Minhtran creates a practice exam for Laude’s CH302 Exam 1 If you find an answer that is incorrect, please e-mail Dr. Laude and he will check it. But rely on your own common sense and knowledge when looking at the answer key. 1. Which one of the following statements regarding substance stability is false: A substance is stable when a. ∆H formation is negative correct b. ∆G formation is negative c. The process of forming cause a total increase in entropy of the universe d. The entropy increase in the system is larger than the entropy increase in the surrounding 2. Given the reaction and thermodynamic data below, find the temperature range for which the reaction is spontaneous: 2N2 + 3H2 ↔2NH3 ∆H = -92.22 kJ/mol ∆S = -390.36 J/mol a. T > 4000K b. T< 36.75C c. T> 36.75C correct d. T>236.24C ∆G = -92.22 kJ– T* (- .39036kJ) In order for this reaction to be spontaneous, ∆G should be negative, or T > -92.22/(- .39036) = 236.24 K 3. Choose the correct answer: a. The first law of thermodynamics says that the entropy of the universe must be positive b. The first law of thermodynamics says that only spontaneous processes have an increase in entropy. c. The second law of thermodynamics says that the entropy of the universe is constant d. The third law of thermodynamics says that the absolute temperature of a perfect crystal at 0K is 0 correct 4. The entropy of the system increases during these chemical reactions: I Water Æ steam II H2(g) + O2(g) Æ H20(g) III C(s)+ O2(g) Æ CO2 (g) IV CO2(g) + H2(g) Æ CH4(g) + H2O (l) a. I and II b. I and III correct c. II and III d. III and IV 5. What is the entropy change when 2.4 g of ice melts at 0ºC and 1atm? (The heat of fusion is 6 kJ/mol) a. 2.93kJ b. 0.8 J c. 0.8 kJ d. 2.93 J correct 2.4 g = .1333 mole ∆H = .13333* 6 = .8 kJ Entropy change: ∆Ssys = ∆H/T = 800 J / 273 = 2.93 J 6. What is the change in surrounding entropy of this reaction at 300K 2N2 + 3H2 ↔2NH3 ∆H = -92.22 kJ/mol ∆S = -390.36 J/mol a. 390.36 J/K b. -390.36 J/Kc. 307.4 J/K correct d. -410.9 J/K ∆Ssurr = -∆H/T = 92 220 J / 300K = 307.4 J/K 7. Which of the compounds listed below is stable?: Compound Free energy of formation(kJ/mol) I SO2(g) -300 II HCN(g) 124.7 III PCl5(g) -405.5 IV NO(g) 86.55 I and III correct 8. Consider this reaction: 2N2 + 3H2 ↔2NH3 ∆H = -92.22 kJ/mol ∆S = -390.36 J/mol When temperature is increased, which way does the equilibrium shift and why? To the left because it is an exothermic reaction 9. Consider this reaction: N2 + 3H2 ↔2NH3 ∆H = -92.22 kJ/mol ∆S = -390.36 J/mol When pressure is decreased, which way does the equilibrium shift and why? a. The reaction shifts to the left. correct b. The reaction shifts to the right. c. There is no change in reaction products. d. More information is needed. To the left because the pressure and entropy of the system increase in that direction 10. Rank the vapor pressure for the compounds below from highest to lowest at room temperature: diamond, KI, C6H12(liquid), H2, CH3OH, CH3CH2CH2CH2OH, HCN(g). Answer: H2, HCN, C6H12, CH3OH, CH3CH2CH2CH2OH, KI, diamond. 11. Which of the answers below is best. NaCl dissolves in water because: a. The structure of NaCl is similar to that of water. b. NaCl only dissolves in warm water, when the molecules have lots of energy and can’t stay together. c. Water is polar and NaCl can dissociate into ions. Correct d. The sodium cations are attracted to the positive end of water molecules 12. (Draw a general phase diagram for a different substance from water) At a constant temperature of 100K as the pressure increases from .001 atm to 150 atm, what happens to this substance: a. It turns from a gas into solid Correct b. It turns from a gas into a liquid into a solid c. It turns into a super critical fluid d. It stays as a solid. 13. Draw the phase diagram of water. At what temperature can the water exist in three simultaneously?a. near 100oC b. near 0oC Correct c. above 100oC d. near 0K 14. What is the heat required to warm up 5g ethanol from 220K to 300K? Ethanol freezes at -26ºC and boils at 80ºC. Solid ethanol has a specific heat of 1 J/gºC and a heat of fusion of 3 kJ/g. Liquid ethanol has a specific heat of 0.5 J/gºC From 220 to 247 (or -26ºC): 5 g. * 1 J/gºC * 27ºC. = 135 J Melting at 247: 5 g * 3kJ/g = 15 kJ or 15 000 J From 247 to 300 : 5g * .5 J/gºC * 53ºC. = 132.5 J Total heat: 15 267.5 J 15. Which of the following affect gas solubility in liquid: I intermolecular force II temperature III pressure IV reaction with water a. I, III and IV b. Only I c. I, II and IV d. All of them correct 16. Which one of these solutions will have the least solubility in hexane (non-polar solvent) a. liquid detergent b. Glucose correct c. oil d. benzene 17. Which of the following solvents will be most miscible which each other? I NH3 liquid II benzene III CH3OH IV water a. water and CH3OH correct b. NH3 liquid and CH3OH c. NH3 liquid and water d. CH3OH and benzene 18. Which colligative property(ies) of the solvent decrease as we add more solute to the solvent: a. Vapor pressure b. Boiling point c. Osmosis d. Freezing point correct 19. Using Raoult’s Law, predict the change in vapor pressure of substance A (in a mixture of A and B) when its mole fraction changes from 0.5 to 0.8. The pure vapor pressure of A at this temperature is 100 torr. ∆P = P1-P2 = .8 * 100 torr - .5 *100 torr = 30 torr20. The vapor pressure of benzene is 76 torr at 20ºC. At what temperature does its pressure triple? (heat of vaporization of benzene is 25kJ/mol ln(3) = 25000/8.314 (1/293-1/T2) solve for T2 = 328K 21. Which one of these solutes would cause the largest increase in boiling point of water: a. .5 mole of NaF b. 1.5 mole glucose correct c. .1 mole chalk (CaCO3) d. .4 mole of H2CO3 22. Putting salt into water makes pasta cook more quickly because: a. salt dissociates into ions, which in turn dissolves the starch in pasta b. salt increases the boiling point of water correct c. salt increases vapor pressure d. the energy released from making that salt solution makes pasta more tender. 23. What is the new freezing point of water when 3 grams of NaCl is added? Kf = 2ºC/m a.
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