U, W10; Exp. 6-7.9 6....

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UOIT Chem. 1800U, W10; Exp. 6-7.9 114 6. ELECTROCHEMISTRY AND VOLTAIC CELLS Objectives 1) To measure the voltages generated by several common galvanic cells; 2) to observe the effect of changing concentration on cell potential; 3) to estimate the formation constant (K f ) for Cu(NH 3 ) 4 2+ . Introduction Electrochemistry is an incredibly important area of chemistry. In its broadest definition electrochemistry is the study of the transfer of electrical charge (or energy) between chemical species or the generation of electrical energy (electrical work) from chemical systems. Originally electrochemistry was considered a part of physical chemistry, but now it plays important roles in almost all areas of chemistry: analytical, synthetic, energy science, and materials science. In this experiment the generation of electricity from chemical reactions will be studied. Devices that generate electricity from chemical reactions are usually referred to as voltaic (or sometimes galvanic) cells. Batteries are one common example of this phenomenon in everyday life. Reduction - Oxidation Reactions If a small amount of lead metal (Pb) is added to a solution of copper sulphate (which is blue) a reaction is observed. The colour of the solution will fade, copper metal will precipitate from the solution and lead will dissolve. The chemical equation for this reaction is: Pb(s) + Cu 2+ (aq) ± Pb 2+ (aq) + Cu(s) (6.1) In a formal sense this reaction can be broken into two “half-reactions”:
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UOIT Chem. 1800U, W10; Exp. 6-7.9 115 Pb(s) ± Pb 2+ + 2e - (6.2) Cu 2+ + 2e - ± Cu (6.3) The first of these involves the loss of electrons and is called “oxidation” (the oxidation half- reaction). The second is the gain of electrons and is called “reduction” (the reduction half-reaction). In the overall reaction Pb is called the “reducing agent” (it causes Cu 2+ to be reduced) and Cu 2+ is the “oxidizing agent” (it causes Pb to be oxidized). The overall reaction is called a “redox” (an abbreviation of reduction-oxidation) reaction. Voltaic Cells If the electrons that are transferred from the reducing agent to the oxidizing agent can be forced to flow through an external circuit, the electrons can be used to do electrical work. This can be accomplished by physically separating the two half-reactions. The oxidation half-reaction takes place at the “anode” and the electrons flow through a wire to the “cathode” where the reduction reaction takes place. To maintain overall electrical neutrality, anions must flow to the anode and cations must flow to the cathode. This is often done using a salt bridge or, as in this experiment, a porous cup. Without the flow of anions and cations the flow of electrons stops. The combination of the anode half-cell, cathode half-cell and porous cup is called a voltaic cell and the overall chemical reaction is called the cell reaction.
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This note was uploaded on 11/11/2010 for the course CHEMISTRY CHEM1800 taught by Professor Krista during the Winter '10 term at UOIT.

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Lab manual_6 - UOIT Chem. 1800U, W10; Exp. 6-7.9 6....

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