Chemistry is the science of substances and their transformations, as well as methods for their preparation. Even the regular school curriculum addresses such an important issue as the types of reactions. The classification, which students are introduced to at a basic level, takes into account the change in the degree of oxidation, the phase, the mechanism of the process, etc. In addition, all chemical processes are divided into non-catalytic and catalytic reactions. Examples of transformations occurring with the participation of a catalyst are found in humans in ordinary life: fermentation, decay. Non-catalytic transformations are much less common.
What is a catalyst?
This is a chemical substance that can change the rate of interaction, but does not participate in it itself. In that case, when the catalyst accelerates the process, we are talking about positive catalysis. In the event that the substance added to the process reduces the reaction rate, it is called an inhibitor.
Types of Catalysis
Homogeneous and heterogeneous catalysis differ in the phase in which the starting materials are located. If the initial components taken for interactions, including the catalyst, are in the same aggregate state, homogeneous catalysis proceeds. In the case when substances different in phase take part in the reaction, heterogeneous catalysis occurs.
Action selectivity
Catalysis is not just a means of increasing the productivity of the equipment, it positively affects the quality of the products obtained. This phenomenon can be explained by the fact that due to the selective (selective) action of most catalysts, a direct reaction is accelerated, side processes are reduced. Ultimately, the resulting products are of great purity, there is no need to further purify the substances. The selectivity of the catalyst gives a real reduction in non-production costs of raw materials, a good economic benefit.
The benefits of using a catalyst in production
What else are characterized by catalytic reactions? The examples considered in a regular high school indicate that the use of a catalyst allows the process to be carried out at lower temperatures. Experiments confirm that it can be used to significantly reduce energy costs. This is especially important in modern conditions, when there is a shortage of energy resources in the world.
Examples of catalytic production
What industry use catalytic reactions? Examples of such industries: the manufacture of nitric and sulfuric acids, hydrogen, ammonia, polymers, oil refining. Catalysis is widely used in the production of organic acids, monohydric and polyhydric alcohols, phenol, synthetic resins, dyes, and medicines.
What is the catalyst
The catalysts can be many substances that are in the periodic system of chemical elements of Dmitry Ivanovich Mendeleev, as well as their compounds. Among the most common accelerators are: nickel, iron, platinum, cobalt, aluminosilicates, manganese oxides.
Catalyst Features
In addition to the selective action, the catalysts have excellent mechanical strength, they are able to withstand catalytic poisons, are easily regenerated (restored).
According to the phase state, catalytic homogeneous reactions are divided into gas-phase and liquid-phase.
We consider these types of reactions in more detail. In solutions, the accelerator of the chemical transformation are hydrogen cations H +, hydroxide ions, OH– ions, metal cations M +, and substances that contribute to the formation of free radicals.
The essence of catalysis
The mechanism of catalysis during the interaction of acids and bases is that there is an exchange between the interacting substances and the catalyst with positive ions (protons). In this case, intramolecular transformations occur. This type of reaction is:
- dehydration (water disengagement);
- hydration (addition of water molecules);
- esterification (ester formation from alcohols and carboxylic acids);
- polycondensation (polymer formation with water cleavage).
The theory of catalysis explains not only the process itself, but also possible side effects. In the case of heterogeneous catalysis, the process accelerator forms an independent phase, some centers on the surface of the reacting substances possess catalytic properties, or the entire surface is involved.
There is also a microheterogeneous process, which involves finding the catalyst in a colloidal state. This option is a transition state from a homogeneous to a heterogeneous type of catalysis. Most of these processes occur between gaseous substances using solid catalysts. They can be in the form of granules, tablets, grains.
The spread of catalysis in nature
Enzymatic catalysis is quite widespread in nature. It is with the help of biocatalysts that protein molecules are synthesized, metabolism is carried out in living organisms. Not a single biological process involving living organisms escapes catalytic reactions. Examples of vital processes: synthesis of body-specific proteins from amino acids; the breakdown of fats, proteins, carbohydrates.
Catalysis Algorithm
Consider the mechanism of catalysis. This process, proceeding on porous solid accelerators of chemical interaction, includes several elementary stages:
- diffusion of interacting substances to the surface of the grains of catalyst from the core of the stream;
- diffusion of reagents in the pores of the catalyst;
- chemisorption (activated adsorption) on the surface of a chemical reaction accelerator with the appearance of chemical surface substances — activated catalyst-reagent complexes;
- atom rearrangement with the appearance of surface catalyst-product combinations;
- diffusion in the pores of the product reaction accelerator;
- diffusion of the product from the surface of the grain of the reaction accelerator into the flow core.
Catalytic and non-catalytic reactions are so important that scientists for many years continue research in this area.
With homogeneous catalysis, there is no need to build special structures. Enzymatic catalysis in the heterogeneous version involves the use of a variety of and specific equipment. For its course, special contact devices have been developed, which are subdivided along the contact surface (in tubes, on walls, catalyst grids); with a filter layer; weighted layer; with a moving dusty catalyst.
Heat transfer in the apparatus is realized in different ways:
- by using external (external) heat exchangers;
- using heat exchangers built into the contact apparatus.
By analyzing the chemical formulas, one can also find reactions in which one of the final products acts as a catalyst, which is formed during the chemical interaction of the starting components.
Such processes are commonly called autocatalytic, the phenomenon itself in chemistry is called autocatalysis.
The rate of many interactions is related to the presence of certain substances in the reaction mixture. Their formulas in chemistry are most often overlooked, replaced by the word "catalyst" or its abridged version. They are not included in the final stereochemical equation, since they do not change from the quantitative point of view after the completion of the interaction. In some cases, small quantities of substances are sufficient to significantly affect the speed of the ongoing process. Situations when the reaction vessel itself acts as an accelerator of chemical interaction are quite acceptable.
The essence of the effect of the catalyst on the change in the speed of the chemical process is that this substance is included in the active complex, and therefore changes the activation energy of the chemical interaction.
During the decomposition of this complex, regeneration of the catalyst is observed. The bottom line is that it will not be spent, will remain unchanged after the end of the interaction. For this reason, an insignificant amount of the active substance is quite enough to carry out the reaction with the substrate (reactant). In reality, insignificant amounts of catalysts are still consumed during chemical processes , since various side processes are possible: its poisoning, technological losses, change in the state of the surface of the solid catalyst. Chemistry formulas do not include catalyst accounting.
Conclusion
Reactions in which the active substance (catalyst) takes part surround the person, and they also occur in his body. Homogeneous reactions are much less common than heterogeneous interactions. In any case, at first the formation of intermediate complexes occurs, which are unstable, are gradually destroyed and regeneration (restoration) of the accelerator of the chemical process is observed. For example, in the interaction of metaphosphoric acid with potassium persulfate, iodic acid acts as a catalyst. When it is added to the reactants, a yellow solution forms. As you approach the end of the process, the color gradually disappears. In this case, iodine acts as an intermediate product, and the process occurs in two stages. But as soon as metaphosphoric acid is synthesized, the catalyst returns to its original state. Catalysts are indispensable in industry, they help to accelerate the transformation, to obtain high-quality reaction products. Biochemical processes in our body are also impossible without their participation.