ln16s08 - Lecture 16 Electrochemistry The Big Picture...

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Lecture 16: Electrochemistry -- The Big Picture Electrochemistry follows the adventures of the electron e - Recall that we first discussed the electron when it came up as a fundamental particle back when discussing quantum mechanics. Also recall that when learning about configurations for atoms and molecules that we obsessed over it. But as we went on to discuss thermodynamics and properties of the bulk, we put it away. It is back now, demanding its own chapter and perhaps its own consideration in thermodynamic terms. After all, we spent six weeks on the fundamental particle called the proton and it has nothing on the electron from a reactivity perspective. How do we know this? Think about it from a charge density perspective as we compare some particles of different charge and size Note that while the mass of an electron is quite small—2000 times smaller than a proton and 50,000 times smaller than a sodium ion: It has the same amount of charge which means that its charge density is vastly larger than any other particle. Seeing as how we have on many occasions we have used charge density to rank reactivity, having stumbled across a particle with 2000 times the charge density of the next guy in line, we should be willing to examine it more carefully. Type of particle e - H + H Na + mass per particle 8.3 x 10 -27 g 1.6 x 10 -24 g 1.6 x10 -24 g 3.7 x 10 -23 g mass per mole 1/2000 g/mole 1 g/mole 1 g/mole 23 g/mole charge per particle 1.614x10 -19 C 1.614 x 10 -19 C 0 1.614 x 10 -19 C charge per mole 9.6 x 10 4 C/mole = 1 F 9.6 x 10 4 C/mole = 1 F 0 9.6 x 10 4 C/mole = 1 F electron
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Time out for charge: Everyone understands mass—you step on scales every day to see how much you weigh and are used to buying goods “by the pound or gram.” And that is why we can talk about the mass of a mole of an atom and you just look at the periodic table and rattle off “uranium weighs 238 grams/mole.” But charge is not so familiar. You are used to the notion of charge in the form of static electricity or getting a shock, but have no real quantitative feel for it. Until now. Very simply: The unit for charge is the Coulomb in the same way the unit of mass is grams. and In the same way you can assign a mass to a mole of a compound—like water is 18 grams/mole, you can assign a mole of charge to something. And the best part is that a mole of charge always has exactly the same value: 1 mole of charge = 1 Faraday = 9.6 x10 4 C/mole And this value is the same whether you have a mole of e - or a mole of H + or a mole of Na +
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The chemistry of the electron—oxidation/reduction reactions: We have just finished a unit on equilibria in which we looked at reactions in which we followed the movement of a proton and reactions in which we followed the movement of cations and anions: Acid base reactions solubility reactions H + proton Na + or Cl - cations or anions NH 3 + HNO 3 NH 4 + + NO 3 - -
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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.

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ln16s08 - Lecture 16 Electrochemistry The Big Picture...

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