In Part I ideal gases and incompressible liquids have been introduced and the basic thermodynamic correlations have been derived. Part II has shown, that real fluids can be subject to a change of aggregate state and that many thermodynamic cycles are based on phase change, e.g. a steam power plant. Furthermore, Part II covered mixture of fluids, e.g. humid air or mixtures of ideal gases. However, within these mixture each component was stable and not part of a chemical reaction, i.e. decomposition of the present atoms and molecules. In Part III the focus now is on chemical reacting systems: First, the stoichiometry of a chemical reaction is investigated, i.e. the principle of mass conservation is applied to reactants and products of a chemical reaction. In doing so it is possible to predict the composition of the products. This is important for instance, when the composition of an exhaust gas of a combustion process needs to fulfil technical thresholds. Second, a chemical reacting system is investigated in terms of an energy balance. In this chapter the heating-value approach is followed with focus on technical combustions, i.e. combustions based on fossil fuels. In Chap. 24 another energetic approach based on absolute enthalpy respectively entropy is introduced. Major advantage of this method is, that the irreversibility of chemical reactions can be quantified.