Vapor Pressure

Vapor Pressure of a Pure Liquid In this experiment the vapor pressure of a liquid is measured at several temperatures. The standard boiling point and the molar enthalpy of vaporization are calculated using the Clausius-Clapeyron equation.

Theory. When the temperature is raisen, the vapor pressure of a liquid increases, because more molecules gain sufficient kinetic energy to break away from the surface of the liquid. When the vapor pressure becomes equal to the pressure above the liquid, the liquid boils. The temperature at witch the vapor pressure of a liquid is equal to the external pressure is called the boiling temperature at that pressure. For the special case of an external pressure of 101 325 Pa, the boiling temperature is called the normal boiling point, Tb. Vapor pressure varies exponentially with temperature. The Clausius-Clapeyron equation expresses the relation between vapor pressure, temperature, and heat of vaporization. The differential form of the Clausius-Clapeyron equation is d ln P 0 ∆H m ,vap = , dT RT 2

(1)

where P0 = vapor pressure (Pa) T = temperature (K) ∆Hm,vap = molar heat of vaporization (J·mol-1) R = gas constant (8.314 J·K-1·mol-1). The procedure of separation of variables and simple integration of equation (1) under the simplifying assumption that enthalpy of vaporization is constant in the measured range of temperature, leads to the result ln P 0 = A −

∆H m ,vap 1 ⋅ , R T

(2)

B , T

(3)

pertinently

ln P 0 = A +

where A and B are constants. For vapor pressure measurement are used ebuliometric, static and saturation methods.

1

Determination of Vapor Pressure by the Isoteniscope The simplest method for determining the vapor pressure of a liquid is to connect a manometer to the liquid vapor system. This is the static or direct method. A simple device for measuring the vapor pressure of a liquids is the isoteniscope (Fig. 1). Essentially, the vapor pressure of a liquid closed inside the isoteniscope by column of the same liquid in U-manometer is compensated by external pressure, which is measured with mercury manometer.

Apparatus. General arrangement of apparatus is in Fig. 2. Apparatus consists from isoteniscope, cooler, pressure container and mercury manometer.

Procedure. The liquid sample is introduced into the dry isoteniscop in such amount that the cylindrical container is about the 2/3 full and the liquid stands about half-way up the limbs of U-tube (inner manometer). The isoteniscop is then immersed in the bath and connected through cooler with a manometer and pressure-regulating container. The thermostat is set the lowest temperature of the temperature range (it is defined by instructor). When the temperature of the liquid in the bath has been come constant, the pressure in the apparatus is reduced by means of the pump. As the pressure falls, the sample boils: the air is gradually carried out of the closed side of the isoteniscope by the vapor. When it is judged by the operator that all air has been removed (about after 5 min.) the pressure is raisen slowly until boiling ceases and the levels of the liquid in the two limbs are approximately equal. At this point the inner manometer shows that the outside pressure is the same as that inside (the vapor pressure of the liquid) the outside pressure is observed on the mercury manometer. The boiling-out process and observation are then repeated until constancy of the result obtained confirms that in fact all the air has been expelled. A new temperature is then selected and the process of boiling and observation is repeated.

Treatment of the experimental data. The molar heat of vaporization is determined from the constant B:

B=−

∆H m,vap R

.

(4)

The normal boiling point is calculated from eq. 3 by substitution of P0 = 101,325 kPa . 2