Practice+Exam+2_Solutions - a The chart below shows the...

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Unformatted text preview: a) The chart below shows the temperature forecast for a warm(er) spring night in Atlanta. At 7:00 p.m. the relative humidity is 72%. At what time will you observe dew formation, assuming that the composition of the atmosphere remains constant throughout the night? Document your work. There will be no credit for guessing. At 7 pm, T = 62 ° F = 16.67 ° C, and from the Antoine eqn., p * = 14.25 mm Hg. 46 48 50 52 54 56 58 60 62 64 h r = 0.72 = p i / p * Æ p i = 10.26 mm Hg T(°F) At the dew pt., p * must be equal to p i , so set p * to 10.26 mm Hg Use the Antoine eqn. to find T = 11.6 ° C = 52.95 ° F ≈ 53 ° F Dew formation starts at ~ 4 AM b) Specific Enthalpy Change in Air Air is sparged into a reactor at room temperature (25 º C) and heated up to 35 º C. The air is allowed to expand during the heating such that the reactor maintains a constant pressure of 1 atm. Given that the specific internal energy of air increases by approximately 259 J/mol as air is warmed, what is the difference in specific enthalpy of air between these two states? Solution : The ideal gas law (PV = nRT) is used to find the specific volume of air at 25 º C (298 K) and 37 º C (310 K). At room temperature: V ˆ = V/n = RT/P = (0.08206 L · atm/mol · K)(298 K)/ 1atm = 24.5 L/mol And at 310 K: V ˆ = V/n = RT/P = (0.08206 L · atm/mol · K)(310 K)/ 1atm = 25.4 L/mol The absolute specific enthalpy cannot be found at either temperature. However the difference in specific enthalpy between the two temperatures can be calculated. Î H ˆ 310K – H ˆ 298K = U ˆ 310K – U ˆ 298K + P ( V ˆ 310K – V ˆ 298K ) = 259 J/mol + 1 atm (25.4 L/mol – 24.5 L/mol)(101.3 J/ L · atm) = 350 J/mol c) Consider a mixture of three condensable chemicals A, B, and C at 50 ° C and 2 atm. Under these conditions all three components are present in the liquid and vapor phase. A and B can be converted into each other, and the system is in equilibrium. Which parameters need to be specified to describe the system unambiguously? Explain your answer in 2 or 3 sentences. From the Gibbs Phase Rule: DOF = c - Π- R + 2 = 3 - 2 - 1 + 2 = 2 Both temperature and pressure are specified so no other parameters can be specified according to the laws of thermodynamics. Nerve gas attack On an episode of the TV show 24, terrorists release nerve gas in the headquarters of CTU (counter terrorism unit). To save CTU, the agents propose to turn on the air conditioning to vent the nerve gas. Of course, the air conditioners aren’t functioning properly and a number of agents die before they can get them to work. Had CTU recruited any chemical engineers as agents, they would have realized that the nerve gas could also be cleared by activating the sprinkler system....
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This note was uploaded on 02/22/2012 for the course CHBE 2100 taught by Professor Staff during the Summer '08 term at Georgia Tech.

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Practice+Exam+2_Solutions - a The chart below shows the...

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