C401Ch18LN2 - Chapter 18 Electrochemistry Part 2 Cell...

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Chapter 18: Electrochemistry Part 2 Cell Potentials and Free Energy Changes for Cells What drives the rxn in a galvanic cell? Or what forces the electrons to move from the anode to the cathode? The driving force is an electrical potential called the electromotive force or emf. The emf for a cell is also simply called the cell potential, E, E cell , or the cell voltage. The E cell potential is read with a voltmeter which is connected to the cell circuit. The - terminal on the voltmeter must be connected to the cell anode, while the + terminal on the voltmeter must be connected to the cell cathode. In the lab, this is actually how the direction of a spontaneous cell rxn is determined: when the voltmeter gives a + reading, the connections are made properly, and the anode and cathode are identified. If the connections are made improperly, a 0 or negative voltage reading is obtained (as rxn is not spontaneous). Obviously, the unit for the cell voltage is the volt, V. • Unit relationships: 1 V = 1J/C (C is coulombs, the electric charge) 1 C = 1 amp•sec or 1 C = 1 A•sec (A or amp is the current in amperes) 1 watt = 1 W = 1 J/sec (watt is the power) Since the voltage is related to energy as well as electric charge, there must be a relationship between the energy of a cell and its cell potential. Relationship between Free Energy, ± G, and E There is a general relationship between ± G and E: ± G = -nFE where n is the number of moles of e - being transferred in the cell, and F is Faraday’s constant: F = 96,500 C/mol e - Notice that if the cell rxn is spontaneous, the E is + but the ± G is – as we would expect!
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