An analytical tool to determine undergraduate students' use of volume and pressure when describing expansion work and technical work

Abstract

In undergraduate chemical thermodynamics teachers often include equations and view manipulations of variables as understanding. Undergraduate students are often not able to describe the meaning of these equations. In chemistry, enthalpy and its change are introduced to describe some features of chemical reactions. In the process of measuring heat at constant pressure, work is often disregarded. Therefore, we investigated how undergraduate students describe expansion work and technical work in relation to enthalpy and its change. Three empirical studies (n_tot = 64, n_tot = 22, n_tot = 10) with undergraduate chemistry students taking their first or fifth chemistry course at two Swedish universities were conducted. Questions on enthalpy and its change, internal energy and its change, heat and work were administered in questionnaires, exam questions, hand-ins and interviews. An analytical matrix was developed and qualitative categories with respect to pressure and volume were formed. The results indicate that work in general and even more so expansion work and technical work are difficult processes to describe and relate to the definition and formula of enthalpy change. Work is mainly described without reference to pressure and volume. The properties of volume are more likely to be described correctly than the properties of pressure. The definition of enthalpy change at constant pressure is generalised to constant volume/varying pressure cases. This study gives further insight into the way in which students use pressure and volume as they describe expansion work and technical work as well as the contextual correctness of these descriptions. The matrix and categories can be used by researchers, teachers and students.