Vapour Pressure of a Pure Liquid

Vapor Pressure of a Pure Liquid: Determination of ΔvapH

Physical Chemistry 231

Lab # 5

Naomi Crane

Sarah Sutton

Lauren

Introduction

Vapor pressure is the partial pressure of a gas phase of a substance in equilibrium with its liquid or solid phase (1). Boiling occurs when the vapor pressure of a substance (acetone) equals the external pressure of the surroundings. By increasing the temperature of the system, one can increase the vapor pressure to that of the surroundings as more molecules in the liquid have enough free energy to break free from the liquid surface and enter the gas phase (1). In this experiment, a vacuum apparatus was used to change the pressure of a boiling liquid, acetone, which results in a temperature change of the liquid. Because the system is evacuated, the measured pressure was equal to the vapor pressure of the liquid.  The apparatus was originally set to 14 kPa. Once the acetone began to boil, the temperature of the acetone was recorded. The pressure was then increased by intervals of 14 kPa until 98 kPa was reached. At each interval, the acetone was allowed to boil, and the temperature vs pressure plot was recorded. Once 98 kPa was attained, the process was repeated in reverse. The dependence of vapor pressure on temperature is given by the Clausius-Clapeyron equation. This equation relates the change in vapor pressure with temperature to the ΔvapH, or the enthalpy of vaporization (1). To use this equation, three assumptions were made, the molar volume of the liquid is negligible compared to that of the vapor, the vapor obeys the ideal gas law, and ΔvapH is independent of temperature (1).

Experimental and Literature Data

Experimental Values of Temperature vs. Vapor Pressure

[pic 1]

Experimental Values of ln Vapor Pressure vs 1/T (K-1 )

[pic 2]

[pic 3]

(Values obtained from reference 2)

(Values obtained from reference 2)

Calculations

ln (p) = -ΔvapH/RT + constant

y = mx + b

m = -ΔvapH/R        ΔvapH = -m x R = (4066.04871) x (8.314 J/Kmol) = 33805 J/Kmol

ΔvapS = ΔvapH / Tvap = 33805 J/Kmol / 329.15 K = 102.7J/K

Results and Discussion        

It was found the experimental ΔvapH         of acetone was 33.81 KJ/mol and the ΔvapS was 102.7 J/K. The literature calculation of the ΔvapH was 36.71 KJ/mol. The actual literature value of ΔvapH for acetone was 30.99.  

Percent Error

% Error = [(Actual – Experimental)/Actual] x 100