The system decreased its volume by 3L, at constant pressure p=3bar and constant temperature. The resulting change in Helmholtz free energy is (in kJ)
Formula: The Helmholtz free energy (A)
=
U - T
S
........... Equation 1
Where
U is the change in internal energy, T is the absolute temperature
and
S is the change in entropy.
According to first and law of thermodynamics: TS
=
U + P
V
i.e.
U - T
S
= -P
V
........... Equation 2
Where
V is the change in volume = -3 L
P = pressure = 3 bar
Now, from equaitons 1 and 2:
A = -P
V
i.e.
A = -3 bar * (-3 L)
i.e.
A = 9 L.bar
i.e.
A = 9*100 J
i.e.
A = 0.9*1000 J
i.e.
A = 0.9 KJ
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