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ERIC Number: EJ1031159
Record Type: Journal
Publication Date: 2014-Mar
Pages: 8
Abstractor: As Provided
Reference Count: N/A
ISSN: ISSN-0021-9584
Expansion Work without the External Pressure and Thermodynamics in Terms of Quasistatic Irreversible Processes
Schmidt-Rohr, Klaus
Journal of Chemical Education, v91 n3 p402-409 Mar 2014
We demonstrate that the formula for irreversible expansion work in most chemical thermodynamics textbooks does not apply during the expansion process. Instead of the "external pressure" P[subscript ext], the pressure P[subscript sys,mb] on the piston or other moving boundary (hence the subscript mb), which is nearly equal to the system pressure P[subscript sys], should be used in the integral over volume. This formula only requires that P[subscript sys](V) and T are well defined, that is, a system of uniform P and T ("uPT") undergoing a "uPT process", which may be irreversible. An instructive example is an expanding gas accelerating a bullet horizontally and performing work without a conventional external pressure. We emphasize that dw = -P[subscript sys,mb] dV ˜ -P[subscript sys] dV is the only useful formula for infinitesimal PV work during a uPT process. The quasistatic approximation P[subscript sys,mb] = P [subscript sys] and dw = -P[subscript sys] dV is usually excellent and enables analyses of irreversible uPT processes, for example, in heat engines; friction in the surroundings and a large piston mass improve the approximation. Slow chemical reactions at constant T and P are quasistatic, and many equations in advanced chemical thermodynamics apply specifically to uPT or quasistatic processes. We show that the equality dS = dq[subscript irr]/T applies in irreversible quasistatic processes without composition change. In short, with well-defined P and T at constant composition, the simple equations for reversible processes are usually excellent approximations even when the process is irreversible.
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Publication Type: Journal Articles; Reports - Descriptive
Education Level: Higher Education; Postsecondary Education
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A