Solved Problems on Thermodynamic Relations

SOLVED PROBLEMS ON THERMODYNAMIC RELATIONS

Problem 5.5

Obtain the expression for ds in terms of dT and dp.

Solution:

Let us assume, the entropy is a function of temperature and pressure.

The enthalpy is given by

dh = Cp dT = Tds + vdp

Dividing the above equation by dT at constant pressure,

⸫ Substituting this in equation (5.108),

Problem 5.6

The equation of state in the given range of pressure and temperature is given by where C is constant. Derive an expression for the change of enthalpy and entropy for this substance during an isothermal process.

Given data:

Solution:

The general equation for finding change in enthalpy (dh) is given by

The general equation for finding ds is given by the equation (5.109) which is already discussed in previous example.

Problem 5.7

Using Calusius Clapeyron equation, estimate the enthalpy of vaporization at 220° C saturation temperature. Take the following data: vg = 0.08603 m3/kg, vf = 0.00119 m3/kg and

Given data:

Ts = 220°C = 220 + 273 = 493 K

vg = 0.08603 m3/kg

v = 0.00119 m3/kg

(dp/dT) = 35 kPa/K

Solution:

Using the Calusius Clapeyron equation,