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SOL3-2011 - ECH 152A Fall 2011 Roland Faller Solutions for...

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ECH 152A, Fall 2011, Roland Faller Solutions for Homework 3, October 10, 2011 Problem 1 . (4 points) (a) What is the definition of a state function? (b) What does “reversible” mean? (c) What is “steady state”? (d) What is the difference between internal energy and enthalpy? Solution (a) a function that ONLY depends on the current state, not on any history (b) The process can be reversed at any time by an INFINITESIMAL change in driving force (c) in steady state changes with time (e.g. flow rates) can be non-zero but then they are constant. (d) Enthalpy is normally used in isobaric and internal energy in isochoric pro- cesses, the difference is pV Problem 2 . (2 point) (a) To a closed system we add 18 kJ of heat while its internal energy decreases by 34 kJ. How much work do we gain or perform? (Everything reversible) (b) If we do not gain/perform work, how much heat is exchanged (internal energy change same as in part (a))? Solution The first law says U = Q + W, → - 34 kJ = 18 kJ + W So W = - 52 kJ. The system performs 52 kJ of work on the environment.
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