General Chemistry II
Exam 3 – Chapter 14 Practice
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After practicing and studying all relevant material, test yourself using this practice exam. Place yourself in a simulated exam environment, preferably under a time constraint. Attempt to complete this practice as you would an exam, using no materials for assistance. Answers will be posted after all have had a chance to complete this trial exam. Check your answers against thy key EVALUATE: If you were able to complete problems on the trial exam, ask yourself o Had you seen the material before? If you have not, go review the relevant sections. o If you had seen the material, were you able to complete the homework and example problems? If not, review and practice until you are able to complete the problems in this situation. o If you had seen the material and were able to complete the homework and example problems, then you should do the following:  Attempt to redo the homework and example problems without assistance, in an exam environment. Continue until you can accomplish this.  Select other problems in the text that are similar and attempt to do them in an exam environment.  Continue this process until you can see a similar problem and solve it without assistance. 1.) Consider the reaction: CCl4(g) + 3 O2(g)  4 ClO(g) + CO2(g) a. Write the equilibrium constant expression for the reaction. b. Using the Thermodynamic Table provided, calculate the Gibbs Free Energy change for this reaction at 298.15K. (Go(ClO(g)) = 97.11 kJ/mol) c. Calculate the equilibrium constant for this reaction. d. Initially, 1 bar of CCl4 and 3 bar of O2 are added to a vessel. What is the total pressure in the system if the reaction: i. Goes exactly half way. ii. Goes to completion. e. If the reaction vessel is compressed to a lesser volume, in which direction will this reaction shift after it has reached equilibrium? 2.) Consider the following reaction, initial conditions and equilibrium constant: Kp = 0.00608 at 225oC CO(g) + 2 H2(g)  CH3OH(g) Initial 3.0 bar 4.0 bar 0.30 bar a. In which direction will this reaction shift? b. Show that when an ICE diagram is prepared, the expression to be solved, given in terms of the common variable, x, is: 0.30
3.0
4.0 2
c. In this expression, give the minimum and maximum possible values for x. d. Using the Recursion Method, or otherwise called the Successive Approximations method, solve the equilibrium expression for x. (Note: this would be given in advance as an exam problem) e. Calculate the equilibrium pressures of all species and the total pressure in the system. 3.) Consider the following reaction carried out in a 2 L pressure vessel at 500 K. Kp = 0.345 COCl2(g)  CO(g) + Cl2(g) a. Calculate Kc for this reaction. b. If 2 moles of CO(g) and 1 mol of Cl2(g) are placed in the vessel and heated to 500K to initiate the reaction, fill in the ICE diagram provided. COCl2(g)  CO(g) + Cl2(g) Initial (Molarity) Change Equilibrium c. Write the equilibrium constant expression for this reaction. d. Using your results in the table and in c., produce an algebraic expression in the form of: Ax2 + Bx + C = 0 e. Using the quadratic equation given, calculate the correct value of x. f.
Calculate the equilibrium pressures of all species. √
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General Chemistry II
Exam 3 – Chapter 14 Practice
Solutions
1.) CCl4(g) + 3 O2(g)  4 ClO(g) + CO2(g) a.
b.
c.
d.
e.
62.04 kJ/mol 1.35 x 10‐11 4.5 bar and 5 bar Reaction will shift to the left. a.
b.
c.
d.
e.
Reaction will shift to the left. 0  x  0.30 5.87 x 10‐3 PCO = 3.00587 bar PH2 = 4.012 bar PCH2OH = 0.294 bar 2.) 3.) a. Kc = Kp(RT)‐1 = 0.00841 b. COCl2(g)  CO(g) + Cl2(g) Initial 0 M 1 M 0.5 M Change +x ‐x ‐x Equilibrium x 1‐x 0.5‐x c. x2 – 1.50841x + 0.5 = 0 d. x = 0.4919 e. PCOCl2 = 0.4919xRT = 20.180 atm, PCO =0.5081 xRT = 20.845 atm, PCl2 = 0.3323 atm (Note: 0.343 = Kp ) General Chemistry II
Exam 3 – Chapter 16 Practice
1.) CaF2 has a Ksp of 3.45 x 10‐11 a. Write the equilibrium equation for this substance in water. b. Calculate the solubility of this substance in water. c. Calculate the solubility of this substance in 0.0050 M F‐1 solution. State any assumptions. d. BONUS: Repeat part c for a solution of 0.00050 M F‐1. Test any assumptions by using a recursion method. 2.) The Ksp values for a series of silver salts are given as follows: Ag2SO4 Ksp = 1.2 x 10‐5. AgCl Ksp = 1.8 x 10‐10. Ksp = 8.5 x 10‐12 Ag2CO3 a. Place these compounds in order of increasing solubility. b. If a solution of Ag+ was added, dropwise to 0.015 M solutions of SO4‐2, Cl‐, and CO3‐2, which solution would begin to form a precipitate first? 3.) Mg(OH)2 has a Ksp of 5.6 x 10‐12. a. Calculate the molar concentration of the [Mg+2] and the [OH‐]. b. If 100.00 mL of a saturated solution of Mg(OH)2 was titrated with 0.00150 M HCl, a reaction will occur between the acid, HCl and the hydroxide, OH‐. How many ml of HCl solution will be required to neutralize all of the OH‐ produced by the Mg(OH)2? General Chemistry II
Exam 3 – Chapter 16 Practice
Solutions
1.) a.
b.
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d.
CaF2(s)  Ca+2(aq) + 2 F‐(aq) 2.05 x 10‐4 M 1.38 x 10‐6 M assuming x << 0.005 7.95 x 10‐6 M. Note that the previous assumption is invalid. Recursion requires about 10‐
12 steps for convergence. 2.) a. AgCl < Ag2CO3 < Ag2SO4. b. AgCl 3.) a. [Mg+2] = 1.1 x 10‐4, [OH‐] = 2.2 x 10‐4 b. 14.9 ml