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Experiment 4 Vapor Pressure of a Pure Liquid Andres Phillips
Experiment Date: October 6th, 2011
Partner: Keval Shah
Report Submitted: October 13 , 2011
th
Ethan Pepmiller
Abstract The temperature dependence of the vapor pressure, the enthalpy of vaporization, and the normal (760 mm) boiling point of 1-propanol were determined by use of the Clausius-Clapeyron equation and the Berthelot equation. The enthalpy of vaporization was determined to be 45.21 kJ/mol ± 0.23 and normal boiling point was determined to be 366.9K ± 4.4. These values were close to the literature values of 41.44 kJ/mol for the enthalpy of vaporization and 370.3K for the normal boiling point.
Introduction The purpose of this report was to determine the temperature dependence of the vapor pressure, the enthalpy of vaporization, and the normal (760 mm) boiling point of 1-propanol by use of the Clausius-Clapeyron and Berthelot equations. The vapor pressure of a substance is the pressure created by the vapor phase. This vapor pressure depends on the nature of the substance and the temperature. The heat of vaporization, ∆Hvap, equals the enthalpy difference between the vapor and liquid phases and represents the enthalpy required to vaporize the liquid, (the lower the value of ∆Hvap, the higher the vapor pressure.) If the substance is heated bubbles form in the liquid when the vapor pressure equals the external pressure and if the external pressure (P) above the liquid is maintained at a constant value such as atmospheric pressure. The Clapeyron equation is denoted by:
∆S – difference between entropies of the two phases at constant pressure and temperature ∆V - difference between volume of the two phases at constant pressure and temperature
- slope of the set of pressures and temperatures at which the two phases are in equilibrium.
Since the change in state is isothermal and ∆G = ∆H - T∆S = 0 at equilibrium, ∆S in the previous equation may be replaced by ∆H/T:
neglecting the molar volume of the liquid phase compared to the vapor phase and recognizing that Vg = ZRT/P. Also dlnP = (1/P)dP and d(1/T) = -(1/T2)dT. The equation can be rearranged and written as:
() and the Clausius-Clapeyron equation:
⁄ Z – the compressibility factor R- the gas constant
A plot of lnP versus 1/T gives a slope which is used to determine the heat of vaporization. If ∆H and Z were constant, then the slope would be linear. However, Z generally decreases with increasing temperature and the variation of ∆H with temperature is given by:
The value of the compressibility factor can be estimated using the Berthelot equation:
⁄ Tc – critical temperature Pc - critical pressure Experimental Methods The experimental procedure given in the handout was followed and no alterations were made. The sample of 1-propanol used was from Acros Organics with 99.5% purity. The apparatus used was set up and adjusted with the help of the instructor as indicated in the manual. After this was done the initial heights of the columns of mercury were recorded. The sample was then heated for different external pressures and the heights of the columns of mercury and the boiling temperature were recorded.
Results Temp Right o C Ht.(mm) 42.5 53.6 62.5 68.4 72.0 76.2 79.7 83.2 87.0 88.9 91.0 93.0 90.7 88.4
Table 2 : Temperature, 1/T, P and ln P used to get the slope and intercept for the calculation of the ΔHvap and normal b.p.
Heat of Vaporization Data 7
6
P n l
5
4
3 2.6E-03
2.8E-03
3.0E-03
3.2E-03
3.4E-03
1/T
Figure 1: Graph of ln P vs 1/T
Calculations & Error Analysis See Appendix Discussion Property Δ Hvap (kJ/mol) Normal Boiling Point
Literature @ 370.3K [2] 41.44 370.3 + 0.5
Experimental 45.21 ± 0.23 366.9 ± 4.4
The values for the heat of vaporization and the normal boiling point tended to agree with the experimental values and as such the experiment can be considered a success. The literature values were taken at a different temperature than that of the experiment, and is the reason for the slight variation in the two values. Trouton's rule states that for liquids with ordinary intermolecular forces (i.e. no hydrogen bonding), ∆Svap= ∆Hvap /Tb ≈ 85 J/mol-K, where Tb is the boiling point. This value does not
agree with the experimental value.1-propanol does not meet the criteria for Trouton’s rule because it does have hydrogen bonding in its intermolecular forces.
References 1) [1] Baum, C. CHM 3011, Experiment 3 Lab Manual for Fall 2011. 2) NIST Chemistry Webbook
