While chemical kinetics is concerned with the rate of a chemical reaction, thermodynamics determines the extent to which reactions occur. In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal (the principle of dynamic equilibrium)  and the concentrations of the reactants and products no longer change. This is demonstrated by, for example, the Haber&ndash;Bosch process for combining nitrogen and hydrogen to produce ammonia. Chemical clock reactions such as the Belousov&ndash;Zhabotinsky reaction demonstrate that component concentrations can oscillate for a long time before finally attaining the equilibrium.