UNIT 7 REVIEW ANSWERS

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1 Heat, Temperature, First Law and Calorimetry UNIT 7 REVIEW ANSWERS 1. Object B has been in a beaker of boiling water for about 5 minutes. Using tongs, Object B is pulled from the beaker and placed in a cup of chilled water. Which of the following would occur in the next few minutes? Select all that apply. a. The temperature of Object B would decrease. b. The temperature of the water would increase. c. Energy would be transferred from Object B to the water. d. There would be a flow of cold from the water to Object B. e. The temperature of the water would eventually become greater than Object B's temperature. 2. The specific heat of a substance is. a. the amount of heat contained in a gram of a substance at a specific temperature. b. the amount of energy required to raise the temperature of the substance by C. c. the amount of energy transferred between two substances of a different temperature d. the amount of energy required to raise the temperature of 1 gram of the substance by 1 C. 3. A material which has a high specific heat will. a. undergo a large temperature change when a relatively small amount of heat is added to it. b. undergo a small temperature change when a relatively large amount of heat is added to it. c. be capable of acquiring a relatively large amount of heat before it finally changes state. d. be capable of undergoing a large temperature change before it finally changes state. 4. What is the difference between temperature and heat? a. Heat is a form of chemical energy and temperature is a form of thermal energy b. Heat is the flow of energy and temperature is a measure of a sample's kinetic energy. c. Heat is a conserved quantity that never changes and temperature is always capable of change. d. Heat is the energy that flows out of an object and temperature is the energy contained in an object. e. Heat and temperature are two identical and interchangeable terms. 5. A ml sample of water (density = 1 g/ml) is observed to increase its temperature from 23.4 C to 38.2 C. How much heat (in Joules) is gained by the water? a b c d e Endothermic, Exothermic, Enthalpy Change 6. Identify the following process as being either endothermic or exothermic: The ice cube placed in a glass of lemonade slowly melts. a. endothermic b. exothermic 7. Which of the following statements are true of exothermic reactions? Select all that apply. a. Energy is released to the surroundings. b. The enthalpy of reactants is less than the enthalpy of products. c. The temperature of the surroundings decreases. d. Heat would be a product term in the balanced chemical reaction.

2 Energy Energy 8. Endothermic reactions cause energy to the surroundings. a. be released to b. be absorbed from c. be conserved by 9. Consider the energy level diagram at the right. The reaction represented by this diagram is a(n) reaction. a. endothermic b. exothermic c. activated d. caloric Rxn Coordinate 10. The energy level diagram at the right represents the energy of the system. Identify each section on the diagram. A. enthalpy of reactants B. enthalpy of products C. change in enthalpy D. activation energy Rxn Coordinate Thermal Stoichiometry 11. The heat of combustion of butane fuel (C 4H 10) is kj/mol. Which one of the following is the proper thermochemical equation for the combustion of butane fuel? a. 2 C 4H 10(g) + 13 O 2(g) kj 8 CO 2(g) + 10 H 2O(g) b. 2 C 4H 10(g) + 13 O 2(g) kj 8 CO 2(g) + 10 H 2O(g) c. 2 C 4H 10(g) + 13 O 2(g) kj 8 CO 2(g) + 10 H 2O(g) d. 2 C 4H 10(g) + 13 O 2(g) kj 8 CO 2(g) + 10 H 2O(g) e. 2 C 4H 10(g) + 13 O 2(g) 8 CO 2(g) + 10 H 2O(g) kj ab. 2 C 4H 10(g) + 13 O 2(g) 8 CO 2(g) + 10 H 2O(g) kj ac. 2 C 4H 10(g) + 13 O 2(g) 8 CO 2(g) + 10 H 2O(g) kj ad. 2 C 4H 10(g) + 13 O 2(g) 8 CO 2(g) + 10 H 2O(g) kj

3 12. Which of the following statement(s) is(are) true of endothermic reactions? Select all that apply. a. q < 0 b. H system < 0 c. H products > H reactants d. Heat is absorbed by the system from the surroundings. e. There is a positive enthalpy change. 13. Identify the following statement as being either TRUE or FALSE: When a thermochemical equation is written for an endothermic reaction, a heat term (e.g., 343 kj) will be included on the reactant side of the equation. a. True b. False 14. The heat of fusion of ice is kj/mole. Suppose that 9.00 grams of water are placed into the freezer. What quantity of heat must be released to freeze this ice? a b c d e Consider the thermochemical equation below: H 2(g) + Cl 2(g) 2 HCl(g) kj If 2.00 mol of H 2 react with unlimited Cl 2, then kj of heat will be released. a. 4.0 b c. 185 d. 370 Heat of Formation and Hess's Law 16. Given the following thermochemical equation 2 H 2(g) + O 2(g) 2 H 2O(g) kj what is the heat of formation (kj/mol) of water? a. 286 b. 572 c d. None of these. 17. The H f values for elements in their stable state at standard conditions is. a. always a positive value b. always a negative value c. zero

4 Phases of Matter, Heating Curves and Phase Diagrams 18. Which of the following traits are characteristic of a liquid? Select all that apply. a. Particles exhibit an orderly arrangement. b. Particles are unable to move about the bulk of the container c. Particles are spread relatively far apart. d. Particles exert very weak forces upon each other. 19. Solid Liquid Gas Type and Degree of Motion #1 c #2 b #3 a Degree of Organization and Order #4 b #5 c #6 a Spatial Separation Between Particles #7 b #8 b #9 a For Questions #1-3, choose from the following choice: a. Particles move freely about the container in random motion at high speeds. b. Particles are relaively free to move; weak forces will impede totally free movement. c. Particles are locked in place and have no freedom of any movement besides vibration. For Questions #4-6, choose from the following choice: a. Particles exhibit no organization whatsoever b. Particles are highly organized and held in place by strong forces. c. Particles are mostly disordered with some regions of localized order. For Questions #7-9, choose from the following choice: twice.) (Yes. You will have to use one answer a. Particles are spaced relatively far apart. b. Particles are spaced relatively close together.

5 Temperature ( C) E D C B A Time (sec) Consider the heating curve above in answering the next several questions. 20. At which of the following times is only liquid present in the sample of this unknown material? Select all that apply. a. 100 s b. 200 s c. 400 s d. 600 s e. 800 s 21. At which of the following times is a gas present in the sample of this unknown material? Select all that apply. a. 100 s b. 200 s c. 400 s d. 600 s e. 800 s 22. At which of the following times is the sample melting? a. 100 s b. 200 s c. 400 s d. 600 s e. 800 s e. The sample is never melting. Consider the phase diagram of carbon dioxide. Use it to answer the next several questions.

6 23. A sample of carbon dioxide is present at standard pressure and -100 C. The temperature is then increased by 100 C. What phase change occurs? Select all that apply. a. melting b. freezing (fusion) c. vaporization d. condensation e. sublimation ab. deposition 24. A sample of carbon dioxide is present at standard temperature and pressure. The pressure is then increased to 100 atm. What phase change occurs? Select all that apply. a. melting b. freezing (fusion) c. vaporization d. condensation e. sublimation ab. deposition 25. A sample of carbon dioxide is present at a pressure of 100 atm and -80 C. The temperature is then increased to room temperature. What phase change occurs? Select all that apply. a. melting b. freezing (fusion) c. vaporization d. condensation e. sublimation ab. Deposition

7 Entropy and Spontaneity and Gibbs Free Energy 26. Which of the following systems has a greater entropy? a. A box of 500 puzzle pieces. b. A completed 500piece puzzle. 27. Identify whether the following process would be characterized by an increase or a decrease in entropy. 2 H 2(g) + 2 O 2(g) 2 H 2O(g) kj a. An increase in entropy b. A decrease in entropy. 28. A spontaneous reaction is a reaction that. a. takes place rapidly b. produces its own energy c. occurs naturally, once activated d. is not planned for 29. Liquid water freezes. This process would be characterized by a. Select all that apply. a. increase in enthalpy b. decrease in enthalpy c. increase in entropy d. decrease in entropy 30. A reaction with a value of Gibbs free energy change ( G) is definitely a reaction. Select all that apply. a. positive, spontaneous b. negative, spontaneous c. positive, nonspontaneous d. negative, nonspontaneous e. Nonsense! No such claim can be made. 31. A reaction is characterized by the following thermodynamic values: H = kj S = J/K This reaction will be a. spontaneous at all temperatures b. never spontaneous c. spontaneous only at low temperatures d. spontaneous only at high temperatures 32. The heat of combustion of sulfur is kj/mol. The product of the reaction is sulfur trioxide. a. Write the balanced thermochemical equation for the combustion of sulfur. Include the heat term in the equation on the proper side. 2S + 3 O 2 2 SO kj or S + 3/2 O 2 SO kj b. Determine the amount of energy released by the combustion of 16.1 g of aluminum. 16.1g S x 1mol S/32.1g S x kj/mol S=198 kj

8 Use the provided table of H f values in solving the following problems: Compound H f (kj/mol) Compound H f (kj/mol) CH 4 (g) HCl(g) CO 2 (g) H 2 O(g) NaCl(s) SO 2 (g) H 2 O(l) NH 4 Cl(s) H 2 S(g) NO(g) H 2 SO 4 (l) NO 2 (g) MgSO 4 (s) SnCl 4 (l) MnO(s) SnO(s) MnO 2 (s) SnO 2 (s) NaCl(s) SO 2 (g) NaF(s) SO 3 (g) NaOH(s) ZnO(s) NH 3 (g) ZnS(s) Determine the H for the following reaction: CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2O(l) [ 1mol( kj/mol) + 2mol(0kJ/mol)]-[1mol(-74.8kJ/mol)+2(0kJ/mol)]= kj 34. Jack and Jill are doing the Heat of Combustion of a Candle lab. The candle is made of paraffin (C 25H 52). They are trying to determine the molar heat of combustion of paraffin in kj/mol. They determine the mass of the paraffin burned by massing the candle before and after the experiment. They use a water-filled metal calorimetry cup to absorb the heat released by the burning paraffin. Their data is shown below. Mass of Candle Before Burning Mass of Candle After Burning Volume of water (density = 1 g/ml) g g ml Initial temperature of water 7.5 C Final temperature of water 42.8 C a. Determine the heat released by the burning paraffin (to three significant digits). PSYW 250.0g x 4.18J/g 0 C x ( C C)=36,900 J

9 b. Determine the moles of paraffin (C 25H 52) that are burned (to three significant digits). PSYW (32.56g-31.49g) x (1 mol/352.0g)= mol c. Determine the molar heat of combustion of paraffin in kj/mol (to three significant digits). PSYW [(36,900J x 1kJ/1000J])/ mol=12,100 kj/mol 35. Calculate H for the reaction CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g), from the following data. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) H = kj C (s) + 2 H 2 (g) CH 4 (g) H = kj H 2 (g) + 2 C (s) + N 2 (g) 2 HCN (g) H = kj 2 NH 3 (g) N 2 (g) + 3 H 2 (g) 91.8 kj 2 CH 4 (g) 2 C (s) + 4 H 2 (g) 2 x 74.9 kj + H 2 (g) + 2 C (s) + N 2 (g) 2 HCN (g) kj 2 CH 4 (g) + 2 NH 3 (g) 2 HCN (g) + 6 H 2 (g) kj CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g) kj 36. Consider the following reaction with the listed H and S values. 2 NO(g) + O 2(g) 2 NO 2(g) ΔH = 114 kj; ΔS = 147 J/K a. Determine the Gibb's free energy change at a temperature of 75 C. G = -114 kj 348K [(-147 J/K) x (1kJ/1000J)]=-63 kj b. Is this reaction spontaneous at 75 C? YES ( Place answer in blank.)

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