AP Chemistry: Chapter 6 Thermochemistry Section 1: Multiple Choice MC#1 CH4(g) + 2 O2(g) ---> CO2(g) + 2 H2O(l); ΔH = - 889.1 kJ ΔHf° H2O(l) = -285.8 kJ / mole ΔHf° CO2(g) = - 393.3 kJ / mole What is the standard heat of formation of methane, ΔHf° CH4(g), as calculated from the data above? (A) -210.0 kJ/mole (B) -107.5 kJ/mole (C) -75.8 kJ/mole (D) 75.8 kJ/mole (E) 210.0 kJ/mole MC#2 Which of the following is a graph that describes the pathway of reaction that is endothermic and has high activation energy? (A) (C) (B) (D) (E) MC#3 H2(g) + 1/2 O2(g) ---> H2O(l) Δ H° = x 2 Na(s) + 1/2 O2(g) ---> Na2O(s) Δ H° = y Na(s) + 1/2 O2(g) + 1/2 H2(g) ---> NaOH(s) Δ H° = z Based on the information above, what is the standard enthalpy change for the following reaction? Na2O (s) + H2O (l) ---> 2 NaOH (s) (A) x + y + z (B) x + y - z (C) x + y - 2z (D) 2z - x - y (E) z - x - y MC#4 The energy diagram for the reaction X + Y ---> Z is shown below. The addition of a catalyst to this reaction would cause a change in which of the indicated energy differences? (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III MC#5 Which of the following best describes the role of the spark from the spark plug in an automobile engine? (A) The spark decreases the energy of activation for the slow step. (B) The spark increases the concentration of the volatile reactant. (C) The spark supplies some of the energy of activation for the combustion reaction. (D) The spark provides a more favorable activated complex for the combustion reaction. (E) The spark provides the heat of vaporization for the volatile hydrocarbon. MC#6 C2H4(g) + 3 O2(g) --> 2 CO2(g) + 2 H2O(g) For the reaction of ethylene represented above, ΔH is - 1,323 kJ. What is the value of ΔH if the combustion produced liquid water H2O (l), rather than water vapor H2O(g)? (ΔH for the phase change H2O(g) --> H2O(l) is -44 kJ mol¯1.) (A) -1,235 kJ (B) -1,279 kJ (C) -1,323 kJ (D) -1,367 kJ (E) -1,411 kJ MC#7 I2(g) + 3 Cl2(g) ---> 2 ICl3(g) According to the data in the table below, what is the value of ΔH° for the reaction represented above? Bond Average Bond Energy (kj/mole) I---I 150 Cl---Cl 240 I---Cl 210 (A) - 870 kJ (B) - 390 kJ (C) + 180 kJ (D) + 450 kJ (E) + 1,260 kJ Section II: Free Response FRQ #1 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 30°C 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, ΔHf°, of propane given that ΔHf° of H2O(l) = -285.3 kJ/mol and ΔHf° 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 . FRQ#2 N2(g) + 3 F2(g) 2 NF3(g) ΔH° = – 264 kJ mol–1; The enthalpy change in a chemical reaction is the difference between energy absorbed in breaking bonds in the reactants and energy released by bond formation in the products. (a) How many bonds are formed when two molecules of NF3 are produced according to the equation in the box above? (b) Use both the information above and the table of average bond enthalpies below to calculate the average enthalpy of the F–F bond. Average Bond Enthalpy Bond (kJ mol-1) N 946 N–F 272 F–F ? FRQ#3 CO(g) + O2(g) CO2(g) The combustion of carbon monoxide is represented by the equation above. (a) Determine the value of the standard enthalpy change, ∆H˚rxn for the combustion of CO(g) at 298 K using the following information. C(s) + O2(g) CO(g) ∆H˚298 = –110.5 kJ mol-1 C(s) + O2(g) CO2(g) ∆H˚298 = –393.5 kJ mol-1 FRQ #4 Enthalpy of Combustion, ∆H° Substance (kiloJoules/mol) C(s) -393.5 H2(g) -285.8 C2H5OH(l) -1366.7 H2O(l) -- (a) Write a separate, balanced chemical equation for the combustion of each of the following: C(s), H2(g), and C2H5OH(l). Consider the only products to be CO2 (g) and/or H2O(l). (b) In principle, ethanol can be prepared by the following reaction: 2 C(s) + 2H2(g) + H2O(l) C2H5OH(l) Calculate the standard enthalpy change, ∆H°, for the preparation of ethanol, as shown in the reaction above. FRQ#5 ∆Hf° Compound (kilocalories/mole) H2O(l) -68.3 CO2(g) -94.1 O2(g) 0.0 C3H8 ? When 1.000 gram of propane gas, C3H8, is burned at 25°C and 1.00 atmosphere, H2O(l) and CO2(g) are formed with the evolution of 12.03 kilocalories. (a) Write a balanced equation for the combustion reaction. (b) Calculate the molar enthalpy of combustion, ∆H°comb, of propane. (c) Calculate the standard molar enthalpy of formation, ∆Hf°, of propane gas. Equations: For the reactions below write a balanced NET ionic reaction and predict what type of question will be asked with the reaction: a. carbon dioxide is bubbled into water b. equimolar solutions of ammonium chloride and potassium hydroxide are mixed. c. hydrogen peroxide is catalytically decomposed. d. the electrolysis of water e. equimolar solutions of barium nitrate and potassium fluoride are mixed.