PChemexam062 - NAME and Section No. Chemistry 391 Exam II...

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––––––––––––––––––––––––––––––– NAME and Section No. 1 Chemistry 391 Fall 2006 Exam II KEY 1. (30 Points) ***Do 5 out of 6 ***(If 6 are done only the first 5 will be graded)*** a). Can the entropy of a universe increase, decrease or remain the same? State the conditions for your conclusions. It can either increase (irreversible process) or remain the same (reversible process). b). A Carnot cycle operates between T H and T C taking in q H at T H and rejecting q C at T C . Write an expression for the efficiency / H ew q = − in terms of q C and q H, and also T C and T H . What is the possible range of values of e ? Since 0 CH dU w q q ==+ + v , () // 1 / 1/ H HCH C H q qqq q q TT =− = + = + The temperatures are positive numbers and / 1 < , so 01 e < < c). State the third law of thermodynamics. Give conditions for its validity. The entropy change for some transformation goes to zero as the temperature goes to zero. Requires all substances to be in internal equilibrium. (No metastable states).
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––––––––––––––––––––––––––––––– NAME and Section No. 2 d). What is the sign of S in a reversible isothermal expansion of an ideal gas? Show your reasoning/derivation. Ideal gas () UUT = . Therefore, for isothermal 0 Uq w =⇒= . For reversible 1 rev dq Sq TT ∆= = . Since its an expansion w <0, so q > 0, and so is S .
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PChemexam062 - NAME and Section No. Chemistry 391 Exam II...

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