Sulfur trioxide polymorphism

Sulfur trioxide is prepared in the laboratory by heating fuming sulfuric acid, condensing its vapors, and collecting in a cool receiver. When vapors are condensed below 27°C in the presence of trace moisture, aU three polymorphic phases of SO3 are produced. They can be separated by fractional distillation. Condensation of the vapors above 27°C forms the hquid variety of gamma-sulfur trioxide. 

Figure 16.17b), as does 0t-SO3 in which the chains are arranged into layers in the solid state. Differences in the thermodynamic properties of the different polymorphs are very small, although they do react with water at different rates. Sulfur trioxide is very reactive and representative reactions are given in scheme 16.95. 

VA Fig. 16.17 The structures of solid state polymorphs of sulfur trioxide contains tetrahedral SO4 units (a) Y-SO3 consists of trimeric units and (b) a- and P-SO3 contain polymeric chains. Colour code S, yellow O, red. 

Sulfur trioxide condenses at 44.5°C to a colorless liquid, which freezes at I6.8°C to transparent crystals. The substance is polymorphous, these crystals being the unstable form (the a-form). The stable form consists of silky asbestos-like crystals, which are produced when the a-crystals or the liquid stands for some time, especially in the presence of a trace of moisture (Fig. 8-3). There exist also one or more other forms of this substance, which are hard to investigate because the changes from one form to another are very slow. The asbestos-like crystals slowly evaporate to SO3 vapor at temperatures above 50°C. 

---