Question

An Insulated piston cylinder device contains 4 L of saturated liquid water at a constant pressure of 150 K PA. water is stirred by a paddle wheel while a current of 9 A flows for one hour through a resistor placed in the water. if 80% of the liquid is vaporized doing this constant pressure process and the paddle does 400 kJ of work during this time. determine the resistance of the resistor?

Answer 1

The (latent) heat of vaporization (∆H_vap) also known as the enthalpy of vaporization or evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance, to transform a given quantity of the substance into a gas.

The enthalpy of vaporization can be written as

It is equal to the increased internal energy of the vapor phase compared with the liquid phase, plus the work done against ambient pressure. Here, the work done, p∆V = 400 kJ. The increase in the internal energy can be viewed as the energy required to overcome the intermolecular interactions in the liquid. The enthalpy of vaporization of liquid water is 40.65 kJ/mol. The amount of water vaporized is m = (80/100) x 4 L = 16/5 L = 16000/(5 x 18) mole = 177.78 mole. Therefore, the enthalpy of vaporization of liquid water will be ∆H_vap = 177.78 x 40.65 kJ = 7226.76 kJ.

The change in internal energy is supplied by the electric heating process. The energy will be i²R = 81R. Therefore, from the above equation