Chemistry 360 Spring 2015 Dr. Jean M. Standard April 15, 2015 Name _____________________________________ Exam 3 – 100 points Note: You must show your work on problems in order to receive full credit for any answers. You must turn in your equation sheet along with this exam in order to receive full credit for the exam. Please turn off cell phones and store them during the exam. 1.) (14 points) Consider the reaction NO 2 ( g) NO ( g) + 1O 2 2 ( g) . At 500 K and a total pressure of 2 bar, pNO 2 / pNO = 2.20. Determine the equilibrium constant K eq for the reaction at 500 K, assuming that there is 1 € mole of NO2 present in the system initially. € € € 2 2.) (14 points) The temperature dependence of the chemical potential for a single component system is shown below for a pressure of 1 atm. S µ L V T Indicate on the diagram above the temperature at which solid-liquid equilibrium occurs (the melting point) and the temperature at which liquid-vapor equilibrium occurs (the boiling point). Now, consider what happens to the diagram above if the pressure is increased. Does the boiling point of the system increase or decrease relative to its 1 atm value? Explain your reasoning on the basis of changes in the chemical potentials. Sketch the behavior of the chemical potentials as a function of temperature for a higher pressure. You may want to use the diagram above and draw in the changes in the chemical potentials for the higher pressure relative to those shown for P=1 atm, or you may draw your own. 3 3.) (14 points) The vapor pressure of liquid sulfur dioxide obeys the relation ln P = 17.9962 − 2999.8 , T where the pressure is in torr and temperature is in Kelvin. a.) Determine the normal boiling point of sulfur dioxide in degrees Kelvin. b.) Calculate the molar enthalpy of vaporization of sulfur dioxide in kJ/mol. 4 4.) (15 points) True/false, short answer, multiple choice. a.) True or False: The Clapeyron equation is valid only for the liquid-vapor phase coexistence curve in one-component systems. b.) True or False: The reaction CO (g) + 3 H2 (g) to the left if carbon monoxide is removed from the system. CH4 (g) + H2O (g) is expected to shift c.) Short answer A maximum or minimum boiling solution for a binary liquid-vapor system is referred to as an __________________________ . d.) Short answer The ____________________________ is defined to be the molar Gibbs free energy of a substance. e.) Multiple Choice For the three graphs shown below, circle the one that exhibits the correct behavior for a typical reaction which reaches chemical equilibrium. G G ξ (a) G ξ (b) ξ (c) 5 5.) (14 points) Mixtures of benzene and cyclohexane exhibit ideal behavior. A solution was created containing 1.5 moles of liquid benzene and 2.5 moles of liquid cyclohexane, and at 50ºC the total vapor pressure of the solution was measured to be 340 torr. Another solution was created containing 1.5 moles of liquid benzene and 3.5 moles of liquid cyclohexane, and at 50ºC the measured total vapor pressure was 370 torr. Calculate the vapor pressures of pure benzene and pure cyclohexane at 50ºC. 6 6.) (14 points) Chlorine oxide, ClO, a key component in the destruction of the ozone layer in the polar stratospheric regions, dimerizes to form Cl2O 2 according to the reaction 2ClO ( g) Cl2O 2 ( g) . At 310 K and a total pressure of 1 bar, ΔH R! = –72.43 kJ/mol and ΔSR! = –144 J mol–1K–1. € € (a.) Determine the standard molar Gibbs free energy ΔGR! and equilibrium constant K eq at 310 K and 1 bar. € (b.) Discuss whether increasing the total pressure (at constant temperature) will increase or decrease the extent of reaction. Explain your answer. (c.) Discuss whether increasing the temperature (at constant pressure) will increase or decrease the equilibrium constant. Explain your answer. 7 7.) (15 points) True/false, short answer, multiple choice. a.) True or False: For the liquid-to-vapor phase transition in a one-component system, the Gibbs free energy change is negative ( ΔG < 0 ). b.) True or False: A chemical equilibrium is an example of a dynamic equilibrium. c.) Short answer The ____________________________ Equation gives the temperature dependence of the equilibrium constant. d.) Short answer The ____________________ of reaction provides a measure of how far a chemical reaction has progressed from reactants to products. e.) Multiple Choice Henry's Law is generally expected to be valid for which component or components of a solution? 1) the solute 2) the solvent 3) both the solute and the solvent 8 CHE 360 PHYSICAL CONSTANTS AND EQUATIONS R = 0.08206 L atm mol–1 K–1 = 8.314 J mol–1 K–1 = 0.08314 L bar mol–1 K–1 1 atm = 101325 Pa = 1.013 bar = 760 torr 1 bar = 105 Pa = 0.98692 atm 1 L atm = 101.3 J; 1 L bar = 100 J 1 cal = 4.184 J Fundamental Equations dU dH dA dG = = = = T dS − P dV T dS + V dP − S dT − P dV − S dT + V dP N € Note: for open or reacting systems, add ∑ µ i dni i=1 € to each of the fundamental equations.