Overview of Condensed Phase Methods

This part includes a discussion of the main experimental methods that have been used to study the energetics of chemical reactions and the thermodynamic stability of compounds in the condensed phase (solid, liquid, and solution). The only exception is the reference to flame combustion calorimetry in section 7.3. Although this method was designed to measure the enthalpies of combustion of substances in the gaseous phase, it has very strong affinities with the other combustion calorimetric methods presented in the same chapter. Most published enthalpies of formation and reaction in the condensed phase were determined by calorimetry (see databases indicated in appendix B). It is therefore not surprising that the discussion of calorimetric methods occupies a large fraction of part II. The heart of a calorimeter is the calorimeter proper (also called measuring system or sample cell), which contains the reaction vessel, where the chemical reaction or phase transition under study occurs. Sometimes the calorimeter proper coincides with the reaction vessel. For example, in the setup shown in figure 6.1a, which is typical of many combustion calorimeters, the reaction vessel is placed inside the calorimeter proper. In the arrangement of figure 6.1b, used in many reaction-solution calorimeters, the calorimeter proper is also the reaction vessel. Normally, a controlled-temperature jacket surrounds the calorimeter proper. Other parts besides thermometers, commonly found in calorimeters, are stirring, heating, cooling, and ignition devices. Some of these devices are placed inside the calorimeter proper or cross its boundaries and are also considered to be part of it. In modern instruments, the data acquisition and many steps of the calorimetric experiments are usually computer-controlled. Calorimeters of many different designs have been constructed and operated. However, these are all variations of a few basic categories. For example, based on the heat exchange mode between the calorimeter proper and the surrounding jacket, it is convenient to distinguish three main classes of calorimeters: adiabatic, heat conduction, and isoperibol. In a perfectly adiabatic calorimeter no heat is transferred between the calorimeter proper and the jacket (the corresponding heat flow rate Φ = dQ/dt = 0, where Q represents the heat exchanged and t is time).