General Chemistry 1 CHM201 Unit 3 Practice Test

Transcription

1 General Chemistry 1 CHM201 Unit 3 Practice Test 1. Heat is best defined as a. a substance that increases the temperature and causes water to boil. b. a form of potential energy. c. a form of work. d. the total energy that a substance has. e. energy transferred as the result of a temperature difference. 2. The kinetic energy associated with the random motion of molecules is called a. motional energy. d. microscopic energy. b. work. e. thermal energy. c. heat. 3. Assuming that the charge of the ions remain constant in all cases, which of the following ion pairs has the greatest electrostatic potential energy (i.e., largest in magnitude)? a. d. b. e. c. 4. The following diagrams illustrate the flow of energy (q) and work (w) in different processes. Which one is definitely an exothermic process? a. c. b. d.

2 5. Which arrow in the following diagrams represents an endothermic phase transition? a. I and II only b. I and III only c. II and IV only d. III and IV only e. All represent exothermic transitions. 6. The first law of thermodynamics implies that a. energy is transferred from the surroundings to the system during a combustion reaction. b. if a system loses energy to the surroundings, then the surroundings must do an equal amount of work on the system. c. if the surroundings gain energy from the system, then the system must lose an equal amount of energy. d. energy is transferred from the system to the surroundings during a combustion reaction. e. if a system does work on the surroundings, then the surroundings must transfer an equal amount of energy to the system. 7. An expanding gas does 175 kj of work on its surroundings at a constant pressure of 5.55 atm. If the gas initially occupied 125 ml, what is the final volume of the gas? (101.3 J = 1 L atm) a L d. 311 L b. 31,500 ml e. 31,500 L c. 311 ml 8. A cooling curve for some substance is shown below. Which of the line segments (I V) represents cooling of the gas? a. I d. IV b. II e. V c. III

3 9. What will be the final temperature of a 10.0 g piece of iron (C P = J/mol ) initially at 25 C, if it is supplied with 9.5 J from a stove? a. d. b. e. c. 10. Using the following data for water, determine the final temperature when g of ice at 10.0 is heated with kj of energy. Boiling point 373 K Melting point 273 K Enthalpy of vaporization 2,260 J/mol Enthalpy of fusion 334 J/mol Specific heat capacity (solid) 2.11 J/(g K) Specific heat capacity (liquid) 4.18 J/(g K) Specific heat capacity (gas) 2.08 J/(g K) a. 309 d. 225 b. 100 e. 325 c Given the following thermochemical equation detailing the combustion of glucose C 6H 12O 6(s) 6O 2(g) CO 2(g) 6H 2O(g) H rxn = 2803 kj/mol C 6 H 12O 6 determine the amount of glucose needed to release 30 kj of energy. a g d. 1,930 g b g e g c. 467 g 12. Determine the enthalpy for the reaction, S(s) + O 2(g) SO 2(g), using the following data. S(s) + 3 / 2 O 2(g) SO 3(g) 2SO 2(g) + O 2(g) 2SO 3(g) ΔH = -395kJ ΔH = -198kJ a. 593 kj d kj b kj e. 989 kj c. 296 kj 13. For which reaction below does the enthalpy change under standard conditions correspond to a standard enthalpy of formation? a. 2H 2(g) C(s) CH 4(g) d. CO(g) H 2O(g) CO 2(g) H 2(g) b. CO 2(g) C(s) 2CO(g) e. CO 2(g) H 2(g) CO(g) H 2O(g) c. 2NO 2(g) N 2O 4(g)

4 14. Ethanol is being promoted as renewable biofuel. Use the standard enthalpies of formation below to determine the standard enthalpy of combustion of ethanol. CH 3CH 2OH(l) + 3O 2(g) 2CO 2(g) + 3H 2O(l) Substance CH 3CH 2OH(l) CO 2(g) H 2O(l) H in kj/mol a. 1,280 kj/mol d. 1,430 kj/mol b. 400 kj/mol e. 1,370 kj/mol c kj/mol 15. If the pressure of a sample of gas triples, by what factor does the volume change, assuming constant temperature? a. 1/3 d. 9 b. 3 e. 1/9 c Four containers, each with the same volume and at the same temperature, are shown in the following diagrams. Which container is at the highest pressure? (Each dot represents one mole of the gas.) a. c. b. d. 17. A balloon vendor at a street fair is using a tank of helium to inflate the balloons. The tank has a volume of 75 L and a pressure of 95 atm at 22 C. The valve is not closed completely and some helium leaks out. After some time, the pressure drops to 85 atm. How many moles of helium have been lost? a mol d. 263 mol b. 1.2 mol e. 31 mol c. 73 mol 18. What is the temperature if 3.0 g of propane (C 3H 8) in a ml container exerts a pressure of 5.5 atm? a C d. 175 C b C e. 264 C c. 121 C 19. Which of the following reactions will result in a negligible change in pressure? a. Na(s) Cl 2(g) NaCl(s) b. CO(g) H 2O(l ) CO 2(g) H 2(g) c. CH 4(g) H 2O(l ) CO(g) 3H 2(g) d. BrNO 2(g) NO(g) NO 2(g) BrNO(g) e. 2Na(s) 2H 2O(g) 2NaOH(s) H 2(g)

5 g of an unknown gas occupies 1.36L at torr and 30.0 C. Which of the following could be the unknown gas? a. CS 2 d. NH 3 b. NO e. NO 2 c. N 2O 21. In which of the following containers is the mole fraction of helium greatest? The white circles represent helium atoms; the dark circles represent argon atoms. a. c. b. d. 22. What is the mole fraction of H 2 in a mixture with a total pressure of 800 torr when the partial pressure of H 2 is 90 torr? a d. 0.5 b. 8.8 e c. 1.0

6 CHM201 Unit 3 Practice Test Answer Section MULTIPLE CHOICE 1. ANS: E This is a definition. PTS: 1 DIF: Easy REF: 5.1 OBJ: Define the term heat. MSC: Remembering 2. ANS: E This is the definition of thermal energy. PTS: 1 DIF: Easy REF: 5.2 OBJ: Identify forms of energy and their characteristics. 3. ANS: B MSC: Remembering Collect and Analyze: There are five distances to choose from, 10pm, 3pm, 5pm, 12pm and 8pm. The charges are the same in each diagram. Electrostatic potential is computed by the followin equation: E ( Q Q ) 1 2 el d The distance, d, is the denominator, and the numerator is constant. As the distance, d, increases, the energy must decrease. The smallest distance must be the largest energy, E. The math doesn t deceive! PTS: 1 DIF: Medium REF: 5.2 OBJ: Describe how the electrostatic potential energy varies with the charge on two particles and the distance between them. MSC: Understanding

7 4. ANS: C In a, b and d,, no heat is leaving the system, so none of these is exothermic. In c, heat is leaving the system, therefore the process is exothermic. Work is not a factor in endothermy and exothermy. PTS: 1 DIF: Easy REF: 5.3 OBJ: Define the terms endothermic and exothermic, and identify processes as exothermic or endothermic. MSC: Understanding 5. ANS: B I - requires an input of energy (products are in a higher energy state), and is therefore endothermic. II - releases energy (products are in a lower energy state), and is therefore exothermic. III - requires an input of energy (products are in a higher energy state), and is therefore endothermic. II - releases energy (products are in a lower energy state), and is therefore exothermic. I and III represent endothermic phase changes. PTS: 1 DIF: Easy REF: 5.3 OBJ: Define the terms endothermic and exothermic, and identify processes as exothermic or endothermic. MSC: Remembering 6. ANS: C This is the mathematical statement of the First Law of Thermodynamics, the Conservation of Energy. E system = E surroundings PTS: 1 DIF: Medium REF: 5.3 OBJ: Identify how the internal energy of a system or the surroundings changes as the result of some process. MSC: Understanding

8 7. ANS: D Collect and Analyze: We are given: w = 175kJ P = 5.55 atm Using, we can substitute, Solve the equation for V f,, A very large amount of work should expand the gas by a very large amount. PTS: 1 DIF: Medium REF: 5.3 OBJ: Explain what is meant by pressure volume work, and calculate pressure volume work for a given situation. MSC: Applying 8. ANS: A The highest temperature always represents the gas phase. I - Cooling gas II - Condensing gas III - Cooling liquid IV - Freezing liquid V - Cooling Solid PTS: 1 DIF: Easy REF: 5.5 OBJ: Plot and analyze a heating/cooling curve in terms of phase changes. MSC: Understanding

9 9. ANS: B Collect and Organize: 10.0 g of Fe C P = J/mol?9 T i = 25 q = 9.5 J Analyze: We are asked to solve for the final temperature, T f. q = nc PΔT and ΔT = T f T I and n = m/?9m, substituting, q = (m/?9m)c P(T f T I) Applying your algebra: Plug in the values and do the math: We added heat, so the temperature went up. PTS: 1 DIF: Easy REF: 5.5 OBJ: Use heat capacity, specific heat capacity, and molar heat capacity appropriately in analysis or calculations. MSC: Applying

10 10. ANS: E Collect and Organize: T i = C m = g q = kj c s (ice) = 2.11 J/g = 1 x 10-3 kj/g ΔH fusion = 334 J/g = kj/g c s (water) = 4.18 J/g = x 10-3 kj/g ΔH vap = 334 J/g = kj/g c s (water) = 2.08 J/g = x 10-3 kj/g Analyze: First the ice must warm to 0 C, so we need the specific heat capacity of ice. We apply and check if we have used all 350 kj. If we haven t, we then check to see how much heat it will take to melt the ice. We apply check again. and If there is energy left, use the specific heat of liquid water. We apply. If there is energy left, we apply. If there is energy left, we solve. for ΔT and add that value to 100 C First heat the ice. Next, melt the ice.. Plenty of heat left. Next, heat the water.. Still plenty of heat left. Now evaporate the water.. Still plenty left.. Now we need to know how much heat we have left.

11 Solve, for ΔT T = T boiling + ΔT = 100 C C = 335 C The change in temperature may be expressed in either scale, since the units are of the same magnitude. We went from a chunk of ice to very hot steam by adding a a large amount of heat. PTS: 1 DIF: Medium REF: 5.5 OBJ: Calculate the energy needed to raise or lower the temperature of a substance through phase transitions. MSC: Applying 11. ANS: B Collect and Organize: ΔH rxn = kj/mol q = -30kJ (exothermic) Analyze: We are asked to determine the mass of glucose needed to produce 30kJ of heat. Since we are given a molar value, we will first have to determine how many moles of glucose are required, so we will have to determine the molar mass of glucose from its formula, C 6H 12O 6. This step is left to the solver (old stuff). Since only a small fraction of the molar value of heat was produced, only a small fraction of a mole of glucose was required. PTS: 1 DIF: Medium REF: 5.6 OBJ: Use thermochemical equations to determine the amount of energy released during a reaction or the mass of a substance needed to generate a given amount of energy. MSC: Applying

12 12. ANS: C Collect and Organize: We are given two reactions to determine the enthalpy for the reaction, S(s) + O 2(g) SO 2(g). S(s) + 3 / 2 O 2(g) SO 3(g) 2SO 2(g) + O 2(g) 2SO 3(g) ΔH = -395kJ ΔH = -198kJ Analyze: We must use Hess s Law. The first reaction has S(s) as a reactant, just as the target reaction. It also has the same coefficient, so it appears we can use this reaction as is. The second reaction has the product (SO 2) of the target, but it appears as a reactant, so we will have to reverse the reaction and negate the enthalpy. Its coefficient is twice that of the target, so we will have divide all the coefficients and the enthalpy. If the sum of the first reaction and the altered second reaction are equal to the target, the sum of the enthalpies will equal the enthalpy of the target reaction. Alter the second equation: Reverse it 2SO 3(g) SO 2(g) + O 2(g) ΔH = +198kJ Divide it by two SO 3(g) SO 2(g) + ½ O 2(g) ΔH = +99kJ Add the two and cancel whatever appears on both sides S(s) + 3 / 2 O 2(g) SO 3(g) SO 3(g) SO 2(g) + ½ O 2(g) S(s) + O 2(g) SO 2(g) ΔH = -395kJ ΔH = +99kJ ΔH = -296kJ Enthalpy is a state function, so we don t care how we got here only where we started and ended up. PTS: 1 DIF: Medium REF: 5.7 OBJ: Relate reactions to each other, and calculate enthalpies of reaction using Hess s law. MSC: Applying

13 13. ANS: A A standard enthalpy of formation is defined as the enthalpy of a reaction in which a compound is formed from its elements in their most stable state. a. methane is formed from hydrogen gas and solid carbon - this is the one! b, carbon dioxide is not an element - NO. c. nitrogen dioxide is not an element - NO d. carbon monoxide is not an element - NO. e. carbon dioxide is not an element - NO. PTS: 1 DIF: Easy REF: 5.8 OBJ: Write or identify the reaction associated with an enthalpy of formation. MSC: Understanding 14. ANS: E Collect and Organize: We are given CH 3CH 2OH(l) + 3O 2(g) Substance CH 3CH 2OH(l) CO 2(g) H 2O(l) 2CO 2(g) + 3H 2O(l) H in kj/mol Analyze: We will use the standard enthalpies of formation to determine the enthalpy of the given reaction. We expect a combustion reaction to be exothermic. PTS: 1 DIF: Medium REF: 5.8 OBJ: Relate enthalpies of formation and enthalpies of reaction. MSC: Applying

14 15. ANS: A Collect and Organize: The pressure triples on a sample of a gas. Analyze: n is constant (a sample of a gas) T is constant (Charles s Law) That makes this a Boyle s Law problem. Volume is inversely proportional to pressure, so, Assume V 1 = 1, P 1 = 1, P 2 = 3 Solve Boyle s law for V 2, It s an inverse relationship. This is all you really have to know. PTS: 1 DIF: Easy REF: 6.3 OBJ: Relate pressure and volume changes: Boyle s law. 16. ANS: D MSC: Understanding From the combined gas law. More moles = more pressure. PTS: 1 DIF: Easy REF: 6.4 OBJ: Describe the predictions of the ideal gas law. MSC: Understanding

15 17. ANS: E Collect and Organize: V = 75L (does not change) P 1 = 95 atm P 2 = 85 atm T = 22 C = 295K (does not change) Incidentally, we already know R = L?9 atm/mol?9 K Analyze: We are only measuring the amount lost, and the change in pressure represents P in the ideal gas equation. PV = nrt Solve for moles: Pressure lost = P 1 P 2 = 95atm 85atm = 10 atm This is simply an application of the Ideal Gas Law. PTS: 1 DIF: Medium REF: 6.4 OBJ: Use the ideal gas law in analysis and calculations. MSC: Applying

16 18. ANS: C Collect and Organize: V = 400.0mL= 0.400L P = 5.5 atm T =? m propane= 3.0g Incidentally, we already know R = L atm/mol K Analyze: We will have to determine the number of moles of propane to use the Ideal Gas Law. PV = nrt Solve for T, Substitute for n, Plug in the values you know (you must compute the molar mass of propane), solve and convert the value to Celsius (the answer selections are in C). 394K 273K = 121 C Another application of the Ideal Gas Law. PTS: 1 DIF: Medium REF: 6.4 OBJ: Use the ideal gas law in analysis and calculations. MSC: Applying

17 19. ANS: D a. More moles of gaseous reactants than products - pressure decrease b. More moles of gaseous products than reactants - pressure increase c. More moles of gaseous products than reactants - pressure increase d. Same number of moles of gaseous products than reactants - pressure should be constant e. More moles of gaseous reactants than products - pressure decrease PTS: 1 DIF: Easy REF: 6.5 OBJ: Relate the quantity of a gas consumed or produced in a chemical reaction to quantities of reactants and products. MSC: Understanding

18 20. ANS: C Collect and Organize: m = 2.34g V = 1.36L P = 750torr = atm T = 30 C = 303K Of course, R = L atm/mol K Analyze: We will have to use the Ideal Gas Law PV = nrt and substitute for n, and solve for molar mass, Plug in the known values and solve Of the selections, c.no 2, has the molar mass (44) closest to the answer Another application of the Ideal Gas Law. PTS: 1 DIF: Medium REF: 6.6 OBJ: Calculate the molar mass of a gas from its density. MSC: Applying

19 21. ANS: C By the definition of mole fraction, X a. b. c. d. PTS: 1 DIF: Easy REF: 6.7 OBJ: Relate partial pressures, mole fractions, masses, and total pressures of gases in a mixture. MSC: Understanding

20 22. ANS: A Collect and Organize: Analyze: The definition of mole fraction: Plug in the values, This is Dalton s Law. PTS: 1 DIF: Easy REF: 6.7 OBJ: Relate partial pressures, mole fractions, masses, and total pressures of gases in a mixture. MSC: Applying