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ERIC Number: EJ1221676
Record Type: Journal
Publication Date: 2019-Jul
Pages: 5
Abstractor: As Provided
ISSN: ISSN-0021-9584
Enthalpy and the Second Law of Thermodynamics
Keifer, David
Journal of Chemical Education, v96 n7 p1407-1411 Jul 2019
The change in enthalpy of a chemical reaction conducted at constant pressure is equal to the heat of the reaction plus the nonexpansion work of the reaction, ?H = qP + w[subscript additional]. After deriving that relationship, most general and physical chemistry textbooks set w[subscript additional] = 0 to arrive at the claim that ?H = qP, and nearly all further discussion of enthalpy assumes that ?H = qP. Setting w[subscript additional] = 0 is viable for spontaneous reactions, but for nonspontaneous reactions, wadditional ? 0 as a consequence of the second law of thermodynamics. Therefore, ?H ? qP for nonspontaneous reactions. Moreover, nonexpansion work is important for many interesting and important spontaneous reactions in biology (e.g., muscular movement, nerve signal transmission) and in modern society (e.g., batteries); incorporating w[subscript additional] into ?H allows for a more accurate discussion of the energy flow in these reactions. In this paper, I show that ?H ? qP for nonspontaneous reactions, and I discuss how ?H must be partitioned between qP and w[subscript additional] for several kinds of reactions according to the second law of thermodynamics. Finally, I suggest how the discussion of enthalpy could be corrected in general and physical chemistry textbooks.
Division of Chemical Education, Inc. and ACS Publications Division of the American Chemical Society. 1155 Sixteenth Street NW, Washington, DC 20036. Tel: 800-227-5558; Tel: 202-872-4600; e-mail:; Web site:
Publication Type: Journal Articles; Reports - Descriptive
Education Level: Higher Education; Postsecondary Education
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A
Grant or Contract Numbers: N/A