Question

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)

Answer 1

Formula: The Helmholtz free energy (A) = U - TS ........... 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 + PV

i.e. U - TS = -PV ........... Equation 2

Where V is the change in volume = -3 L

P = pressure = 3 bar

Now, from equaitons 1 and 2: A = -PV

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