EML3100 Thermo - Summer 2001 - Web Lesson

Entropy changes in solids liquids and ideal gases - PAGE 6

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Now we will approach the solutions to

and

in a manner similar to that which we used for determing internal energy changes and enthalpy changes for ideal gases.

I.   The least accurate method would be to assume that Cvo and Cpo are constant with temperature, and pulling them through their respective integrals, giving


Values for Cvo and Cpo can be found in table A-20.

II.   A more accurate method would be to get equations for Cvo and Cpo as functions of temperature, and integrate using those equations, giving


Temperature dependent equations leading to Cvo and Cpo as functions of temperature can be found in table A-21.

III.   The most accurate method for determining the entropy change for ideal gases is to used the values for entropy at each of the given temperatures, and be sure to make an adjustment for pressure, and calculate the change exactly, as shown in


Values for so can be found in tables A-22 and A-23.
NOTE -
the value soT, is for the entropy at a temperature "T" and a pressure of 1 bar.
Because entropy for ideal gases depends on more than just temperature (unlike internal energy and enthalpy), we must make an "adjustment" if the pressure at the states is not 1 bar.
This should explain the - Rln(p2/p1) term in the equation.

You should now be able to complete homework #17

Have a great holiday, and I'll see you on July 10th.

-Dr. P.H.
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