A model of thermodynamics of a reacting mixture of rarefied gases and a suspension of condensed species is developed by using statistical physics methods. An NVT  ensemble is considered for determining the detailed equilibrium chemical composition, and the minimum of the free energy of the mixture of possible species is found numerically. Tabular data for the species are used for determining the enthalpy and free energy of chemical compounds. An algorithm that allows the Chapman–Jouguet detonation parameters to be determined for a wide range of combustible mixtures is developed. The model is tested through comparisons of the predicted and experimental detonation velocities. Good agreement for mixtures with oxygen excess is demonstrated. For compositions with the formation of a significant amount of condensed carbon, the predicted and experimental detonation velocities agree reasonably well.