Question

You have a material (not unlike water) whose molar specific heat Cp at atmo-

spheric pressure is 30J/(mole K) in the gaseous phase, 75 J/(mole K) in the

liquid phase, and 35 J/(mole K) in the solid phase. Furthermore, the material

condenses at 373 K with a latent heat of 41 000 J/mole and freezes at 273

K with a latent heat of 6000 J/mole. Assuming that it behaves as an ideal

gas at 600 K, find (a) its enthalpy at 600 K, (b) its molar enthalpy at 500 K,

374 K, 372 K, 323 K, 274 K, 272 K, and 200 K.

(At constant pressure, H = E + pV = Q, so you don’t have to worry

about changes in volume at phase transitions.)

Answer 1

Reference temperature is taken as 200K.its enthalpy at 200K=0

from reference temperature 200K to 273, it is sensible heat of solid. At 273K, it is latent heat of fusion. From 273K liquid state to 373K, it is sensible heat and at 373 K is is latent heat at constant temperature and from 373K to final temperature it is super heat.

1. Sensible heat of solid= specific heat* temperature difference

2. heat of fusion = 6000 J/mole

3. sensible heat of liquid = specific heat* temperature difference

4. heat of vaporization = 41000 J/mole

5. sensible heat of vapor = specific heat* temperature difference

all these heats will have to acounted when enthalpy has to be calculated (depending upon the temperature)

for different temperatures, the enthalpy is calculated and shown in the table below