As you know, a chemical reaction is the transformation of one substance, which is usually called the starting material into another, which will already differ from it in chemical composition, as well as structure. In the first case, they talk about reagents, and in the second - about the reaction products. This is known to everyone else from the school chemistry course. Also known from the course of school chemistry is the fact that during such a reaction the composition of chemical elements does not change, in this case only the distribution of nuclei and electrons occurs, as a result of which new chemical substances are formed.
All chemical reactions can be divided into classes. Homogeneous reactions should be noted first. These are reactions that occur in one phase. Homogeneous reactions, examples of which are also easy to find in any textbook on chemistry, imply the occurrence of a wide variety of homolytic reactions, characterized by the decay of molecules into radicals, substitution and cleavage, rearrangement, and so on. And this happens, as a rule, in the liquid phase. The speed of a homogeneous reaction, if the volume of substances remains constant, will depend on the law of the acting masses. Homogeneous reactions that occur between reactants that are initially in different phases become possible only if diffusion occurs between them. Then the reaction rate will increase significantly. If the diffusion of the starting materials is slow, then the homogeneous reaction itself will occur at the interface of the diffusion phases. All the features of such reactions, as well as their mechanisms, will depend on the conditions under which the reactants will act.
The reaction rate will depend on some factors, such as: the nature of the substances that enter into the reaction, their concentration, the temperature at which the reaction occurs, as well as the presence of a specific catalyst in it. It helps the reaction proceed with greater speed and with less activation energy. As for the concentration of substances, it is clear that the more substances enter the reaction, the more they will collide and, as a result of this, the reaction rate will increase.
A vivid example of a homogeneous system is any mixture of gases. All gases at high temperature will dissolve in large quantities in each other. This also applies to a mixture of nitrogen and oxygen. One more example can be given showing what a homogeneous reaction is. It is nothing but an aqueous solution containing sodium chloride, magnesium sulfate, nitrogen and oxygen. The system passes in this case only one phase. If the reaction proceeds in a homogeneous system, then it proceeds in its entirety. If, say, when draining, as well as when mixing solutions of sulfuric acid and sodium thiosulfate, turbidity will occur. It will occur due to sulfur, which will appear in the entire volume of the solution.
During the course of a particular homogeneous reaction, a certain amount of heat is released. The thermal effect of the reaction will depend on the amount of reagents taken in the reaction, as well as on what products result from the reaction. The question with what speed a homogeneous reaction will proceed will depend on what period of time it proceeds. A whole science is involved in clarifying these issues. It is called Kinetics. Its most important postulate is the fact that for the reaction to take place, the molecules that participate in it must constantly come into contact with each other. However, not every such contact can lead precisely to a chemical reaction. Homogeneous reactions are only those in the course of which contacting particles will have such a supply of energy that will exceed a threshold value.