Question

The following data are available for liquid ethanol at 20 C:

M.W. = 46.07, Density = 0.7893 g/cc, Coefficient of thermal expansion = 1.12x10^-3 K^-1, Isothermal compressibility = 1.0x10^-4 bar^-1

(a) Estimate the difference (Cp-Cv) for liquid ethanol at 20 C in J/mol K

(b) 10 liters of liquid ethanol at 20 C is compressed isothermally from initial pressure of 1 bar to a final pressure of 50 bar. For this process, calculate the enthalpy and entropy change of the liquid.

Answer 1

We have the relationship:

Cp= Cv+ (TV²/)

Therefore, Cp- Cv= (TV²/)

Putting the above mentioned value in the equation,

Cp- Cv= (20+273)V(1.12x10^-3)²/ 1.0x10^-4

= 293× V× 125.44/ 76.8

= 3.675× V.

Here since the mass is not given i am assuming it to be 1 mole i.e 46g and Density = 0.7893 g/cc

therefore Volume = 46/0.7893 = 58.2mL

Cp- Cv= 3.675× 58.2 = 214.17

entropy change is given by

P1 = 1 bar P2 = 50 bar

= -32.524 J K^-1 mol^-1