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DP Study Guide for Chapter 14

DP Study Guide for Chapter 14 - DP's Study Guide for...

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DP’s Study Guide for Chapter 14 General Chemistry, 4th Ed.”, Hill, Petrucci, McCreary, and Perry (ISBN 0-13- 140313-3) 14.1 Dynamic Equilibrium: Another lie from CHEM 1 was that reactants proceed to products and that’s it. There was the idea of a limiting reagent and when it is all gone the reaction quits, that is almost true but not exactly. Reactants do come together to form products, at rates we discussed in Chapter 13, but near the end stages of the process some of the products reform into the reactants. After some time the amounts of products and reactants appear to be constant. What is really going on is a dynamic equilibrium: where the products are being formed at the same rate the products are coming together to remake the reactants. 14.2 The question is, how much of the reactants are left and how much of the products are left after equilibrium is achieved? Calculating each of these amounts is not straight forward, (see section 14.5) but the equilibrium constant expression is used for this purpose. The equation is: c a ts ac oducts K ] tan [Re ] [Pr = or the equilibrium concentration of products raised to their stoichiometric coefficients divided by the equilibrium reactant concentrations raised to their stoichiometric coefficients. You have now been exposed to two of the main themes in chemistry: kinetics and equilibrium. Rate constants are k, “little k’s” and are coefficients out in front of the concentrations in the rate equations. Equilibrium constants, K, “big K’s” are used at a certain time during the life of a reaction. This is when the forward rate of the reaction equals the reverse rate of the reaction. See below where DP redoes Chapter 13.9. Test your self to see if you have the right idea by working self assessment questions 2,4, and 5. I would now like to return to Chapter 13.9 and work some of the kinetic problems involving equilibrium. There is a mechanistic case with a “Fast Reversible Step” followed by a slow step. Now that you know what equilibrium is, Fast Reversible step can be fully explained. When a reaction is in equilibrium the rates of the forward and reverse reaction need not be considered because they are the same. In other words chemical equilibrium says noting about how long it took to reach the equilibrium, it only describes what is going on right now, which “appears” to be nothing. The most important thing you get from section 13.9 is that at equilibrium, Rate forward = Rate backward.
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