(E) half as fast as methane.

Transcription

1 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. 1. Use the information in the table to calculate the enthalpy of this reaction. C 2 H 6 (g) + 7/2 O 2 (g) 2CO 2 (g) + 3H 2 O(l) Reaction H f, kj mol 1 2C(s) + 3H 2 (g) C 2 H 6 (g) 84.7 C(s) + O 2 (g) CO 2 (g) H 2 (g) + ½ O 2 (g) H 2 O(l) (A) 764 kj (B) 1560 kj (C) 1664 kj (D) 3120 kj (E) -595 kj 2. Under which conditions will a gas behave most ideally? (A) low P and high T (B) low P and low T (C) high P and low T (D) high P and high T (E) a gas will be have ideally at all conditions 3. Which statement is true for a reaction at equilibrium? (A) All reaction ceases. (B) The reaction has gone to completion. (C) The rates of the forward and reverse reactions are equal. (D) The amount of product equals the amount of reactant. (E) The forward reaction ends and the reverse reaction begins. 4. A sample of neon gas has a volume of 248 ml at 30. C and a certain pressure. What volume would it occupy if it were heated to 60. C at the same pressure? (A) 226 ml (B) 273 ml (C) 278 ml (D) 496 ml (E) 124 ml 5. A gas is collected in the flask shown here. What is the pressure exerted by the gas if the atmospheric pressure is 735 mmhg? (A) 42 mmhg (B) 693 mmhg (C) 735 mmhg (D) 777 mmhg (E) 84 mmhg 6. For the reaction: 2SO 2 (g) + O 2 (g) qe 2SO 3 (g) ΔH < 0 Which change(s) will increase the fraction of SO 3 (g) in the equilibrium mixture? 1. Increasing the pressure 2. Increasing the temperature 3. Adding a catalyst (A) 1 only (B) 2 only (C) 3 only (C) 1 and 3 only (D) 1, 2 and 3 7. A sample of oxygen gas and a sample of an unknown gas are weighed separately in the same evacuated flask. Use the data given to find the molar mass of the unknown gas (assume experiments are carried out at the same pressure and temperature). Mass of evacuated flask g Mass of flask + oxygen g Mass of flask + unknown gas g (A) 22 g/mol (B) 38 g/mol (C) 44 g/mol (D) 84 g/mol (E) 66 g/mol

2 8. Given these reactions: A 2B ΔH = 40 kj B C ΔH = -50 kj 2C D ΔH = -20 kj Calculate ΔH for the reaction; D + A 4C. (A) 100 kj (B) 60 kj (C) 40 kj (D) 100 kj (E) -30 kj 9. C 2 H 6 (g) + 7/2O 2 (g) 2CO 2 (g) + 3H 2 O(g) ΔH = kj If the enthalpy of vaporization for H 2 O(l) is 44.0 kj/mol, what is ΔH for this reaction if H 2 O(l) is formed instead of H 2 O(g)? (A) kj (B) kj (C) kj (D) kj (E) kj 10. A gas mixture at 27 C and 760 mm Hg contains 1.0 g each of He, H 2, N 2 and CO 2. How do their average molecular speeds compare? (A) He = H 2 = N 2 = CO 2 (B) CO 2 < H 2 = N 2 < He (C) He < H 2 < N 2 < CO 2 (D) CO 2 < N 2 < He < H 2 (E) H 2 < He < N 2 < CO For which of these is ΔH f not equal to zero? (A) Br 2 (l) (B) Fe(s) (C) I 2 (s) (D) O 3 (g) (E) Xe(g) 12. Helium is often found with methane, CH 4. How do the diffusion rates of helium and methane compare at the same temperature? Helium diffuses (A) sixteen times as fast as methane. (B) four times as fast as methane. (C) twice as fast as methane. (D) at the same rate as methane. (E) half as fast as methane. 13. The enthalpy change for which reaction represents the standard enthalpy of formation for hydrogen cyanide, HCN? (A) H(g) + C(graphite) + N(g) HCN(g) (B) ½H 2 (g) + C(graphite) + ½N 2 (g) HCN(g) (C) HCN(g) ½H 2 (g) + C(graphite) + ½N 2 (g) (D) H 2 (g) + 2C(graphite) + N 2 (g) 2HCN(g) (E) ½H 2 (g) + C(graphite) + N(g) HCN(g) 14. What is the molar mass of a gas if 10.0 grams of it occupy 4.48 liters at 273 K and kpa (1.00 atm)? (A) 2.00 g/mol (B) 25.0 g/mol (C) 50.0 g/mol (D) 100. g/mol (E) 75.0 g/mol 15. For the formation of one mole of each of these gases from their elements, which reaction is most endothermic? (A) CO (ΔH f = kj mol 1 ) (B) NO 2 (ΔH f = kj mol 1 ) (C) O 3 (ΔH f = kj mol 1 ) (D) SO 2 (ΔH f = kj mol 1 ) (E) I 2 (ΔH f = kj mol 1 ) 16. The specific heats of several metals are given in the table. If the same number of Joules were applied to the same mass of each metal, which metal would show the greatest temperature change? Substance 1 Specific Heat, J g 1 C Al Au Cu Hg (A) Al (B) Au (C) Cu (D) Hg (E) all would be equal

3 17. 4Li(s) + O 2 (g) 2Li 2 O(s) At 25 C, H for this reaction is kilojoules per mole of Li 2 O(s) formed. What mass of Li should be reacted with excess O 2 (g) in order to release 150. kj? (A) g (B) 1.74 g (C) 3.48 g (D) 6.98 g (E) 7.52 g 18. A 2.00 liter evacuated container has a mass of g. When the container is filled with an unknown gas at 800. mm Hg pressure and 25.0 C the mass is g. What is the molar mass of the gas (in g mol -1 )? (A) 28 (B) 31 (C) 54 (D) 56 (E) Consider this reaction. 2N 2 H 4 (l) + N 2 O 4 (l) 3N 2 (g) + 4H 2 O(g) H = 1078 kj How much energy is released by this reaction during the formation of 140. g of N 2 (g)? (A) 1078 kj (B) 1797 kj (C) 3234 kj (D) 5390 kj (E) kj 20. Which pair of gases has the same average rate of diffusion at 25 C? (A) He and Ne (B) N 2 and O 2 (C) N 2 O and CO 2 (D) NH 3 and HCl (E) SF 6 and Xe 21. Calculate the amount of energy released when mol of diborane, B 2 H 6, reacts with oxygen to produce solid B 2 O 3 and steam. Substance ΔH f, (kj.mol -1 ) B 2 H 6 (g) 35 B 2 O 3 (s) H 2 O(l) -285 H 2 O(g) -241 (A) 203 kj (B) 216 kj (C) 330 kj (D) 343 kj (E) 124 kj 22. How much heat is required to raise the temperature of 100. g of Fe 2 O 3 from 5.0 C to 25.0 C? (Specific heat Fe 2 O 3, J g -1 C -1 ) (A) 1.58 kj (B) 1.27 kj (C) kj (D) kj (E) kj 23. A gas has a volume of 6.0 L at a pressure of 0.80 atm. What is the volume if the pressure is changed to 0.20 atm at constant temperature? (A) 1.5 L (B) 3.0 L (C) 12 L (D) 24 L (E) 0.96 L 24. Given the thermochemical equations: Br 2 (l) + F 2 (g) 2BrF(g) ΔH = -188 kj Br 2 (l) + 3F 2 (g) 2BrF 3 (g) ΔH = -768 kj Determine ΔH for the reaction: BrF(g) + F 2 (g) BrF 3 (g) ΔH =? (A) -956 kj (B) -580 kj (C) -478 kj (D) -290 kj (E) 580 kj 25. The enthalpy change of which reaction corresponds to ΔH f for Na 2 CO 3 (s) at 298 K? (A) 2Na(s) + C(s) + 3/2O 2 (g) Na 2 CO 3 (s) (B) Na 2 O(s) + CO 2 (g) Na 2 CO 3 (s) (C) 2Na + (aq) + CO 2-3 (aq) Na 2 CO 3 (s) (D) Na 2 (g) + CO 3 (g) Na 2 CO 3 (s) (E) 2Na + (aq) + 2OH (aq) + CO 2 (aq) Na 2 CO 3 (s) + H 2 O 26. When a bomb calorimeter is used to determine the heat of reaction, which property of the system under investigation is most likely to remain constant? (A) number of molecules (B) pressure (C) temperature (D) volume (E) number of moles 27. For the reaction shown, which is closest to the value of ΔH? 2Cr 3+ (aq) + 3Ni(s) 2Cr(s) + 3Ni 2+ (aq) Substance ΔH f (kj.mol -1 ) Cr 3+ (aq) -143 Ni 2+ (aq) -54 (A) 124 kj (B) 89 kj (C) -89 kj (D) -124 kj (E) 197 kj

4 28. Which equation represents the reaction for the standard enthalpy of formation, ΔH f, for B 5 H 9 (g) at 298 K and 1 atm? (A) 5B(s) + 9H(g) B 5 H 9 (g) (B) 2B(s) + 3BH 3 (g) B 5 H 9 (g) (C) 5/2 B 2 (g) +9/2H 2 (g) B 5 H 9 (g) (D) 5 B(s) +9/2H 2 (g) B 5 H 9 (g) (E) 5/2 B 2 (g) + 9H(g) B 5 H 9 (g) 29. Substance ΔH f (kj mol -1 ) H 2 O(g) Calculate the change in enthalpy, ΔH, for the combustion of 11.2 L of hydrogen gas, measured at 0 C and 1 atm pressure, to form H 2 O(g). (A) 60.5 kj (B) 121 kj (C) 484 kj (D) 2710 kj (E) -242 kj 30. A g sample of a gas in a 100-mL flask exerts a pressure of 600 mmhg at 14 C. What is the gas? (A) chlorine (B) nitrogen (C) krypton (D) xenon (E) oxygen 31. What pressure (in atm) will be exerted by a 1.00 g sample of methane, CH 4, in a 4.25 L flask at 115 C? (A) (B) (C) (D) 7.50 (E) 8.46 Questions 32 and 33 should both be answered with reference to this reaction, for which ΔH is negative. 2NO(g) + O 2 (g) qe 2NO 2 (g) 32. Which would increase the partial pressure of NO 2 (g) at equilibrium? (A) decreasing the volume of the system (B) adding a noble gas to increase the pressure of the system (C) removing some NO(g) from the system (D) adding an appropriate catalyst (E) none of the above 33. At a certain temperature the equilibrium concentrations for this system are: [NO] = 0.52M; [O 2 ] = 0.24M; [NO 2 ] =0.18M. What is the value of K C at this temperature? (A) (B) 0.50 (C) 1.4 (D) 2.0 (E) What is the standard enthalpy of formation of MgO(s) if kj is evolved when g of MgO(s) is formed by the combustion of magnesium under standard conditions? (A) kj mol 1 (B) kj mol 1 (C) kj mol 1 (D) kj mol 1 (E) kj mol 1 H 2 (g) + I 2 (s)qe 2HI(g) ΔH = kj 35. Which would increase the equilibrium quantity of HI(g)? Assume the system has reached equilibrium with all three components present. I. increasing pressure II. increasing temperature III. decreasing volume (A) I only (B) II only (C) III only (D) Both I and II (E) I, II and III 36. Three balloons are filled with the same number of atoms of He, Ar, and Xe, respectively. Which statement is true under the same conditions of temperature and pressure? (A) The balloons contain the same mass of gas. (B) All balloons have the same volume. (C) The densities of the three gases are the same. (D) The average speed of the different types of atoms is the same. (E) All gases have the same root mean square velocity.

5 37. Consider this reaction. 2NO(g) + Cl 2 (g) qe 2NOCl(g) H = kj What conditions of temperature and pressure will produce the highest yield of NOCl at equilibrium? T P (A) high high (B) high low (C) low high (D) low low (E) medium medium 38. The mass of 560 cm 3 of a gas at 0 C and 1 atm is 1.60 g. Which gas could it be? (A) O 2 (B) CO 2 (C) SO 2 (D) Cl 2 (E) Xe 39. Substance Specific Heat Capacity (J g -1 C -1 ) Au H 2 O 4.18 A gold ring that weighs 3.81 g is heated to 84.0 C and placed in 50.0 g of H 2 O at 22.1 C. What is the final temperature? (A) 22.2 C (B) 24.0 C (C) 26.5 C (D) 53.1 C (E) 30.5 C 40. Oxygen, which is 16 times as dense as hydrogen, diffuses: (A) 1/16 times as fast. (B) 1/4 times as fast. (C) 4 times as fast. (D) 16 times as fast (E) equally as fast as hydrogen.

6 Name AP Chemistry / / AP Chemistry Practice Exam #2 Part II: Essays. Show ALL work below. Work should be done in a clear and orderly fashion #6 NH 4 HS(s) <===> NH 3 (g) + H 2 S(g) For this reaction, H = + 93 kilojoules The equilibrium above is established by placing solid NH 4 HS in an evacuated container at 25 C. At equilibrium, some solid NH 4 HS remains in the container. Predict and explain each of the following. (a) The effect on the equilibrium partial pressure of NH 3 gas when additional solid NH 4 HS is introduced into the container. (b) The effect on the equilibrium partial pressure of NH 3 gas when additional H 2 S gas is introduced into the container. (c) The effect on the mass of solid NH 4 HS present when the volume of the container is decreased. (d) The effect on the mass of solid NH 4 HS present when the temperature is increased.

7 #3 Consider the hydrocarbon pentane, C 5 H 12 (molar mass g). (a) Write the balanced equation for the combustion of pentane to yield carbon dioxide and water. (b) What volume of dry carbon dioxide, measured at 25 C and 785 mm Hg, will result from the complete combustion of 2.50 g of pentane? (c) The complete combustion of 5.00 g of pentane releases 243 kj of heat. On the basis of this information, calculate the value of H for the complete combustion of one mole of pentane. (d) Under identical conditions, a sample of an unknown gas effuses into a vacuum at twice the rate that a sample of pentane gas effuses. Calculate the molar mass of the unknown gas.

8 #2 A rigid 5.00 L cylinder contains 24.5 g of N 2 (g) and 28.0 g of O 2 (g). (a) Calculate the total pressure, in atm, of the gas mixture in the cylinder at 298 K. (b) The temperature of the gas mixture in the cylinder is decreased to 280 K. Calculate each of the following. (i) The mole fraction of N 2 (g) in the cylinder (ii) The partial pressure, in atm, of N 2 (g) in the cylinder (c) If the cylinder develops a pinhole-sized leak and some of the gaseous mixture escapes, would the ratio moles : Nitrogen in the cylinder increase, decrease, or remain the same? Justify your answer. moles : Oxygen A different rigid 5.00 L cylinder contains mol of NO(g) at 298 K. A mol sample of O 2 (g) is added to the cylinder, where a reaction occurs to produce NO 2 (g). (d) Write the balanced equation for the reaction. (e) Calculate the total pressure, in atm, in the cylinder at 298 K after the reaction is complete.

9 #1 At elevated temperatures, SbCl 5 gas decomposes into SbCl 3 gas and Cl 2 gas, as shown by the following equation. SbCl 5 <===> SbCl 3 + Cl 2 (a) An 89.7-gram sample of SbCl 5 (molecular weight 299.0) is placed in an evacuated 15.0-liter container at 182 C. (1) What is the concentration in moles per liter of SbCl 5 in the container before any decomposition occurs? (2) What is the pressure in atmospheres of SbCl 5 in the container before any decomposition occurs? (b) If the SbCl 5 is 29.2 percent decomposed when equilibrium is established at 182 C, calculate the value for either equilibrium constant, K p or K c, for this decomposition reaction. Indicate whether you are calculating K p or K c. (c) In order to produce some SbCl 5, a 1.00-mole sample of SbCl 3 is first placed in an empty 2.00-liter container maintained at a temperature different from 182 C. At this temperature K c equals How many moles of Cl 2 must be added to this container to reduce the number of moles of SbCl 3 to mole at equilibrium?