#Chem 162-2008 Recitation Quiz II

#Chem 162-2008 Recitation Quiz II - Chem 162-2008, Quiz 2,...

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Chem 162-2008, Quiz 2, Version A1 1. (10 pts.) Given the reaction Pb(s) + 2Cr 3+ (aq) Pb 2+ (aq) + 2Cr 2+ (aq) K c = 3.2 x 10 -10 What are the equilibrium concentrations of all species when lead is added to 0.100M Cr 3+ (aq)? Pb(s) + 2Cr 3+ (aq) Pb 2+ (aq) + 2Cr 2+ (aq) Initial Y 0.100M 0 0 Change -X -2X +X +2X Equilibrium Y-X 0.100 – 2X +X +2X ([Pb 2+ ][Cr 2+ ] 2 )/[Cr 3+ ] 2 = 3.2 x 10 -10 ([X][2X] 2 )/[0.100-2X] 2 = 3.2 x 10 -10 Simplify to avoid quadratic equation. (Use small K rule.) ([X][2X] 2 )/[0.100] 2 = 3.2 x 10 -10 X = 9.28 x 10 -5 M [Pb 2+ ] = 9.28 x 10 -5 M [Cr 2+ ] = 1.86 x 10 -4 M [Cr 3+ ] = 0.100 – 9.28 x 10 -5 = 0.0999M = 0.100M 2.(6pts) In the same reaction, what effect will the following have on the equilibrium position? A. Adding Pb B. Adding Cr 3+ C. Adding Pb 2+ (A) According to LeChatelier, adding Pb(s) won’t have any effect because Pb is a solid and therefore not part of the equilibrium expression. (B) Adding Cr 3+ will drive the reaction to the right, according to LeChatelier. (C) Adding Pb 2+ will drive the reaction to the left, according to LeChatelier. 3.(4pts.) Explain briefly what a dynamic equilibrium is. ET Note: To explain what “dynamic equilibrium” is, the student not only needs to explain what “dynamic equilibrium” is, but also demonstrate that he/she knows what “equilibrium” is. Equilibrium is a state in which the concentrations of all reactants and products remain constant with time. Static equilibrium is when we look at the reactants and products from a distance (a macroscopic point of view) and see no change. Dynamic equilibrium is when we look at the reactants and products from close up, and see a constant interchange of reactants and products, in which the forward rate equals the reverse rate.
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Quiz 2, Version A2 1.(10pts) Given the reaction Pb(s) + 2Cr 3+ (aq) Pb 2+ (aq) + 2Cr 2+ (aq) K c = 3.2 x 10 -10 What are the equilibrium concentrations of all species when lead is added to 0.200M Cr 3+ (aq)? Pb(s) + 2Cr 3+ (aq) Pb 2+ (aq) + 2Cr 2+ (aq) Initial Y 0.200M 0 0 Change -X -2X +X +2X Equilibrium Y-X 0.200 – 2X +X +2X ([Pb 2+ ][Cr 2+ ] 2 )/[Cr 3+ ] 2 = 3.2 x 10 -10 ([X][2X] 2 )/[0.200-2X] 2 = 3.2 x 10 -10 Simplify to avoid quadratic equation. (Use small K rule.) ([X][2X] 2 )/[0.200] 2 = 3.2 x 10 -10 X = 1.47 x 10 -4 M [Pb 2+ ] = 1.47 x 10 -4 M [Cr 2+ ] = 2.94 x 10 -4 M [Cr 3+ ] = 0.200 – 2.94 x 10 -4 = 0.200M 2.(6pts.) In the same reaction, what effect will the following have on the equilibrium position? A.
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#Chem 162-2008 Recitation Quiz II - Chem 162-2008, Quiz 2,...

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