A phenomenological treatment for the thermodynamic and kinetic description of chemical reactions in the formation of clusters is developed. This theory is valid for isothermal processes in closed systems. It takes into account the consumption of the reactants as the reaction proceeds. The surface of the Helmholtz free energy of the reaction is calculated dependent on cluster size and cluster concentration. This surface has a bottom; the bottom begins in the initial state of the system (only reactants are present). It ends at the stable equilibrium (bulk phase in equilibrium with reactants and products). The bottom is assumed to be the trajectory of the reaction from the initial state to the stable equilibrium of the system. A kinetic rate equation for the reaction is derived. The rate equation is tested by experimental results. These results are obtained for the combustion of ethine in a low pressure flame by a molecular beam mass spectrometrical technique. Good agreement is found between the experimental results and the solution of the rate equation. Various parameters of the reaction can be calculated.