ln17s08 - Lecture 17 : Getting quantitative and looking at...

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Unformatted text preview: Lecture 17 : Getting quantitative and looking at electrochemical cells in action Everything we want to learn in the way of quantitative information about electrochemistry can be found in the familiar diagram below: ! The e- e- e- e- is the amount of charge, q, and can be related to mass through a simple stoichiometry problem ! The potential hill, E, is the value on your batteryit depends on the half cell potentials and the concentrations ! Thermodynamic values like w and DG are realized from q times E. " " " " " e e e e e These three general categories of calculations are examined in order on the following pages: E(V) (this is the voltage (V) difference measured by a voltmeter) This is the number of charges, as per unit time, the current. 1. For electrochemistry to do much work, the size of the hill (V) and the amount of charge (q) both need to be large. 2. Oh, this must mean work= w= qV Everything you wanted to know about measuring charge: basic stuff they should have told you in your high school physics class Electrical Charge. The fundamental unit of charge, q, is measured in Coulombs. One electron has a charge of 1.602 x 10-19 Coulombs (C). Therefore, one mole of electrons has a special amount of charge F= Faraday (Coulombs/mole) = q (Coulombs) / n (moles) F = 9.649 x 10 4 C/mole So q = nF So electrons are our currency for electrochemical measurements in the same way that protons were our currency for acid/base reactions. A Faraday is simply an amount of charge in the same way a dozen is an amount of eggs and we can do routine stoichiometry calculations with this charge to mole conversion factor. Example: In a redox reaction involving the reduction of Fe +++ to Fe ++ , a total of 9.649 x 10 3 C are used up. How much Fe +++ was reduced? 1mole Fe +3 1mole Fe +2 55.85g (9.649 x 10 3 C)( ####### ) ( #### ) ( #### ) = 5.585g Fe +3 9.65 x 10 4 C/mol 1mole Fe +3 1mole Fe +2 Note that for this reaction, a single electron is transferred from the Fe +2 to the Fe +3 . This is analogous to a single H + transfer in a monoprotic acid. Adding time to the equation and get CURRENT. The measure of current as a function of time is the ampere: One Ampere = one Coulomb/second Having current measured as a function of time means we can talk about a rate of reaction (kinetics stuff, something we havent considered since we started equilibrium.) Example: What is the current measured in a circuit involving the half cell reaction Sn +4 + 2e- $ Sn +2 which is occurring at a rate of 4.2 x 10-3 mole/hr? 1hr 2 mole e- 9.65 x 10 4 C i = (4.24 x 10-3 mole/hr) ( ### )( ##### )( ##### ) = 0.227A 3600s 1 mole Sn +4 mole e- By the way, what do you think about this magnitude of current? Will it kill you? Will it power a small city or a small toy? Id say, thats a pretty nasty shockright at the edge of where a DC voltage can kill you. Standard Potentials....
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This note was uploaded on 04/09/2008 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas at Austin.

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ln17s08 - Lecture 17 : Getting quantitative and looking at...

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