A general theory for solid state reactions, particularly of organics, that is based on the previously developed concept of chemical pressure is presented. Elastic multipoles are used to formulate the theory, subsequently yielding quantities that can be calculated using experimental data such as atomic displacement parameters, optical and acoustic phonon dispersion, or elastic constants. From this treatment, quantitative descriptions of the reaction cavity, steric compression, and reaction-induced stress are obtained. Analogies to atomic and frontier orbitals are noted. Further insight is gained into phonon assistance, the definition of homogeneity of a reaction, and the role of the product molecules, as perturbations, on the activation energy. The correlation of the chemical pressure to structural changes associated with the reaction is also examined. The generality and unifying aspects of the theory are demonstrated by application to detonation, thermal, and photochemical reactions in solids.