2. What is the equilibrium constant for the overall reaction?

Transcription

1 Ch 15 and 16 Practice Problems - KEY The following problems are intended to provide you with additional practice in preparing for the exam. Questions come from the textbook, previous quizzes, previous exams, and other sources. A solutions manual is supplied in a separate document. 1. For the following reaction 2SO 3(g) 2SO 2(g) + O 2(g), K c = 1.3 x 10-5 at 500 K. If [SO 3 ] = [SO 2 ] = [O 2 ] = M at 500 K, which one of the following statements is true? A. The system is at equilibrium, thus no concentration changes will occur. B. The concentrations of SO 3 and O 2 will increase as the system approaches equilibrium. C. The concentration of SO 3 will increase as the system approaches equilibrium. D. The concentrations of SO 2 and SO 3 will fall as the system moves toward equilibrium. E. The concentrations of SO 2 and O 2 will increase as the system approaches equilibrium. Answer: C 2. What is the equilibrium constant for the overall reaction? overall reaction SO 3(g) + NO 2(g) SO 2(g) + NO 3(g) K c =? elementary reaction 2 SO 2(g) + O 2(g) 2 SO 3(g) K c1 = 1.89 x elementary reaction NO 2(g) + ½O 2(g) NO 3(g) K c2 = 7.14 x Consider two solutions of formic acid HCHO 2 (pk a = 3.74) which were prepared with concentrations of 0.19 M and M HCHO 2. a. Which has a lower ph? b. Which has a higher percentage ionization?

2 4. For the following solutions, determine the [H 3 O + ], [OH - ], and the ph. a. A 0.35 M HNO 2 solution (pk a = 3.14)

3 b. A 0.35 M HNO 3 solution c. A M HNO 2 solution d. A 0.16 M KOH solution

4 e. A 0.16 M KCN solution (pk b = 4.79) f. A 0.16 M H 2 SO 3 solution (pk a1 = 1.89, pk a2 = 7.21)

5 5. Cobalt phosphate (Co 3 (PO 4 ) 2, g, g/mol) and chromium(iii) chloride (CrCl 3, 3.46 g, g/mol) were added to enough water to make a 100 ml solution. At equilibrium, [Co 1+ ] eq = M. 5a. Calculate the equilibrium constant. 5b. Later, chromium(iv) oxide (CrO 2, g, g/mol) was added to the solution. Determine the concentration of Co 1+ after equilibrium was re-established. (Assume no change in solution volume with the addition of the chromium(iv) oxide.)

6 6. In the reaction below, identify the conjugate base of HSO 4 is. SO 4 2 HSO 4 (aq) + OH (aq) SO 4 2 (aq) + H 2 O (l) 7. Place the follow in order of increasing acidity (HC 2 H 3 O 2, NH 4 Cl, HClO 4, HCHO 2 ) Increasingly strong acid NH 4 Cl < HC 2 H 3 O 2 < HCHO 2 < HClO 4

7 8. The reaction below is at equilibrium, how will the equilibrium will shift to the left, the right, or remain unchanged when the system is modified. Place an X in the appropriate box. Use same starting equilibrium conditions for each individual change. SO 2 (g) + NO 3 (g) SO 3 (g) + NO 2 (g) Hº rxn = -15 kj/mol Equilibrium will shift to the Change to the system Left Unchanged Right SO 2 is added. X The volume is increased. X NO 2 is added. X The temperature is decreased. X 9. Morphine (C 17 H 19 NO 3, molar mass = g/mol) is used to manage pain and is often administered intravenously where the concentration is 1 mg morphine per ml of solution. In the General Chemistry laboratory you prepare an analogous solution from morphine and de-ionized water only. Calculate the ph of the solution you have prepared.

8

9 10. Calculate the percent ionization for 100 ml of a 0.1 M benzoic acid solution with ph = (Benzoic acid, HC 7 H 5 O 2 is a white solid with MW = g/mol) HC 7 H 5 O 2(aq) + H 2 O (l) C 7 H 5 O 2 - (aq) + H 3 O + (aq) pk a = Which of the following graphs shows the correct relationship between the molarity of a weak acid and its percent ionization? A B C D Answer: C 12. For the following chemical equations, balance the equations and then write the equilibrium expression (Kc) and the reaction quotient (Q). a. NO (g) + O 2(g) N 2 O 3(g) b. SF 6(g) + SO 3(g) SO 2 F 2(g) c. C 2 H 6(g) + O 2(g) CO 2(g) + H 2 O (g)

10 13. In a 5.0 L flask, 0.25 atm HBr (g), 0.15 atm H 2(g), and atm Br 2(g) were combined. The temperature was raised to 425 C and allowed to come to equililbrium. What are the equilibrium partial pressures of HBr, H 2, and Br 2? 2HBr (g) H 2(g) + Br 2(g) K p = 4.18 x 10-9 at 425 C 14. The formation of NO (g) from its elements is not favored K c = 4.10 x 10-3 at 2000 C. What will be the equilibrium concentrations of N 2 and O 2 after mol of NO are added to a 1.5L flask and the temperature raised to 2000 C? What are the masses in grams of N 2 and NO in the flask at equilibrium? N 2(g) + O 2(g) 2NO (g)

11 15. During the combustion of H 2, hydroxyl radical ( OH) is formed by the reaction, H (g) + ½O 2(g) OH (g) K c =? Use the following reactions to determine K c for the above reaction. ½H 2(g) + ½O 2(g) OH (g) K c = 0.58 H 2(g) 2H (g) K c = 1.6 x Write K a expressions for the following: a. HCN b. HCO 3 - c. HBrO 2 d. H 3 PO 3

12 17. Choose which solution will have a lower ph when 0.15 moles of following solutes are is dissolved in enough water to make a 250 ml solution. a. H 3 PO 4 or H 3 AsO 4 b. HBrO or HBrO 2 c. Co(NO 3 ) 2 or Co(NO 3 ) 3 d. FeCl 3 or AlCl Identify the Lewis acid and Lewis Base among the reactants in each of the following reactions. a. Ag + + 2NH 3 [Ag(NH 3 ) 2 ] + b. AlCl 3 + Cl- [AlCl 4 ] - c. CN - + H 2 O HCN + OH - d. Ni CO [Ni(CO) 4 ] 2+ e. H 2 SO 4 + NH 3 HSO NH 4 +

13 19. Strong acids like HCl fully dissociate into ions in aqueous solution. Can a weak acid like acetic acid (pk a = 4.74) dissociate to greater than 90 % in aqueous solution? A. Yes at any concentration. B. Yes only at low concentration (<1 x 10-2 M). C. Yes only at high concentration (>1 M). D. No, weak acids do not fully dissociate at any concentration. Answer: B 20. Like water, other compounds undergo autoionization. For methanol (CH 3 OH), the autoionization constant (K met ) is 2.0 x10-17 at 25 C. Calculate the [CH 3 O - ] and [CH 3 OH 2 + ] in pure methanol at 25 C.

14 21. Dinitrogen tetraoxide (N 2 O 4, g, g/mol) was added to a 10 L flask and the system was allowed to come to equilibrium according to the reaction below at 430 C. At equilibrium [NO 2 ] eq = M. 21a. Calculate the equilibrium constant. 21b. The gas was compressed to ½ its original volume with no temperature change. Find the [N 2 O 4 ] eq after compression.

15 22. For each of the following identify the acid, base, conjugate acid, and conjugate base. a. HC 7 H 4 NO 4(aq) + H 2 O (l) C 7 H 4 NO 4 - (aq) + H 3 O + (aq) b. NH 4 Cl (aq) + H 2 O (l) NH 3(aq) + H 3 O + (aq) + Cl - (aq) c. KCN (aq) + HCHO 2(aq) HCN (aq) + K + (aq) + CHO 2 - (aq)

16 23. Acid strength decreases in the series HI > HSO 4 > HF > HCN. Which of the following anions is the weakest base? A. I B. 2 SO 4 C. F D. CN E. It cannot be determined from the available information. Answer: A