Lecture+Ch+10+B

Lecture+Ch+10+B - Chem 1C Q The Crystal Ball SPRING 2009...

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Chem 1C “The Crystal Ball” Q
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SPRING 2009 CHEMISTRY ADD/DROP ADDS/DROPS: THE INSTRUCTOR DOES NOT SIGN ANY ADD OR DROP CARDS. During the first two weeks of classes, use http://www.reg.uci.edu/registrar/soc/webreg.html (WebReg) to add, drop, or change your class. The deadline to change grade option (letter grade to P/NP and vice versa) is April 10. The last day to drop this course is May 8. Students will not be permitted to drop this course after this date. These deadlines will be strictly enforced for all students. After May 8 , all enrollment questions will be handled at the Chemistry Undergraduate Affairs Office (1101 Natural Sciences II, phone 824- 7077) during posted hours.
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Ch 10 – Recap Last time we talked about : - Equilibria - Collision theory - Rates of reaction - Equilibrium constant PCl 3 (g) + Cl 2 (g) PCl 5 (g) Rate = k[X] the rate law, where k is the rate constant And how it depends on the balanced reaction equation- “K eq ”= [C] eq c [D] eq d [A] eq a [B] eq b = aA + bB cC + dD Now we’re going to predict the future! [products] p [reactants] r
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Ch 10 – The reaction quotient The reaction quotient, Q (AKA “The Crystal Ball”) The law of mass action and the equilibrium constant define a very specific ratio of products to reactants a system must establish when moving to an equilibrium position. We can use the value of K eq to predict which direction a reaction will move to establish this equilibrium position. Let’s consider the following balanced general reaction: A + B C + D K C = 4 Now, we create a mixture which has the following concentrations: [A] = 3.0 [B] = 2.0 [C] = 3.0 [D] = 1.0 Do we have an equilibrium established? Let’s check: [C] [D] [A] [B] = [3.0] [1.0] [3.0] [2.0] = 0.5 4!!! Obviously this is NOT an equilibrium position; the reaction needs to do something in order to establish equilibrium.
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What if we allow the reaction to complete one “cycle,” or one “mol of reaction” in the forward direction? Concentrations will change to: [A] = 2.0 [B] = 1.0 [C] = 4.0 [D] = 2.0 And is equilibrium established? [C] [D] [A] [B] = [4.0] [2.0] [2.0] [1.0] = 4.0 YES! Based on this, we predict the reaction will make more products in order to establish an equilibrium position. The REACTION QUOTIENT , Q , is thus based on any non-equilibrium conditions which have been used in the expression for K eq . So for the reaction we’ve been discussing: Q = [C] [D] [A] [B] A + B C + D (non-equilibrium [ ]’s used) Ch 10 – The reaction quotient
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Q is called the “crystal ball,” since it can be used to predict a reaction’s future. If Q < K
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This note was uploaded on 11/04/2009 for the course CHEM Chem 1C taught by Professor Farmer during the Spring '09 term at UCL.

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Lecture+Ch+10+B - Chem 1C Q The Crystal Ball SPRING 2009...

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