2. When determining the ΔH rxn from ΔH f o, which of the following is not necessary in the calculation.

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Barrie Summers
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1 Ch 6 and 7 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. Solutions are supplied in a separate document. 1. A 327 g piece of Cu (C sp = J/g C) at 87 C was placed into an insulated container with a 109 g piece of Fe (C sp = J/g C) at 62 C. The insulated container started out at 22 C and has a heat capacity of 32.1 J/ C. What is the temperature when all pieces have reached thermal equilibrium? 2. When determining the ΔH rxn from ΔH f o, which of the following is not necessary in the calculation. A. reaction stoichiometry B. compounds with correct phases C. the mass of reactants and products in the reaction D. ΔH o f for an element as you find it in nature is 0 kj/mol Answer: the mass of reactants and products in the reaction 1

2 3. In a bomb calorimetry experiment, a g sample of phenanthrene (C 14 H 10, 178 g/mol) was combusted with excess oxygen gas in the reaction below: C 14 H 10 (s) O 2 (g) 14 CO 2 (g) + 5 H 2 O(l) ΔH comb =? kj/mol The experimental data for the calorimetry experiment are given in the table at the right. From the available data, determine the enthalpy of combustion of phenanthrene. Give your answer in kj/mol. Initial temperature ( C) Final temperature ( C) Mass of H 2 O used (g) C cal (J/ C)

3 4. Congratulations!!! You ve been hired by a rogue nation s dictator to help get their nuclear program started. Yellow cake uranium (UO 2 ) was purchased on the black market and needs to be converted to UF 6. As a general chemistry student, you have figured out that you can execute reaction #1 and devise a reaction set up that will collect the UF 4 over water. If excess hydrogen fluoride was reacted with 1.00 g UO 2 and the UF 4(g) was collected over water, what was your percent yield given the data on the UF 4(g) you collected. (Hopefully your percent yield was high, you re working for a dictator after all.) UF 4 gas sample data Temperature 33 C Volume 75.4 ml P bar mmhg P H2O (33 C) mm Hg UO 2(s) + 4HF (g) UF 4(g) + 4H 2 O (l) (1) UF 4(g) + F 2(g) UF 6(g) (2) 5. Based on the kinetic molecular theory, which of the following statements is true? A. All gas molecules in a sample have the same speed and kinetic energy. B. Two different gases at the same temperature have the same root mean squared speed. C. For two samples of the same gas, the one with the higher temperature will have the lower root mean squared speed. D. Two different gases at the same temperature have the kinetic energy. E. None of these statements is true. Answer: Two different gases at the same temperature have the kinetic energy. 3

4 6. Convert the following: a atm to mm Hg b. 269 kpa to atm c. 269 kpa to torr d. 866 torr to atm 7. What is the effect of the following on the volume of 1 mol of an ideal gas? a. The pressure is reduced by a factor of 3 with no change in temperature. b. The pressure changes from 730 mm Hg to 198 kpa, and the temperature changes from 55 C to 404 K. c. The temperature is doubled (100 to 200 C) and the pressure is halved (760 mm Hg to 380 mm Hg). 4

5 8. If the following reactions were carried out inside rigid sealed flasks, for which of the reactions would an increase in pressure be observed inside the flask? (All reactions are balanced. Assume no change in temperature or flask volume.) I. H 2(g) + F 2(g) 2HF (g) II. Na 2 CO 3(s) + 2HCl (aq) 2NaCl (aq) + 2H 2 O (l) + CO 2(g) III. 2SO 2(g) + O 2(g) 2SO 3(g) IV. H 2 0 (g) H 2(g) + ½ O 2(g) Answer: Na 2 CO 3(s) + 2HCl (aq) 2NaCl (aq) + 2H 2 O (l) + CO 2(g) H 2 0 (g) H 2(g) + ½ O 2(g) 9. A mixture of CO 2 (12.36 g) and SO 3 (32.89 g) are in a sealed container with a total pressure of 0.69 atm. Calculate the partial pressure of each gas in the container. 5

6 10. During sleep, a person inhales O 2 and exhales CO 2 and H 2 O. Assuming the P CO2 = 28 mm Hg and P H2O = 47.1 mm Hg at 37 C and that 280 L of breath is exhaled per hour. What masses of CO 2 and H 2 O are exhaled during 8 h of sleep? 6

7 11. The enthalpy of reaction (ΔH rxm o ) for the preparation of thionyl chloride (SOCl 2 ) and trichlorophosphate (POCl 3 ) is an example of a reaction that cannot be studied with a bomb calorimetry. Use the equations below with known ΔH rxn to determine ΔH rxm for the preparation of SOCl 2 and POCl 3. 2 SO 2 (g) + 2 P(s) + 5 Cl 2 (g) 2 SOCl 2 (l) + 2 POCl 3 (l) ΔH rxn o =? ΔH rxn (kj/mol) SOCl 2(l) + H 2 O (l) SO 2(g) + 2 HCl (g) HCl (g) + O 2(g) 2 Cl 2(g) + 2 H 2 O (l) PCl 3(l) + 1/2 O 2(g) POCl 3(l) P 2 O 5(s) + 3 Cl 2(g) 2 POCl 3(l) + 3/2 O 2(g) P (s) + 3/2 Cl 2(g) PCl 3(l)

8 12. The thermite reaction (below) is used to weld railroad rails together because of the tremendous amount of heat released in approximately 30 seconds. If the reaction below were run with 100 g Fe 2 O 3 (159.7 g/mol) and excess Al, what would be the final temperature of the molten iron (Fe (l) ) if the iron to be melted is the product of the reaction plus an additional 300 g of iron from the adjacent rails. The reactants and rails are at 23 C to begin with, and iron s melting point is 1538 C. ΔH fus (Fe) = kj/mol. Fe 2 O 3(s) + 2Al (s) 2Fe (s) + Al 2 O 3(s) ΔH rxn = kj/mol 8

9 13. If L samples of each of the following gases were at STP, which would have the greatest mass? Ar, H 2, CO, O 2, HF Answer: Ar 14. Use the ideal gas law to determine the requested gas parameter with respect to a sample of carbon monoxide with starting conditions of L, 747 mm Hg, 37 C. a. Determine the moles of carbon monoxide. b. From a, the sample was compressed to one-half its volume, the Celsius temperature was doubled, and one-third of the moles were removed. Is the pressure now higher or lower than in a? Support your answer with a calculation. 9

10 15. Uranium enrichment to isolate radioactive 235 U for the purposes of nuclear power plants or nuclear weapons is done with sophisticated centrifuge technology which separates 235 UF 6(g) and 238 UF 6(g) based on their densities. Show with a calculation that a gas effusion experimental setup might also be used to separate these uranium isotopes at 25 C. Which isotope would have a higher concentration in the receiver flask? 16. How many grams of phosphorous react with 28.6 L of O 2 at STP to form phosphorous pentaoxide as in the reaction below? P 4(s) + O 2(g) P 2 O 5(s) (unbalanced) 10

11 17. Draw an enthalpy diagram for: a. the combustion of acetylene (C 2 H 2 ) in oxygen to produce CO 2 and H 2 O b. the vaporization of ethyl alcohol (C 2 H 5 OH) from liquid into the gas phase. 18. Neon gas effuses at a rate times that of gaseous white phosphorous in the same apparatus under the same conditions. What is the molecular formula for white phosphorous? 11

12 19. If the mercury (d = 13.6 g/ml) in the manometer was replaced with carbon tetrachloride (d = 1.58 g/ml), what would be the difference in height? (The atmospheric pressure is 720 mmhg and the pressure of the gas is 708 mmhg in the picture.) Answer: 103 mm 20. The nutritional calorie (Calorie) is equivalent to 1 kcal and one pound of body fat is equivalent to 4.1 x 10 3 Calories. a. Express this energy in J and kj. b. If a runner burns, 2120 kj/h, how long does it take to burn 1.0 lb of body fat? 12

13 21. Calculate the percent yield for a 331 ml sample of a nitrogen dioxide collected over water by the reaction of a g pre-1982 penny (100 % Cu) with 187 ml of 1.3 M HNO 3. The barometric pressure was 743 mm Hg and the temperature was 27.5 C. Cu (s) + 4 HNO 3(aq) Cu(NO 3 ) 2(aq) + 2 NO 2(g) + 2 H 2 O (l) 13

14 22. Strike anywhere matches contain tetraphosphorous trisulfide (P 4 S 3 ) which combusts in oxygen to form P 4 O 10(s) and SO 2(g). How many milliliters of SO 2, measured at 747 mm Hg and 37 C, can be produced from burning g of P 4 S 3? 23. How much heat is released when 291 g of H 2 O 2 decomposes according to the reaction below? 2 H 2 O 2(l) 2 H 2 O (l) + O 2(g) ΔH rxn = kj/mol H 2 O 2 14

15 24. Combine the following reactions to obtain the overall reaction ClF (g) + F 2(g) ClF 3(l). ΔH rxn (kj) 2 ClF 3(l) + 2 O 2(g) Cl 2 O (g) + 3 OF 2(g) OF 2(g) 2 F 2(g) + O 2(g) ClF (g) + O 2(g) Cl 2 O (g) + OF 2(g)

16 25. Nitroglycerine, C 3 H 5 (NO 3 ) 3(l) is a powerful explosive that produces the following gases: N 2, H 2 O, CO 2, and O 2. a. Write a balanced chemical equation for conversion of nitroglycerine to its products during detonation. b. Calculate ΔH f for nitroglycerine given the ΔH rxn = x 104 kj/mol for the detonation. 26. Calculate the root mean squared speed of carbon monoxide at -32 C. 16

(E) half as fast as methane.

(E) half as fast as methane. Name AP Chem / / AP Chem Practice Exam #2 Part I: 40 Questions, 40 minutes, Multiple Choice, No Calculator Allowed Bubble the correct answer on the BLUE SIDE of your scantron for each of the following.