2. Homogeneous Reaction System
A homogeneous reaction system is a kind of ideal reaction system where the reactant molecules evenly distribute in the system. In a macroscopic sense, there are no significant concentration differences in different regions. For every time point, the reaction rate is the same throughout the system. In this kind of reaction system, besides temperature, the only dominant factor for reaction rate is the molecular concentration along with the change therein.
In contrast, a heterogeneous (sometimes also called inhomogeneous) reaction system has a clear concept of the reaction interface. The molecular reactions only occur on interfaces of this kind. In this kind of system, the change in concentration is no longer the dominant factor for reaction rate.
In fact, outside of the interface, the reactant always maintains the initial concentration and the reaction rate is always zero. Besides temperature, the factor which dominates the reaction rate with respect to the interface is simply the geometry property of the interface itself, together with its evolution over time (expanding, contracting, thickening) and space (one-dimensional, two-dimensional, three-dimensional).
Both homogeneous and heterogeneous systems are ideal mathematical models. The real chemical reaction systems are often more complicated. But for small-sized reactions (like thermal analysis measurements with ~10mg sample amount), and under the assumption of idealized mass & heat transfer, most reactions can be attributed to one of these two systems.
In the field of thermal analysis, normally the uniform liquid-phase reactions (like reactions in solution) can be classified as homogeneous reactions, whereas the reactions involving multi-phases (gas/solid, liquid/solid, gas/liquid, solid/solid, liquid/liquid) shall be heterogeneous. But some uniform solid-phase reactions with no clear concept of reaction interface sometimes can also be simplified to a homogeneous model.
Once the kinetic model for a small-sized reaction system is obtained, a further correction for mass and heat transfer must be applied in order to scale up for actual industry application.
Note: Here, the terms ‘homogeneous’ and ‘heterogeneous’ only refer to the phase states of reactant and product. It is irrelevant whether the material composition is chemically pure substance or not.
One example would be the crystallization of a solid: although the material may be “pure” in the chemical sense, if the phase state of the crystalized region differs from that of the amorphous region, the reaction is heterogeneous.
Another example is the curing of fiber-reinforced resin. Although macroscopically the material is a composite containing various compositions (resin, fiber, etc.), on a smaller scale the distribution of fiber may also not be so uniform. However, if one ignores the interaction of resin and fiber, and supposes the curing reaction only takes place within the liquid resin, then it can still be classified as a homogeneous reaction.