Enthalpy and Entropy of Evaporation from Measured Vapor Pressures Using a Programmable Desk Calculator Recently, a Fortran computer program was presented which correlated up to fifty pieces of pressure-temperature data.' Its output consisted of the slope and intercept of the Clausius-Clapeyron equation, together with their standard deviations, the standard error of estimate, the correlation coefficient and the student t factor. The following program (written for the Hewlett-Packard 9100 A desk calculator) is similar to that of reference 1, but has several advantages: (1) The number of data is unlimited. (2) A computer is not necessary. Any of the currently available, small-memory programmable desk calculators may be used. (3) The error treatment is more flexible. The program calculates the heat of evaporation, AH, in cal/male, of a solid or liquid and the corresponding entrapy change. AS, in cal/deg male, by means of an unweighted least-squares fit of the Clausius-Clapeyron equation lag P
= 2.303R --AH + - T1
AS 2.303R
The data is input as pressures in torr and temperatures in degrees centigrade. In addition, the Student t factor far any desired level of confidence, for the number of data points minus two, is an input parameter. The statistical errors in AH and AS, EAHand Ens, respectively, are also calculated, along with the correlation coefficient, so that the results may be quoted as A H + E m a n d AS + Ees, a t the level of confidence selected. In general, if the temperature range of the vapor pressure measurements is sufficiently small (less than about 100' for solids and less than about 30" for liquids not close to their critical points) AH may be expected to be constant to within experimental error. The calculated value of AS is less accurate than AH, however, and corresponds to the entropy of evaporation to an equilibrium vapor pressure of one atmosphere, assuming the enthalpy of evaporation remains constant over the required temperature interval. AH and AS are traditionally quoted far the mean temperature of the interval. Large systematic errors may occur a t higher pressures, since ideal gas behavior is assumed, as well as near the critical point, where AHchsnges rapidly with temperature. As an example, the data for water was correlated a t ten-degree intervals from 0 to 100'C a t a 90% confidence level. The result is AH = 10347 + 77 cal/mole The measured value is 10240 cal/mole a t 50°C. The program with instructions is available upon request. 'Sehlessinger, G. C., andDeMiehiell, R. L., J. CHEM. EDUC., 47,119 (1970). Douglas M. McEaehern Center for Research & Advanced Studies National Polytechnic Institute A.P. 14-740, Mexico 14, D. F.
190
/
Journal of Chemical Education
