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Exam 4-solutions - Version 226 Exam 4 Sutclie(52380 This...

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Version 226 – Exam 4 – Sutcliffe – (52380) 1 This print-out should have 24 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Consider the following compounds and their thermodynamic data: Compound Δ H f S Δ G f ( kJ mol ) ( J mol · K ) ( kJ mol ) CH 4 - 75 186 - 50 CH 2 O - 108 218 - 102 C 6 H 5 NH 2 87 166 319 C 2 H 4 52 68 219 Using this data, which of the following an- swers includes the compounds that are ther- modynamically unstable? 1. CH 4 , CH 2 O, C 2 H 4 2. C 6 H 5 NH 2 , C 2 H 4 correct 3. CH 4 , C 2 H 4 4. Cannot be determined from the data pro- vided. 5. CH 2 O, C 6 H 5 NH 2 6. All of the compounds are thermodynami- cally stable. Explanation: 002 10.0 points A battery does 35 kJ of work driving an elec- tric motor. The battery also releases 7 kJ of heat. What is the change in internal energy of the system? Assume the motor is not part of the system. 1. - 42 kJ correct 2. - 35 kJ 3. +28 kJ 4. +42 kJ 5. - 28 kJ Explanation: Δ U = q + w = - 7 kJ + ( - 35) kJ = - 42 kJ . 003 10.0 points The first law of thermodynamics states that 1. energy is neither created nor destroyed. correct 2. energy is the capacity to do work. 3. doing work is defined as causing move- ment against a resisting force. 4. heat flows from a warmer body to a cooler body. Explanation: Definition 004 10.0 points A drinking mug made from Pyrex glass is placed in the freezer and chilled to - 10 C. It is then removed from the freezer and 200 mL of cool water at 12 C is poured inside. What will be the final temperature of the resulting system, assuming NO heat is exchanged with the surroundings? The specific heat of Pyrex is 0 . 753 J / g · C and the mass of the mug is 455 g. Recall that the density of water is 1 g/mL. 1. 3.15 C 2. 0.644 C 3. 5.60 C correct 4. 16.09 C Explanation: T m = - 10 C V w = 200 mL T w = 12 C C m = 0 . 753 J / g · C m m = 455 g ρ w = 1 g/mL c m = 0 . 753 J / g · C
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Version 226 – Exam 4 – Sutcliffe – (52380) 2 The mass of the water is m w = (1 g / mL) (200 mL) = 200 g . Heat flows from the water into the mug. Both end up at the same temperature (assum- ing no heat exchange with the surroundings), so q m = - q w m m C m Δ T m = - m w C w Δ T w m m C m ( T f - T m ) = - m w C w ( T f - T w ) T f = m m C m T m + m w C w T w m m C m + m w C w Since m m C m + m w C w = (455 g)(0 . 753 J / g · C) + (200 g)(4 . 18 J / g · C) = 1178 . 62 J / C , T f = (455 g)(0 . 753 J / g · C)( - 10 C) 1178 . 62 J / C + (200 g)(4 . 18 J / g · C)(12 C) 1178 . 62 J / C = 5 . 60476 C . 005 10.0 points What is the entropy at T = 0 K for one mole of chloroform (CHCl 3 ), which can orient four ways? 1. 0 J/K 2. 1.38 J/K 3. - 11 . 5 J/K 4. 11.5 J/K correct 5. 1 . 9 × 10 23 J/K Explanation: 006 10.0 points An astronaut opens a can of compressed gas on a spacewalk. There is no liquid in the can. Assume space is a perfect vacuum. For the system (the can and contents), w is 1. + 2. - 3. 0 correct Explanation: Expansion of the gases in a vacuum (free expansion) does not require any work to be done by or on the system.
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