Enthalpy and Entropy Values Deduced from Gibbs Free Energy Measurements

Authors

Robert F. Brebrick, Marquette UniversityFollow

Document Type

Article

Publication Date

1976

Publisher

Humana Press

Source Publication

High Temperature Science

Source ISSN

1080-1278

Abstract

An overlooked feature of second-law enthalpy and entropy values is illustrated using recent data for the dissociation of As₄(g). Evaluation of a measure-of-fit, δ, for an experimental set of equilibrium constant-temperature values at values of ΔH₂₉₈ and ΔS₂₉₈ other than those minimizing δ² gives contours of constant δ that are long, very narrow ellipses. The value of δ at the contour corresponding to a 95% probability that the true values for ΔH₂₉₈ and ΔS₂₉₈ fall within this contour is calculated assuming a normal error distribution. Within an uncertainty of a few hundred calories in the enthalpy the values of ΔH₂₉₈ and ΔS₂₉₈ fall along a segment of a best-fit line that covers 8 kcal or more in ΔH₂₉₈. The best-fit line for each set has a slope equal to the reciprocal of the average value of 1/T for the set and an intercept simply- related to ΔG at this average temperature. Thus the most certain result of an experimental set of ΔG-T values is the value of ΔG at this average temperature, i.e., the determination of a best-fit line. Similar results follow from a conventional, linear least-squares analysis if the correlation coefficient as well as the standard deviations between ΔH₂₉₈ and ΔS₂₉₈ are calculated and properly Interpreted. The best-fit line feature leads naturally to a correlation between second law ΔH₂₉₈ and ΔS₂₉₈ values from different sets that are equally free of systematic errors, It is shown that the best-fit-line feature is very pronounced whenever the range (standard deviation) in the experimental values of 1/T is small compared to the average value. It Is therefore most likely characteristic of many ΔG-T sets.

Comments

High Temperature Science, Vol. 8 (1976): 11-23. Publisher link.

Recommended Citation

Brebrick, Robert F., "Enthalpy and Entropy Values Deduced from Gibbs Free Energy Measurements" (1976). Mechanical Engineering Faculty Research and Publications. 298.
https://epublications.marquette.edu/mechengin_fac/298