BCIT Fall 2017 Chem 3310 Exam #2 Name: Attempt all questions in this exam. Read each question carefully and give a complete answer in the space provided. Part marks given for wrong answers with partially correct reasoning/calculations. Constants and equations are given below. A Periodic Table and Formula Sheet are attached at the back. Total points = 50 Page 1 of 12
Section I: (12 points total, 1 point each) Choose the BEST answer to the following questions. 1. A gas absorbs 10.0 J of heat and then performs 20.0 J of work. The change in internal energy of the gas is a) 30.0 J b) 10.0 J c) +10.0 J d) +30.0 J e) none of these 2. Which statement is true of a process in which one mole of a gas is expanded from state A to state B? a) When the gas expands from state A to state B, the surroundings are doing work on the system. b) The amount of work done in the process must be the same, regardless of the path. c) It is not possible to have more than one path for a change of state. d) The final volume of the gas will depend on the path taken. e) The amount of heat released in the process will depend on the path taken. 3. Consider an ideal gas in a 1.0 L bulb at STP that is connected via a valve to another 1.0 L bulb that is initially evacuated. The valve between the two bulbs is opened and the gas expands isothermally into second flask. How many of q, w, E and H are equal to 0 for this expansion? a) 0 b) 1 c) 2 d) 3 e) 4 4. Consider the reaction: 2H 2 (g) + O 2 (g) ) H 2 O() ΔH = 572 kj Which of the following is true? a) The reaction is exothermic. b) The reaction is endothermic. c) The enthalpy of the products is less than that of the reactants. d) Heat is absorbed by the system. e) Both A and C are true. 5. Which of the following does not have a standard enthalpy of formation equal to zero at 25 C and 1.0 atm? a) F 2 (g) b) Al(s) c) H 2 O() d) None have a standard enthalpy equal to zero. e) They all have a standard enthalpy equal to zero. Page 2 of 12
6. Which of the following statements is true? a) As long as the disorder of the surroundings is increasing, a process will be spontaneous. b) For all process S sys and S surr have opposite signs. c) If S sys = S surr, the process is at equilibrium d) Statements a, b and c are all false. e) Statements a, b and c are all true. 7. For a non spontaneous endothermic process, which of the following must be true? a) G must be negative. b) S must be positive. c) S must be negative. d) Two of the above (A C) must be true. e) None of the above (A C) must be true. 8. For the process S 8 (rhombic) S 8 (monoclinic) at 110 C and 1 atm, H = 3.21 kj/mol and S = 8.70 J mol 1 K 1. Which of the following is correct? a) This reaction is spontaneous at 110 C (S 8 (monoclinic) is stable). b) This reaction is spontaneous at 110 C (S 8 (rhombic) is stable). c) This reaction is nonspontaneous at 110 C (S 8 (rhombic) is stable). d) This reaction is nonspontaneous at 110 C (S 8 (monoclinic) is stable). e) More information is needed to determine if the reaction is spontaneous. 9. In which process is S expected to be positive? a) a reaction that forms a solid precipitant from aqueous solutions b) an ideal gas being compressed at a constant temperature and against a constant pressure c) water freezing below its normal freezing point d) a spontaneous endothermic process at a constant temperature and pressure e) none of these 10. For a certain process, at 300. K, G = 10.0 kj and H = 20.0 kj. If the process is carried out reversibly, the amount of useful work that can be performed is a) 30.0 kj b) 10.0 kj c) +10.0 kj d) +30.0 kj e) none of these Page 3 of 12
11. The reaction, COCl 2 (g) CO(g) + Cl 2 (g), is non spontaneous at 25 C and 1 atm. Which of the following changes could be used to make this a spontaneous reaction? a) decrease T b) keep T at room temperature but add a catalyst c) increase T d) none of a c would work e) more information is needed 12. The synthesis of the pesticide thionyl chloride, OSCl 2, proceeds according to: SO 3 (g) + SCl 2 () OSCl 2 () + SO 2 (g) If G = 106 kj/mol for this reaction at 25 C, what is the K eq at 25 C? a) 7.21x10 12 b) 3.50x10 14 c) 3.81x10 18 d) 1.19x10 20 e) none of the above Section II: Calculations and reasoning must be shown (38 points total) 13. How much heat is required to raise the temperature of a 4.48 g sample of iron (C Fe = 0.450 J g 1 K 1 ) from 25.0 C to 79.8 C? (2 points) 14. Consider the evaporation of water at 25 C and 1 atm pressure, H 2 O() H 2 O(g). For this process what are the signs for ΔH, ΔS, and ΔG? Circle the correct answer (3 points) ΔH Negative Zero Positive Need more information ΔS Negative Zero Positive Need more information ΔG Negative Zero Positive Need more information Page 4 of 12
15. Consider the following reaction at 25 C for which the following data are relevant: 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) H = 110.02 kj/mol H f S N 2 O 5? 355.3 J mol 1 K 1 NO 2 33.15 kj/mol 239.9 J mol 1 K 1 O 2? 204.8 J mol 1 K 1 a) Calculate H f for N 2 O 5. (2 points) b) Calculate S for the reaction. (2 points) c) Sketch a ΔG versus temperature graph. Over what temperature range will this reaction be spontaneous at constant temperature and pressure? (4 points) Page 5 of 12
16. 0.75 moles of an ideal gas initially at 20.3 C undergoes a reversible isothermal expansion from 3.7 L to 8.4 L. What is the value of q? (3 points) 17. The boiling point of bromine, Br 2, is 58.8 C. For Br 2 () Br 2 (g), H = 30.9 kj/mol. What is S vap for Br 2 at 58.8 C? (2 points) 18. What is ΔH for the following reaction? (3 points) 2H 2 O 2 (g) 2H 2 O(g) + O 2 (g) Page 6 of 12
19. Calculate S for the following reaction done at 1 atm pressure. (6 points) H 2 (g, 25 C) + Cl 2 (g, 25 C) 2HCl(g, 300 C) Substance S (cal mol 1 K 1 ) at 25 C C p (cal mol 1 K 1 ) where T is temperature (K) H 2 (g) 31.21 6.95 0.20x10 3 T + 4.81x10 7 T 2 Cl 2 (g) 53.29 7.58 + 2.42x10 3 T 9.65x10 7 T 2 HCl(g) 44.65 6.74 + 0.43x10 3 T + 3.70x10 7 T 2 Page 7 of 12
20. Given the following data: molar mass of NH 3 = 17.03 g/mol 2NH 3 (g) N 2 (g) + 3H 2 (g) ΔH = 92 kj/mol 2H 2 (g) + O 2 (g) 2H 2 O(g) ΔH = 484 kj/mol a) Calculate ΔH for the reaction to make ammonia, NH 3. (2 points) 2N 2 (g) + 6H 2 O(g) 3O 2 (g) + 4NH 3 (g) b) Based on the enthalpy change, is energy released from the reaction or is energy required to make the reaction proceed? Circle the correct answer. (1 point) Released from reaction Energy must be put in for reaction to proceed c) How much energy is required to make 1.00 kg of NH 3 at constant pressure? (2 points) d) Based on the enthalpy change, is this reaction useful for the synthesis of ammonia, NH 3? Explain your reasoning. (1 point) Page 8 of 12
21. Given the following information at 25 C, Zn 2+ (aq) + 2e Zn(s) E = 0.763 V ZnS(s) Zn 2+ (aq) + S 2 (aq) K sp = 2.5x10 22 What is the standard half cell reduction potential, E for ZnS(s) + 2e Zn(s) + S 2 (aq)? (5 points) Page 9 of 12
Formula Sheet: Gas constant, R = 8.314 J mol 1 K 1 = 1.987 cal mol 1 K 1 = 0.08206 L atm mol 1 K 1 Faraday's constant, F = 96485 C mol 1 J = C V T(K) = T( C) +273.15 PV = nrt q = m c T w P dv q = n C T wnrt ln wnrt ln E = q + w E = nc v T H = E + PV H=q p H = nc p T H = E + (PV) H = (Energy of bonds broken) (Energy of bonds formed) H o =n prod H f o (products) n react H f o (reactants) C v = 1.5R E = 1.5nRT E = 1.5(PV) C p C v = R ln p p H R 1 T 1 T S universe = S system + S surroundings S o =n prod S o (products) n react S o (reactants) S q T dq nc T T dt S nc ln S nr ln S nr ln S G o = H o TS o G = H TS G = G o + RT ln Q G o = RT ln K = nfe G o =n prod G f o (products) n react G f o (reactants) 1 1 1 ln Page 10 of 12
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