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2.15 PROBLEMS 33

Using the following data, show the pressure at which graphite and diamond are in
equilibrium at a temperature of 25 C. The data for these two phases of carbon at 25 C and

1 bar are given in the following table:

Graphite Diamond
Speciﬁc Gibbs energy, g/(kJ/kg) 0 269
3
Speciﬁc volume, v/(m /kg) 0.446 10 3 0.285 10 3
1
Isothermal compressibility, k/(bar ) 2.96 10 6 0.158 10 6

It may be assumed that the variation of kv with pressure is negligible, and the lower value of the
solution may be used.
[17,990 bar]
P2.15 Van der Waals equation for water is given by

0:004619T 0:017034
p ¼
v 0:0016891 v 2
3
where p ¼ pressure (bar), v ¼ speciﬁc volume (m /kmol), T ¼ temperature (K).
Draw a p–v diagram for the following isotherms: 250 C, 270 C, 300 C, 330 C, 374 C,

390 C.

Compare the computed speciﬁc volumes with Steam Table values and explain the differences
in terms of the value of p c v c =<T c :

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