1 2004 B 2 Fe(s) + 3 2 O 2(g) Fe 2 O 3 (s) H f = -824 kj mol 1 Iron reacts with oxygen to produce iron(iii) oxide as represented above. A 75.0 g sample of Fe(s) is mixed with 11.5 L of O 2 (g) at 2.66 atm and 298 K. (a) Calculate the number of moles of each of the following before the reaction occurs. (i) Fe(s) (ii) O 2 (g) (b) Identify the limiting reactant when the mixture is heated to produce Fe 2 O 3. Support your answer with calculations. (c) Calculate the number of moles of Fe 2 O 3 produced when the reaction proceeds to completion. (d) The standard free energy of formation, G f of Fe 2 O 3 is 740. kj mol 1 at 298 K. (i) Calculate the standard entropy of formation S f of Fe 2 O 3 at 298 K. Include units with your answer. (ii) Which is more responsible for the spontaneity of the formation reaction at 298K, the standard enthalpy or the standard entropy? The reaction represented below also produces iron(iii) oxide. The value of H for the reaction is 280 kj per mol. 2 FeO(s) + 1 2 O 2(g) Fe 2 O 3 (s) (e) Calculate the standard enthalpy of formation, H f of FeO(s).
2 2005 D [repeated in electrochem] AgNO 3 (s) Ag + (aq) + NO 3 (aq) The dissolving of AgNO 3 (s) in pure water is represented by the equation above.. (a) Is G for the dissolving of AgNO 3 (s) positive, negative, or zero? Justify your answer. (b) Is S for the dissolving of AgNO 3 (s) positive, negative, or zero? Justify your answer. (c) The solubility of AgNO 3 (s) increases with increasing temperature. (i) What is the sign of H for the dissolving process? Justify your answer. (ii) Is the answer you gave in part (a) consistent with your answers to parts (b) and (c) (i)? Explain. The compound NaI dissolves in pure water according to the equation NaI(s) Na + (aq) + I (aq). Some of the information in the table of standard reduction potentials given below may be useful in answering the questions that follow. Half-reaction E (V) O 2 (g) + 4 H + + 4 e- 2 H 2 O(l) 1.23 I 2 (s) + 2 e- 2 I 0.53 2 H 2 O(l) + 2 e- H 2 (g) + 2 OH -0.83 Na + + e- Na(s) -2.71 (d) An electric current is applied to a 1.0 M NaI solution. (i) Write the balanced oxidation half reaction for the reaction that takes place. (ii) Write the balanced reduction half-reaction for the reaction that takes place. (iii) Which reaction takes place at the anode, the oxidation reaction or the reduction reaction? (iv) All electrolysis reactions have the same sign for G. Is the sign positive or negative? Justify your answer.
3 2006 B CO(g) + 1 2 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) + 1 2 O2(g) CO(g) H 298 = 110.5 kj mol-1 C(s) + O2(g) CO2(g) H 298 = 393.5 kj mol -1 (b) Determine the value of the standard entropy change, S rxn, for the combustion of CO(g) at 298 K using the information in the following table. Substance S 298 (J mol -1 K -1 ) CO(g) 197.7 CO2(g) 213.7 O2(g) 205.1 (c) Determine the standard free energy change, G rxn, for the reaction at 298 K. Include units with your answer. (d) Is the reaction spontaneous under standard conditions at 298 K? Justify your answer. (e) Calculate the value of the equilibrium constant, Keq, for the reaction at 298 K.
4 2007 part A, question #2 o N2(g) + 3 F2(g) 2 NF3(g) H 298 278 J K 1 mol 1 = 264 kj mol 1 o ; S 298 = The following questions relate to the synthesis reaction represented by the chemical equation in the box above. o (a) Calculate the value of the standard free energy change, G 298 for the reaction. (b) Determine the temperature at which the equilibrium constant, Keq, for the reaction is equal to 1.00. (Assume that H and S are independent of temperature.) (c) Calculate the standard enthalpy change, H, that occurs when a 0.256 mol sample of NF3(g) is formed from N2(g) and F2(g) at 1.00 atm and 298 K. 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. (d) How many bonds are formed when two molecules of NF3 are produced according to the equation in the box above? (e) Use both the information in the box above and the table of average bond enthalpies below to calculate the average enthalpy of the F F bond. Bond Average Bond Enthalpy (kj mol -1 ) N N 946 N F 272 F F?
5 2007 part A, form B, question #1 A sample of solid U308 is placed in a rigid 1.500 L flask. Chlorine gas, Cl2(g), is added, and the flask is heated to 862 C. The equation for the reaction that takes place and the equilibriumconstant expression for the reaction are given below. U308(s) + 3 Cl2(g), 3 UO2Cl2(g) + O2(g) K P (P UO 2 Cl 2 ) 3 (P O2 ) (P Cl2 ) 3 When the system is at equilibrium, the partial pressure of Cl2(g) is 1.007 atm and the partial pressure of UO2Cl2(g) is 9.734 10-4 atm (a) Calculate the partial pressure of O2(g) at equilibrium at 862 C. (b) Calculate the value of the equilibrium constant, KP, for the system at 862 C. (c) Calculate the Gibbs free-energy change, G, for the reaction at 862 C. (d) State whether the entropy change, S for the reaction at 862 C is positive, negative, or zero. Justify your answer. (e) State whether the enthalpy change, H, for the reaction at 862 C is positive, negative, or zero. Justify your answer. (f) After a certain period of time, 1.000 mol of O2(g) is added to the mixture in the flask. Does the mass of U308(s) in the flask increase, decrease, or remain the same? Justify your answer.
6 2003 D Answer the following questions that relate to the chemistry of nitrogen. (a) Two nitrogen atoms combine to form a nitrogen molecule, as represented by the following equation. 2 N(g) N2(g) Using the table of average bond energies below, determine the enthalpy change, H, for the reaction. Bond Average Bond Energy (kj mol 1 ) N N 160 N=N 420 N N 950 (b) The reaction between nitrogen and hydrogen to form ammonia is represented below. N2(g) + 3 H2(g) 2 NH3(g) H = 92.2 kj Predict the sign of the standard entropy change, S, for the reaction. Justify your answer.
7 2001 B 2 NO(g) + O2(g) 2 NO2(g) H = -114.1 kj, S = -146.5 J K -1 The reaction represented above is one that contributes significantly to the formation of photochemical smog. (a) Calculate the quantity of heat released when 73.1 g of NO(g) is converted to NO2(g). (b) For the reaction at 25 C, the value of the standard free-energy change, G, is -70.4 kj. (i) Calculate the value of the equilibrium constant, Keq, for the reaction at 25 C. (ii) Indicate whether the value of G would become more negative, less negative, or remain unchanged as the temperature is increased. Justify your answer. (c) Use the data in the table below to calculate the value of the standard molar entropy, S, for O2(g) at 25 C. Standard Molar Entropy, S (J K -1 mol -1 ) NO(g) 210.8 NO2(g) 240.1 (d) Use the data in the table below to calculate the bond energy, in kj mol -1, of the nitrogenoxygen bond in NO2. Assume that the bonds in the NO2 molecule are equivalent (i.e., they have the same energy). Bond Energy (kj mol -1 ) Nitrogen-oxygen bond in NO 607 Oxygen-oxygen bond in O2 495 Nitrogen-oxygen bond in NO2?
8 1996 B C 2H 2(g) + 2 H 2(g) C 2H 6(g) Information about the substances involved in the reaction represented above is summarized in the following tables. Substance S (J/mol K) H f (kj/mol) C 2H 2(g) 200.9 226.7 H 2(g) 130.7 0 C 2H 6(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 mole Kelvin, calculate the standard molar entropy, S, of C 2H 6 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? (c) Calculate the value of the equilibrium constant, K, for the reaction at 298 K. (d) Calculate the value of the C C bond energy in C 2H 2 in kilojoules per mole.
9 2012 A sample of CH3CH2NH2 is placed in an insulated container, where it decomposes into ethene and ammonia according to the reaction represented above. Substance Absolute Entropy, S, in J/( mol K) at 298 K CH3CH2NH2(g) 284.9 CH2CH2(g) 219.3 NH3(g) 192.8 a. Using the data in the table above, calculate the value, in J/(molrxn K), of the standard entropy change, ΔS, for the reaction at 298 K. b. Using the data in the table below, calculate the value, in kj/molrxn, of the standard enthalpy change, ΔH, for the reaction at 298 K. Bond C C C = C C H C N N H Average Bond Enthalpy (kj/mol) 348 614 413 293 391
c. Based on your answer to part (b), predict whether the temperature of the contents of the insulated container will increase, decrease, or remain the same as the reaction proceeds. Justify your prediction. An experiment is carried out to measure the rate of the reaction, which is first order. A 4.70 10 3 mol sample of CH 3CH 2NH 2 is placed in a previously evacuated 2.00 L container at 773 K. After 20.0 minutes, the concentration of the CH 3CH 2NH2 is found to be 3.60 10 4 mol/l. Calculate the rate constant for the reaction at 773 K. Include units with your answer. Calculate the initial rate, in M min 1, of the reaction at 773 K. If 1/[CH 3CH 2NH 2] is plotted versus time for this reaction, would the plot result in a straight line or would it result in a curve? Explain your reasoning.
10 2013 MgO(s) + 2 H + (aq) Mg 2+ (aq) + H2O(l) A student was assigned the task of determining the enthalpy change for the reaction between solid MgO and aqueous HCl represented by the net-ionic equation above. The student uses a polystyrene cup calorimeter and performs four trials. Data for each trial are shown in the table below. Trial Volume of 1.0 M HCl (ml) Mass of MgO(s) Added (g) Initial Temperature of Solution (qc) Final Temperature of Solution (qc) 1 100.0 0.25 25.5 26.5 2 100.0 0.50 25.0 29.1 3 100.0 0.25 26.0 28.1 4 100.0 0.50 24.1 28.1 (a) Which is the limiting reactant in all four trials, HCl or MgO? Justify your answer. (b) The data in one of the trials is inconsistent with the data in the other three trials. Identify the trial with inconsistent data and draw a line through the data from that trial in the table above. Explain how you identified the inconsistent data. For parts (c) and (d), use the data from one of the other three trials (i.e., not from the trial you identified in part (b) above). Assume the calorimeter has a negligible heat capacity and that the specific heat of the contents of the calorimeter is 4.18 J/(g Cq). Assume that the density of the HCl(aq) is 1.0 g/ml. (b) Calculate the magnitude of q, the thermal energy change, when the MgO was added to the 1.0 M HCl(aq). Include units with your answer. (c) Determine the student s experimental value of Hq for the reaction between MgO and HCl in units of kj/molrxn. (d) Enthalpies of formation for substances involved in the reaction are shown in the table below. Using the information in the table, determine the accepted value of Hq for the reaction between MgO(s) and HCl(aq). Substance H (kj/mol) f MgO(s) 602 H2O(l) 286 H + (aq) 0 Mg 2+ (aq) 467 (e) The accepted value and the experimental value do not agree. If the calorimeter leaked heat energy to the environment, would it help account for the discrepancy between the values? Explain.