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.

Undergraduate students' conceptions of enthalpy, enthalpy change and related concepts

“…if DV = 0 then pDV = 0 even if the context is Dp a 0 and DV = 0). Nilsson and Niedderer (2012) show that students often correctly relate volume to work, but pressure is not mentioned often. Also, Carson (2001) and Sözbilir (2001) describe results similar to those presented here.…”

“…Thus far, no evidence of this conception has been found, and Sözbilir (2001) only describes results similar to Conception F7, as does Carson (2001). However, when analysed with Nilsson and Niedderer's (2012) matrix, the examples given would yield both incomplete and incorrect descriptions of the expansion work. Therefore, it is possible that this conception deepens previous results regarding why students argue that enthalpy change is heat at constant volume.…”

“…Carson (2001) describes that ''students had difficulty in associating work with chemical reactions'' (p. 118), and presents findings such as work equals bond breaking, the presence of a piston or raising weights. Nilsson and Niedderer (2012) show that in their two groups 59% and 37% of the answers regarding work are left blank. This could be interpreted as students having difficulty seeing the work actually being done.…”

“…This study supports this finding, since students tend not to focus on constant pressure and expansion work as they argue why the heat must be the same. Nilsson and Niedderer's (2012) matrix shows how students use pressure and volume when explaining work, expansion work and technical work, and stress the importance of using the general definition of enthalpy change since it is difficult for students to argue for a constant volume/ varying pressure case if they have only been introduced to DH = DU + pDV. This finding is in accordance with Carson's (2001) and Sözbilir's (2001) results that enthalpy change is heat at constant volume.…”

“…Five of 22 students in Carson's (2001) interview study explicitly state constant volume, and Sözbilir's (2001) analysis of questionnaire responses groups this description as one of four that together form one misunderstanding. Recently, Nilsson and Niedderer (2012) have analysed how students use pressure and volume when describing work, expansion work and technical work. Their contextual analysis shows that about 86% and 69% of the descriptions offered by two groups of students, respectively, are either incomplete or incorrect.…”