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http://chem1.csu.edu.tw/teacher/0448/uploads/lesson1/mass_energy_balanc http://chem1.csu.edu.tw/teacher/0448/uploads/lesson1/mass_energy_balanc Class: Name: Student No. A cylinder with a movable piston contains 4.0 liters of a gas at 30 4 bar. The piston is slowly moved to compress the gas to 8.0 bar. (50%) (a) Considering the system to be the gas in the cylinder and neglecting write and simplify the close-system energy balance. Do not assume that the process is isothermal in this part. (b) Suppose instead that the process is adiabatic and that now that U increase as T increase. Is the final system temperature greater than, equal to or less than 30 ? (Briefly state your reasoning.)

Explanation / Answer

(a) For any closed system, energy balance gives the First Law of Thermodynamics, that is

U = Q + W

W will be the pressure volume work, we have W= (PV)

However, since this is a reversible process, we have the delta replaced by integral. Hence,

W = Pdv

Thus, U = Q + Pdv

Since we do not know the nature of compression, we cannot estabilish a relation between P and V.

(b) Since the system is adabatic, Q=0. Hence, all the work done ON THE SYSTEM here is translated into Internal energy U. Now, work done on system here is positive, and hence change in internal energy is positive, which implies increase in temperature.

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