Chemodynamics is the dynamics of chemical reactions, or the study of that. In astronomy, the term is used in modeling to indicate a model includes chemical reactions between some of the modeled material. The term is also used to describe such reactions in systems such as galaxies or planet systems.
Chemical reactions take place at temperatures less than the material's Plasma temperature, which generally varies by nucleus size. Much of what is studied in astronomy is too hot, e.g., stars, and much observation is of Electromagnetic Radiation (EMR) generated by heating the elements, limiting our ability to observe chemical reactions at such a distance.