3. General chemical behaviour of condensed phase-gas phase systems at high temperature (Brewer’s rules)
Aim: To describe the behaviour and properties of high temperature vapours on a thermodynamic basis.
Topic description and teaching suggestions: A vapour coexisting with a condensed phase at high temperature may be constituted of more than one species. The relative chemical stability of, say, two vapour species may change with temperature. Brewer rationalized the unexpectedly complex molecular character of high temperature vapours on the basis of simple thermodynamic arguments. The so-called Brewer’s first rule essentially predicts that if a high temperature saturated vapour system contains several molecular species, then the lower concentration species will increase in relative importance as the temperature is increased. Typical cases are those where the monomer/dimer ratio in the vapour decreases on increasing temperature: the “double” role played by the vaporization enthalpy in determining the partial vapour pressures at any given temperature (Gibbs-Helmoltz equation) and their temperature dependence (van’t Hoff equation) can be underlined by assuming equal vaporization entropies as a first approximation. The role of entropy in determining the dominant species at high temperature can be analysed in more detail on the basis of translational and internal (roto-vibrational and electronic ) contributions. Many examples illustrate this rule. Just to mention a few, the vaporization of carbon-graphite to Cn with n=1-7 species, the vaporization of BeO (s) to BeOn (g) with n=1-6, etc. A good example of species with unusual oxidation states of the elements are: AlO, Al2O, AlO2, Al2O2 produced in the vaporization of alumina. A second Brewer’s rule indicates limitations on solid-gas reactions and states that a gas will react endothermically with a solid to produce a significant yield of reaction product only if the reaction produces at least as many moles of gas as are consumed in the reaction. Here again this behaviour depends on the trends in reaction entropy. The interplay of enthalpy and entropy effects in determining the most important gaseous products in various gas-solid reactions can be discussed. Typical example is a solid metal which at high temperature reacts with a