452 Midterm 1 2008 - University of Washington Department of...

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University of Washington Department of Chemistry Chemistry 452/456 Summer Quarter 2008 Midterm Examination Key July 25, 2008 Blue books are required. Answer only the number of questions requested. Chose wisely! If you answer more than the number of problems requested the excess problems will NOT be graded. You may keep this exam copy as a souvenir. Good luck Avagadro’s Number=N A =6.023x10 23 Universal gas constant=R=8.31J/mole-K=0.0821L-atm/mole-K 1J=1 Nt-m=1kg-m 2 /s 2 1atm=1.01x10 5 Pa=1.01x10 5 Nt/m 2 1000L=1m 3 101J=1L-atm. 1) (18 points) Define 3 out of the following 6 terms as they are used in the field of thermodynamics and/or mechanics. 1.1) Define the term adiabatic as it is used in thermodynamics. When is the entropy change for an adiabatic process zero and when is it not zero? Answer: A change that proceeds without q exchange, i.e. q=0. If the process is reversible and adiabatic then q rev =0. Then 0 rev q Sd T T δ == . But for an irreversible change 0 irrev q = and because rev irrev qq > 0 S 1.2) Define the thermal expansivity β and the isothermal compressibility κ as derivatives involving, state variables P, V, T. These two quantities together with what four other physical properties are used to evaluate thermodynamic derivatives like T U V ⎛⎞ ⎜⎟ ⎝⎠ , etc. Give two examples. Answer: thermal expansivity 1 P V VT β = and isothermal compressibility 1 T V VP κ =− . The other four areP, V, T and the hear capacity at constant pressure P P H C T = . Two examples: 2 V U CC T V T and () 1 T H P
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Others: T S V P β ⎛⎞ =− ⎜⎟ ⎝⎠ and P P C S TT = and T U TP V κ = …the list goes on… 1.3) Define the chemical potential and discuss how we use this quantity to define equilibrium. Give two examples. Answer: Any of the following possibilities will do: ,, jjjj i iiii SVn SPn TVn TPn UHAG nnnn µ ∂∂∂ ==== ∂∂∂∂ where i refers to the ith component of a multi-component system. The variable ni is the number of moles of component i. the chemical potential is used as a citerion for equilibrium. Given two phases at equilibrium: α U the criterion for equilibrium is αβ = . Given a chemical reaction equilibrium: AB CD C D ν νν + + U The equilibrium criterion is AA BB CC DD µν + =+ 1.4) Define the heat capacity C V . How is the heat capacity related to mechanical; degrees of freedom of a system? When is this relationship valid? Also, rank the importance of translational, rotational, vibrational, and electronic motions to the heat capacity for T=300K. 1.5) Unless you drive an automobile equipped with a diesel engine, your car’s engine probably operates according to a thermodynamic cycle called the Otto cycle. Diagram the reversible version of the Otto cycle on a P-V axis system, and explain each step. Does your car’s engine operate reversibly? If it did, what would be the advantage(s)? Explain.
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452 Midterm 1 2008 - University of Washington Department of...

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