**ERIC Number:**EJ1031159

**Record Type:**Journal

**Publication Date:**2014-Mar

**Pages:**8

**Abstractor:**As Provided

**Reference Count:**N/A

**ISBN:**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.

Descriptors: Science Instruction, Chemistry, Thermodynamics, Equations (Mathematics), Scientific Concepts, College Science, Misconceptions, Teaching Methods

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: eic@jce.acs.org; Web site: http://pubs.acs.org/jchemeduc

**Publication Type:**Journal Articles; Reports - Descriptive

**Education Level:**Higher Education; Postsecondary Education

**Audience:**N/A

**Language:**English

**Sponsor:**N/A

**Authoring Institution:**N/A