Vapor Pressure and Heat of Vaporization

Introduction

When a liquid is placed in a container, and the container is sealed tightly, a portion of the liquid will evaporate. The newly formed gas molecules exert pressure in the container, while some of the gas condenses back into the liquid state. If the temperature inside the container is held constant, then at some point equilibrium will be reached. At equilibrium, the rate of condensation is equal to the rate of evaporation. The pressure at equilibrium is called vapor pressure, and will remain constant as long as the temperature in the container does not change.

In mathematical terms, the relationship between the vapor pressure of a liquid and temperature is described in the Clausius-Clayperon equation,

where ln P is the natural logarithm of the vapor pressure, ΔH_vap is the heat of vaporization, R is the universal gas constant (8.31 J/mol•K), T is the absolute, or Kelvin, temperature, and C is a constant not related to heat capacity. Thus, the Clausius-Clayperon equation not only describes how vapor pressure is affected by temperature, but it relates these factors to the heat of vaporization of a liquid. ΔH_vap is the amount of energy required to cause the evaporation of one mole of liquid at constant pressure.

Objectives

In this experiment, you will

• Measure the pressure inside a sealed vessel containing a volatile liquid over a range of temperatures.
• Determine the relationship between pressure and temperature of the volatile liquid.
• Calculate the heat of vaporization of the liquid.