South Pasadena AP Chemistry Name 6 Thermodynamics Period Date U N I T T E S T P R A C T I C E Part 1 Multiple Choice You should allocate 25 minutes to finish this portion of the test. No calculator should be used. A periodic table and data table will be provided. Select the answer that best responds to each question. 1. Naphthalene (C 10H 8), a key ingredient in traditional mothballs, undergoes sublimation at 25 C: C 10H 8 (s) C 10H 8 (g) If a mothball is placed in an insulated container with an inert gas sample at 40 C, what may be observed? (A) The temperature of the gas decreases slightly. (B) The temperature of the gas increases slightly. (C) The temperature of the gas stays the same. (D) The temperature of the mothball increases slightly. 2. A sample of H 2O (g) is placed in a cylinder with a piston against a constant external pressure of 2.0 atm. How is the internal energy of the system changing when the sample condenses? (A) The change in internal energy of the system cannot be determined from the information given. (B) The internal energy of the system is decreasing. (C) The internal energy of the system is increasing. (D) The internal energy of the system remains constant. 3. A cup with water at room temperature is placed in a tub of ice at 0 C. Which of the following takes place? (A) The attractions between molecules in the ice are weakened, increasing the motion of water molecules in the cup. (B) The attractions between molecules in the ice are weakened, decreasing the motion of water molecules in the cup. (C) The attractions between molecules in the ice are strengthened, increasing the motion of water molecules in the cup. (D) The attractions between molecules in the ice are strengthened, decreasing the motion of water molecules in the cup. 4. When 0.100 g benzoic acid (HC 7H 5O 2, molar mass 122) and excess oxygen is ignited in a bomb calorimeter, the temperature of the water changes from 25.000 C to 25.225 C. The heat capacity of the calorimeter is 603 J/ C. What is the energy of this sample of benzoic acid? (A) 13.5 J (B) 135 J (C) 1350 J (D) 15,100 J 5. Using the enthalpies of formation given below, calculate the H combustion for propane, C 3H 8. H f (kj/mol) H 2O(l) 286 CO 2(g) 394 C 3H 8(g) 104 (A) +576 kj/mol (B) 576 kj/mol (C) 2222 kj/mol (D) 2330 kj/mol
6. As ammonium nitrate is dissolved in water, the temperature of the system decreases. This process can be thought of in the following steps: + I. Break ionic bonds between NH 4 and NO 3 ions. II. Break hydrogen bonds between water molecules. III. Form ion-dipole attractions between water + and NH 4 and NO 3 ions. Which of the following explains the change in temperature observed? (A) The sum of the energies required in steps I and II is greater than the energy released in step III. (B) The sum of the energies required in steps I and II is less than the energy released in step III. (C) All three steps release energy to the surroundings. (D) All three steps require energy to be absorbed. 7. Consider the following potential energy graph for the C C bond: 8. Given the following equations for the enthalpies of formations, H f: P 4 (s) + 5 O 2 (g) P 4O 10 (s) H f = x H 2 (g) + ½ O 2 (g) H 2O (l) H f = y 3/2 H 2 (g) + 1/4 P 4 (s) +2 O 2 (g) H 3PO 4 (aq) H f = z Which of the following expressions represent the enthalpy of reaction, H rxn, for the following reaction? P 4O 10 (s) + 6 H 2O (l) 4 H 3PO 4 (aq) (A) 4z x 6y (B) 4z x + 6y (C) x + 6y 4z (D) z x y 9. Use the following data to find the standard entropy change for the following reaction, Cu(s) + ½ O 2(g) CuO(s). S [Cu(s)] = 33 J/K mol S [O 2(g)] = 205 J/K mol S [CuO(s)] = 42 J/K mol (A) 111.5 J/mol K (B) 93.5 J/mol K (C) +111.5 J/mol K (D) +93.5 J/mol K Which of these describes two interacting C atoms that are separated by 0.100 nm? (A) The attractions between the atoms are stronger than the repulsions between the atoms, and the internuclear distance will decrease. (B) The repulsions between the atoms are stronger than the attractions between the atoms, and the internuclear distance will increase. (C) The attractions are maximized and repulsions are minimized, so the internuclear distance will not change. (D) There are no attractive or repulsive forces between the atoms because they are separated from each other. 10. In which of the following processes is the standard entropy of reaction, S, expected to be negative? (A) (NH 4) 2CO 3 (s) 2 NH 3 (g) + H 2O (l) + CO 2 (g) (B) Pb(NO 3) 2 (aq) + 2 HCl (aq) PbCl 2 (s) + 2 HNO 3 (aq) (C) P 4O 10 (s) + 6 H 2O (l) 4 H 3PO 3 (aq) (D) CO 2 (s) CO 2 (g) 11. NH 4Cl (s) was placed in a container filled with water, and the temperature dropped considerably. The reaction proceeds according to the following equation: + NH 4Cl (s) NH 4 (aq) + Cl (aq) What are the signs for H, S, and G for the process? (A) H < 0 S < 0 G > 0 (B) H < 0 S > 0 G < 0 (C) H > 0 S < 0 G > 0 (D) H > 0 S > 0 G < 0
12. For the reaction below, H rxn = 114 kj/mol and S = 146.5 J/mol-K. For which temperatures is the reaction thermodynamically favorable? Assume that the values do not change with temperature. 2 NO (g) + O 2 (g) 2 NO 2 (g) (A) The process is thermodynamically favorable at all temperatures (B) The process is only thermodynamically favorable above 778 K (C) The process is only thermodynamically favorable at 778 K (D) The process is only thermodynamically favorable below 778 K 13. Given the following information, calculate G for the reaction below at 25 C: 2 H 2O 2(l) 2 H 2O(l) + O 2(g) Compound H (kj/mol) S (J/K mol) H 2O 2(l) 190 110 H 2O(l) 290 70 O 2(g) 205 (A) 37,500 kj/mol (B) 3,330 kj/mol (C) 237 kj/mol (D) 203 kj/mol 14. Consider the following reaction: SnCl 4(l) + 2 H 2O(l) SnO 2(s) + 4 HCl(g) At 25 C, H = +133.0 kj/mol and S = +401.5 J/mol K. What is true about the reaction at this temperature? (A) Q > K (B) Q = K (C) Q < K (D) Nothing can be stated about the equilibrium of this reaction. 15. The free energy change for a given reaction is 25 kj. What is the equilibrium constant at 300 K? (A) e 0 = 1.0 (B) e 1 = 2.7 (C) e 10 = 4.4 10 5 (D) e 10 = 2.3 10 4
Part 2 Free Response You should allocate 30 minutes to finish this portion of the test. You may use a scientific calculator. A periodic table and data table will be provided. Respond to each part of the questions completely. Be sure to show your work clearly for questions that involve calculations. 16. Gaseous acetaldehyde, CH 3CHO (molar mass 44.05 g/mol), undergoes combustion according to the following reaction: 2 CH 3CHO (g) + 5 O 2 (g) 4 CO 2 (g) + 4 H 2O (l) When 79.62 g of CH 3CHO is consumed, 4,309 kj of energy was released. The process takes place in a container against an external pressure of 101.3 kpa and temperature of 25 C. The volume of the container decreases by 66.3 L. (1 kpa L = 1 J) (a) What is the value of w and E of the system in J?
(b) What is the value for H rxn for this process in kj/mol? Assume that the work of the system is negligible. (c) Using the structure of acetaldehyde and the average bond energies given, determine the approximate bond energy of the C=O bond. H O H C C H H Bond C H C C O H O=O Average Bond Energy 413 kj/mol 347 kj/mol 467 kj/mol 495 kj/mol (d) Acetaldehyde is formed from ethane, C 2H 4. Use the following reactions to determine the value of S rxn in J/mol-K for the combustion of acetaldehyde. 2 C 2H 4 (g) + O 2 (g) 2 CH 3CHO (g) S 1 = 323.86 J/mol-K C 2H 4 (g) + 3 O 2 (g) 2 CO 2 (g) + 2 H 2O (l) S 2 = 267.68 J/mol-K (e) What is the value of G rxn for the combustion of acetaldehyde in kj/mol? Is the process spontaneous? Why or why not? (f) If G f for CO 2 (g) and H 2O (l) are 394.36 kj/mol and 237.13 kj/mol, respectively, what is the value of G f for CH 3CHO (g)? (g) At what temperatures is this process spontaneous?
Potential Energy 17. Consider the catalyzed isomerization of cis-2-butene gas to produce trans-2-butene gas, represented below, and the thermodynamic data provided below. H f (kj/mol) S (J/mol K) cis-2-butene 7.0 +301 trans-2-butene 11.2 +296 (a) Compare the bonds between cis-2-butene and trans-2-butene. Assume that the corresponding bonds between the compounds have the same bond energy except for the C=C bond. The potential energy graph for the C=C bond for each structure is shown below. Identify to which structure each bond correlates. Explain briefly. Inter-nuclear separation (b) Find the values for G for this reaction at 298 K. (c) Calculate the value of the equilibrium constant, K, at 298 K.
South Pasadena AP Chemistry Name 6 Thermodynamics Period Date Part 1: Multiple Choice Format: 15 questions, four answer choices: (A)-(D) Expected time: 25 minutes Allowed resources: Periodic Table, Equations and Constants. No calculators. U N I T T E S T B L U E P R I N T Q Lesson Topic Objective 1 6.0 Endo/Exo Describe a process as exothermic vs. endothermic (e.g. value of q, direction of energy flow, graph, thermochemical equation). 2 6.1 Heat vs. Work Explain how the internal energy of a system can change with heat flow and expansion work. 3 6.0 Molecular Describe the particulate behavior of a change in temperature or state. Descriptions 4 6.1 H = q/n Calculate the heat involved in the change in temperature of an object. Calculate the heat involved in the change in state of a sample. Calculate the heat involved in a physical or chemical process using H. 5 6.1 H f Determine the H rxn using H f of substances, and by analyzing energy of covalent bonds formed and broken. 6 6.1 Bond Energy and H Determine the H rxn using H f of substances, and by analyzing energy of covalent bonds formed and broken. 7 6.1 PE of Covalent Bond Explain the bond length and energy of a covalent bond using a potential energy graph. 8 6.1 Hess s Law Calculate the value of H rxn using Hess s Law 9 6.2 Entropy Predict whether a physical or chemical process is increasing or decreasing in entropy. 10 6.2 Entropy Change Predict whether a physical or chemical process is increasing or decreasing in entropy. 11 6.2 Spontaneity Determine under what conditions a process is spontaneous (thermodynamically favorable) using the value of G. 12 6.2 Spontaneity Determine under what conditions a process is spontaneous (thermodynamically favorable) using the value of G. 13 6.2 G = H T S Calculate the value of G rxn using G f or Hess s Law. 14 6.3 G vs. Q Determine the spontaneous direction of a reversible process. 15 6.3 G vs. K eq Find the value of K eq from thermodynamic data. Part 2: Free Response Format: o 1 long question (5-8 parts) o 1 short questions (2-4 parts) Expected time: 30 minutes Allowed resources: Periodic Table, Equations and Constants, and scientific calculators. Topics: Any