EML3100 Thermo - Summer 2001 - Web Lesson

Entropy changes in solids liquids and ideal gases - PAGE 1

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For saturated liquids, saturated vapors and superheated vapors, we are able to calculate an entropy change by looking up the entropy for each state in the thermodynamic tables. But it is not so straight-forward for solids liquids and ideal gases...

Remember,

,
Q = dU + W
and
W = pdV

therefore,
TdS = dU + pdV

Now, remembering that H = U + pV,

dH = dU + pdV + Vdp, or
dU = dH - pdV - Vdp

Substituting for dU in TdS = dU + pdV, we get,
TdS = dH - pdV - Vdp + pdV, or
TdS = dH - Vdp
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