AP Problems Winter Break 1982 Water is added to 4.267 grams of UF6. The only products are 3.730 grams of a solid containing only uranium, oxygen and fluorine and 0.970 gram of a gas. The gas is 95.0% fluorine, and the remainder is hydrogen. (a) From these data, determine the empirical formula of the gas. (b) What fraction of the fluorine of the original compound is in the solid and what fraction in the gas after the reaction? (c) What is the formula of the solid product? (d) Write a balanced equation for the reaction between UF6 and H2O. Assume that the empirical formula of the gas is the true formula 1996 Represented above are five identical balloons, each filled to the same volume at 25 C and 1.0 atmosphere pressure with the pure gases indicated. (a) Which balloon contains the greatest mass of gas? Explain. (b) Compare the average kinetic energies of the gas molecules in the balloons. Explain. (c) Which balloon contains the gas that would be expected to deviate most from the behavior of an ideal gas? Explain. (d) Twelve hours after being filled, all the balloons have decreased in size. Predict which balloon will be the smallest. Explain your reasoning. 1990 B Standard Free Energies of Formation at 298 K G f 298 K, kJ mol-1 Substance C2H4Cl2(g) -80.3 C2H5Cl(g) -60.5 HCl(g) -95.3 Cl2(g) 0 Average Bond Dissociation Energies at 298 K Bond Energy, kJ mol-1 C-H 414 C-C 347 C-Cl 377 Cl-Cl 243 H-Cl 431 The tables above contain information for determining thermodynamic properties of the reaction below. C2H5Cl(g) + Cl2(g) (a) (b) (c) (d) C2H4Cl2(g) + HCl(g) Calculate the H for the reaction above, using the table of average bond dissociation energies. Calculate the S for the reaction at 298 K, using data from either table as needed. Calculate the value of Keq for the reaction at 298 K. What is the effect of an increase in temperature on the value of the equilibrium constant? Explain your answer. 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. 1 C(s) + O2(g) CO(g) ∆H˚298 = –110.5 kJ mol-1 2 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. S˚298 Substance -1 -1 (J mol K ) 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. 1996B Explain each of the following observations in terms of the electronic structure and/or bonding of the compounds involved. (a) At ordinary conditions, HF (normal boiling point = 20ºC) is a liquid, whereas HCl (normal boiling point = 114ºC) is a gas. (b) Molecules of AsF3 are polar, whereas molecules of AsF5 are nonpolar. (c) The N-O bonds in the NO2- ion are equal in length, whereas they are unequal in HNO2. (d) For sulfur, the fluorides SF2, SF4, and SF6 are known to exist, whereas for oxygen only OF2 is known to exist.
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