Question

Describe (qualitatively) how standard enthalpy and entropy of vaporization of water will change with temperature?

Answer 1

The enthalpy of vaporization of water is defined as the amount of heat energy required to vaporize a certain mass of water. Usually, the enthalpy of vaporization is recorded at the normal boiling point of water. The enthalpy of vaporization depends on the pressure of the water. At a pressure of 1 atmosphere, the enthalpy of vaporization is known as the standard enthalpy of vaporization. The standard enthalpy of vaporization is assumed to be independent of temperature for practical purposes. However, the truth is that the enthalpy of vaporization is indeed temperature-dependent. However, the change in standard enthalpy of vaporization of water with small increases in temperature is extremely small and hence, can be ignored. The standard enthalpy of vaporization increases with temperature because during vaporization, liquid water molecules are converted to gaseous molecules and hence, the intermolecular forces become weak. Due to weak intermolecular forces, the molecules are less attracted to each other and move about randomly. With further increase in temperature, the forces become ever weaker and hence, the motion increases.

The standard entropy of vaporization of water is the increase in entropy upon vaporization of water. Again, the standard entropy of vaporization is recorded at the normal boiling point of water. When water is vaporized, the molecules move from the liquid to the vapor state and hence, the intermolecular forces become weak. Consequently, the random motion increases. Entropy is a measure of the randomness of the system; hence, the entropy of vaporization should increase with temperature. However, for small temperature changes, the increase can be ignored and the standard entropy of vaporization is assumed to be constant.