The chemistry textbook authors (Engel & Reid, 2010) define both open and closed (as well as isolated) systems: “If a system can exchange matter with the surroundings, it is called an open system; if not, it is a closed system” (p. 2). Living cells are described as examples of open systems. However, the chemistry textbook pays very little attention to open systems; the isenthalpic Joule-Thomson expansion in an open system is dealt with briefly in the text (p. 59). In fact, the textbook authors go to some length to recontextualise an open system such as the heating of an open beaker containing a liquid (a typical chemistry laboratory setting) as a closed
99 system. This is managed by imagining a closed adiabatic boundary around it that does not allow the transfer of heat to the outside. (See Fig. 2.4, Engel & Reid, 2010, p. 20).
Engel and Reid justify this by explaining that only the part of the surroundings closest to the system interacts with the system:
Defining the surroundings as the rest of the universe is impractical, because it is not realistic to search through the whole universe to see if a mass has been raised or lowered and if the temperature of a reservoir has been changed. (p. 19)
In order to show that limiting interest in the ‘surroundings’ of a system to the immediate surroundings is a generalisable
assumption, Engel and Reid propose an imaginary isolated composite system consisting of a rigid sealed reaction vessel with diathermal walls immersed in an inner water bath which is in turn immersed in an outer water bath that separates it from the rest of the universe (see Fig 2.3 p. 19). The outer water bath is kept at the same temperature as the inner water bath, so no heat will flow from the inner to the outer water bath. In this way the composite system (reaction vessel + inner water bath) is isolated from the rest of the universe, which can be disregarded in any process. This fairly elaborate thought experiment is used to argue that only the immediate surroundings of a system are of interest. Effectively
a ‘new’ closed system (system II) is created from the open system: system I is the beaker; system II is [system I + immediate surroundings]. This is an example of the high value placed on generalisability in the chemistry text, and the argument is based on plausible empirical
conditions, even if they are virtual in this case.
In what at first glance looks like an interest in particulars, the chemistry authors briefly refer to examples of some real-life thermodynamics processes in the introduction to the textbook. The reason for this is explicitly stated as a motivation to demonstrate the usefulness and relevance of thermodynamics as a subject area. The examples could, interestingly enough, be considered typical (chemical) engineering examples: the first example refers to the yield from a plant built to synthesise ammonia gas from H2 and N2 being insufficient to make the process profitable.
According to thermodynamics principles, the yield at equilibrium can be increased by increasing pressure and decreasing temperature. (The economic viability of a process is a typical engineering concern.) The second example describes a brief to use methanol to power a
FIGURE 2.3
An isolated composite system is created in which the surroundings to the system of interest are limited in extent. The walls surrounding the inner water bath are rigid.
100 car. An engineer designs an internal combustion engine that runs on the combustion of
methanol. Another engineer designs a fuel cell. The claim is that the vehicle will travel further using the fuel cell. Thermodynamics makes it possible to compare the efficiencies of the two methods. (Improving the efficiency of a product is a classical engineering problem.) The third example refers to the need for a new battery to power a hybrid car. To provide the required voltage a significant number of electrochemical cells have to be connected in series, but because space is limited in the car, as few as possible need to be used. Lists of possible cell reactions and tabulated values of thermodynamic functions can be used to determine the number of cells needed. (This type of optimisation application is frequently used in engineering problems.) It is important to note that apart from this brief mention in the introduction, the chemistry textbook authors never return to these types of problems anywhere in the textbook. The particulars present in the examples above are recruited in the service of the emphasis on universals. Problems in the textbook are always presented in a generalised, idealised laboratory setting.
The specialisation modality of the knowledge orientation in chemistry displayed here is therefore towards the universals or generalisations, rather than particular, specific instances. Any ‘devices’ (such as a piston and cylinder device) are presented as idealised conceptual tools for thinking about generalised conditions with no reference to real-life problem settings, and the secondary modality of idealisation is therefore directed towards the abstract-ideal theorisation of a laboratory setting.
Both mechanical engineering and chemical engineering distinguish between open and closed systems, which are dealt with extensively in the texts. In all instances the context is specific engineering devices and processes. The physics and chemistry textbooks, however, confine their discussions to closed systems, and although the chemistry text defines an open system, it is presented in idealised laboratory contexts, removed from the complexities of a real-world setting.
Principal Mode: universals Secondary Modality: idealisation, Mode: abstract-ideal theorisation
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