#Chem 162-2008 Recitation Quiz II

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

This preview shows pages 1–3. Sign up to view the full content.

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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
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.
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page1 / 8

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

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online