A chemical reaction is called the process of transformation of substances, during which there is a change in their structure or composition. As a result of this process, the transition of the starting materials, or reagents, to the final products takes place. To date, a very clear classification of chemical reactions has been formed.
Description of reactions using equations. Signs of a chemical reaction
There are several classifications, each of which takes into account one or more features. For example, chemical reactions can be separated, paying attention to:
- the amount and composition of reagents and end products;
- state of aggregation of initial and final substances (gas, liquid, solid form);
- number of phases;
- the nature of the particles that are transported during the reaction (ion, electron);
- thermal effect;
- the possibility of the reaction in the opposite direction.
It is worth noting that chemical reactions are usually written using formulas and equations. Moreover, the left side of the equation describes the composition of the reagents and the nature of their interaction, and on the right side you can see the final products. Another very important point - the number of atoms of each element in the right and left parts should be equal. Only in this way is the law of conservation of mass observed .
As already mentioned, there are many classifications. Here will be considered the most commonly used.
Classification of chemical reactions according to the composition, quantity of initial and final products
Compound reactions: several substances enter into them, which combine to form a more complex substance. In most cases, this reaction is accompanied by heat.
Decomposition reactions: the starting reagent is a complex compound, which during decomposition forms several simpler substances. Such reactions can be both redox and occur without a change in valency.
Substitution reactions - are the interaction between a complex and a simple substance. In the process, some atom of a complex substance is replaced. Schematically, the reaction can be displayed as follows:
A + BC = AB + C
Exchange reactions are a process during which two initial reactants exchange components together. For instance:
AB + SD = HELL + CB
Transfer reactions - characterized by the transfer of an atom or group of atoms from one substance to another.
Classification of chemical reactions: reversible and irreversible processes
Another important characteristic of reactions is the possibility of the reverse process.
So, reactions are called reversible, the products of which can interact with each other, forming the same starting materials. As a rule, this feature must be displayed in the equation. In this case, two opposite arrows are placed between the left and right sides of the equation.
In an irreversible chemical reaction, its products are not able to react with each other - at least under normal conditions.
Classification of chemical reactions by thermal effect
Thermochemical reactions are divided into two main groups:
- exothermic processes during which heat (energy) is released;
- endothermic processes that require external energy absorption.
Classification of chemical reactions by the number of phases and phase characteristics
As already mentioned, the state of aggregation of substances is also of great importance for the complete characterization of a chemical reaction. According to these signs, it is customary to distinguish:
- gas reactions;
- reactions in solutions;
- chemical processes between solids.
But the initial and final products do not always refer to any one state of aggregation. Therefore, reactions are classified based on the number of phases:
- single-phase or homogeneous reactions are processes whose products are in the same state (in most cases, such a reaction proceeds either in the gas phase or in solution);
- heterogeneous reactions (multiphase) - reagents and final products can be in different aggregate states.