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# ln6s08 - Lecture 6 An introduction to RICE calculations An...

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Lecture 6: An introduction to RICE calculations An introduction to how to do equilibrium calculations using the RICE set up will be introduced. This method will be used for the next two months in every conceivable chemical system to calculate system concentrations and K values. Sadly, you will get so used to this kind of calculation, and the approximations that simply its application, that you will not remember the young lasses and lads you were when I first introduced this material. But trust me, no matter how sophisticated you get in working equilibrium calculations, it all is based on the concepts in this lecture. But first, time out for a big picture reminder that equilibria is just thermodynamics. -nFE° = Δ G° = -RTlnK Ch 12 Ch7 Ch 9-11 In comparing these formulas, think of a reaction as falling down a free energy hill, a gradient of energy release or concentration change that drives toward an equilibrium at the bottom of the hill. A harpoonrightleftB In the picture, note that whether you start with all A or all B, or somewhere in between, you can find a Q value. When Q is less than K, the reaction shifts from left to right. When Q is greater than K, the reaction shifts from right to left Q. When Q reaches K, you are at equilibrium. AAA AAA BB AA BBB BBB A B Q = [ B ] [ A ] Q changes from K 0 Equilibrium Q=K Q = [ B ] [ A ] Q changes from ∞ → K Δ G

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Another way to present this concept is by graphing the relative amounts of the reactants, A and B in this case, to show: on the left side, the values when A is in excess and Q<K in the middle, the time during which Q=K and there is no change in the rate of A or B forming on the right side, the values when B is in excess and Q>K You will learn that finding whether Q is greater or less than K is a useful trick to understand what a reaction is doing.
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