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Campbell's Rule for Estimating Entropy Changes in Gas-Producing and Gas-Consuming Reactions and Related Generalizations about Entropies and Enthalpies

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Norman C. Craig

Department of Chemistry, Oberlin College, Oberlin, OH 44074-1083

J. Chem. Educ., 2003, 80 (12), p 1432

DOI: 10.1021/ed080p1432

Publication Date (Web): December 1, 2003

Section:

History, Education, and Documentation

Abstract

Learning how to estimate thermodynamic quantities and how to use estimates extends the usefulness of thermodynamic analyses of chemical reactions. Estimating helps chemists understand why chemical reactions occur and why the outcomes change with temperature. A new generalization with the suggested name of Campbell's rule, where Δ_rS° ≈ 140 J/(K mol rxn) per net mole of gas produced in a reaction, is introduced. This rule enables estimating entropies of reaction from the net change in moles of gas whenever the formation or combination of aqueous ions is not involved. A number of patterns in molar entropies are identified. These patterns are the basis for making good estimates of molar entropies of unknown or transient species from entropies of known species of the same phase, same bond type, same shape, similar composition, and similar mass. For several other types of processes, including reactions involving ions, the sign of Δ_rS° is considered. For molar enthalpies and enthalpies of reaction we have fewer patterns than for entropies on which to build. The principal exceptions are recognizing the patterns in bond dissociation enthalpies and estimating enthalpies of reaction for gas phase processes. A simple scheme for estimating enthalpies of reaction for the gas phase is described.

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Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Physical Chemistry

Keywords (Subject):

Gases

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This article has been cited by 2 ACS Journal articles (2 most recent appear below).

Campbell's Rule for Estimating Entropy Changes
William B. Jensen
Journal of Chemical Education2004 81 (11), 1570
- Campbell's Rule for Estimating Entropy Changes
  William B. Jensen
  Journal of Chemical Education2004 81 (11), 1570
  In a recent article Norman Craig has proposed the rule-of-thumb that the approximate value of the entropy of reaction is related to the net moles of gas consumed or generated in the reaction.
  Abstract | Hi-Res PDF | PDF w/ Links
Campbell's Rule for Estimating Entropy Changes (the author replies)
Norman C. Craig
Journal of Chemical Education2004 81 (11), 1571
- Campbell's Rule for Estimating Entropy Changes (the author replies)
  Norman C. Craig
  Journal of Chemical Education2004 81 (11), 1571
  The author revises an aspect of selecting a numerical value for Campbell’s rule that was overlooked in his original presentation.
  Abstract | Hi-Res PDF | PDF w/ Links