AP Chemistry Unit 4 Test Review Topics Covered: Thermochemistry Spontaneity, Entropy, & Free Energy Kinetics (Extra Credit) Section I: Multiple Choice (~50%) 1. A cube of ice is added to some hot water in a rigid, Energy Entrop. insulated container, which is then sealed. There is no (A) Remains constant Remains constant heat exchange with the surroundings. What has happened to the total energy and the total Entrop. (B) Remains constant Decreases when the system reaches equilibrium? (C) Remains constant Increases (D) Decreases Increases (E) Increases Decreases 2. When solid ammonium chloride, NH4Cl(s) is added ΔH ΔS to water at 25 °C, it dissolves and the temperature of A Postive Positive the solution decreases. Which of the following is true for the values of ΔH and ΔS for the dissolving B) Positive Negative process? C) Positive Equal to zero D Negative Positive E) Negative Negative Questions 3-7: The set of lettered choices is a list of symbols used in thermodynamics and thermochemistry. Select the one lettered choice that best fits each statement. A choice maybe used more than once or not at all. a. Ea b. ΔS c. Kc d. ΔG e. ΔH 3. Its value is negative for any exothermic reaction. 4. Its value is negative for any spontaneous reaction. 5. Its value changes when a catalyst is added to the system 6. Its value represents the change in randomness as the reaction proceeds. 7. Its value is determined by using concentrations at equilibrium in the mass action expression. 8. For which of the following is ΔH most nearly equal to ΔE? a. C2H5OH (l) C2H5OH (g) b. N2O4 (g) 2NO2 (g) c. H2 (g) + ½ O2 (g) H2O (g) d. H2 (g) + Cl2 (g) 2HCl (g) e. BaO2 (s) 2BaO2 (s) + ½ O2 (g) 9. Which applies to any reaction that proceeds spontaneously to form products from initial standard conditions? I. Keq > 1 (the equilbrium constant here represents that products are favored) II. ΔH < 0 III. ΔG < 0 a. I only b. II only c. I and II only d. II and III only e. I, II, and III 10. Is a gas in its standard state at 298 K a. Lithium b. Fluorine c. Bromine d. Uranium e. Nickel Section II: Free Response (~50%) Part A: Calculator may be used. CLEARLY SHOW THE METHOD USED AND STEPS INVOLVED IN ARRIVING AT YOUR ANSWERS. It is to your advantage to do this, because you may earn partial credit if you do and you will receive little or no credit if you do not. Attention should be paid to significant figures. Be sure to write all your answers to the questions on the lined pages following each question in this booklet. 1. C6H5OH(s) + 7 O2(g) 6 CO2(g) + 3 H2O(l) When a 2.000-gram sample of pure phenol, C6H5OH(s), is completely burned according to the equation above, 64.98 kilojoules of heat is released. Use the information in the table below to answer the questions that follow. Standard Heat of Absolute Entropy, Formation, Hâ; S, at 25C Substance at 25C (kJ/mol) (J/molòK) C(graphite) 0.00 5.69 CO2(g) -393.5 213.6 H2(g) 0.00 130.6 H2O(l) -285.85 69.91 O2(g) 0.00 205.0 C6H5OH(s) ? 144.0 (a) Calculate the molar heat of combustion of phenol in kilojoules per mole at 25C. (b) Calculate the standard heat of formation, Hâ, of phenol in kilojoules per mole at 25C. (c) Calculate the value of the standard free-energy change, G, for the combustion of phenol at 25C. (d) If the volume of the combustion container is 10.0 liters, calculate the final pressure in the container when the temperature is changed to 110.C. (Assume no oxygen remains unreacted and that all products are gaseous.) 2. C2H2(g) + 2 H2(g) C2H6(g) Information about the substances involved in the reaction represented above is summarized in the following tables. Substance S (J/molK) Hf (kJ/mol) C2H2(g) 200.9 226.7 H2(g) 130.7 0 C2H6(g) ---- -84.7 Bond Bond Energy (kJ/mol) C-C 347 C=C 611 C-H 414 H-H 436 (a) If the value of the standard entropy change, S, for the reaction is -232.7 joules per moleKelvin, calculate the standard molar entropy, S, of C2H6 gas. (b) Calculate the value of the standard free-energy change, G, for the reaction. What does the sign of G indicate about the reaction above? (d) Calculate the value of the CC bond energy in C2H2 in kilojoules per mole. 3. Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking. (a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and H2O(l). (b) Calculate the volume of air at 30C and 1.00 atmosphere that is needed to burn completely 10.0 grams of propane. Assume that air is 21.0 percent O2 by volume. (c) The heat of combustion of propane is -2,220.1 kJ/mol. Calculate the heat of formation, Hf, of propane given that Hf of H2O(l) = -285.3 kJ/mol and Hf of CO2(g) = -393.5 kJ/mol. (d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of water (specific heat = 4.18 J/g.K), calculate the increase in temperature of water. Part B: No calculator maybe used. For each of the following three reactions, in part (i) write a balanced equation for the reaction and in part (ii) answer the question about the reaction. In part (i), coefficients should be in terms of lowest whole numbers. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solutions as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. 1. (a) A 0.1M solution of potassium iodide is mixed with a 0.1M solution of lead II nitrate. (i) Balanced equation: (ii) What evidence of a reaction will be observed? Explain. (b) Sulfur dioxide is bubbled into water. (i) Balanced equation: (ii) Will the resulting product have a pH < 7, pH > 7, or pH = 7? Explain. (c) The complete combustion of hydrogen. (i) Balanced equation: (ii) Is this a spontaneous reaction? Explain. 2. BCl3(g) + NH3(g) Cl3BNH3(s) The reaction represented above is a reversible reaction. (a) Predict the sign of the entropy change, S, as the reaction proceeds to the right. Explain your prediction. (b) If the reaction spontaneously proceeds to the right, predict the sign of the enthalpy change, H. Explain your prediction. (c) The direction in which the reaction spontaneously proceeds changes as the temperature is increased above a specific temperature. Explain.
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