This document discusses reaction rates for different phase systems in chemical reactions. It defines reaction rate as the rate at which a chemical loses its identity per unit time and volume. The symbol used for reaction rate is ri. Reaction rates are calculated differently depending on the phase system. For a fluid-solid system, the rate is calculated based on the mass of solid (ri*). For a gas-solid system on a unit surface area, it is calculated based on the surface area (ri**). If considering a gas-solid system based on the volume of solid, it is calculated using the volume of solid (ri***). Finally, the reaction rate can be calculated based on the volume of the entire reactor (ri****). The different definitions of