In modern science, there are chemical and nuclear reactions that occur as a result of the interaction of the starting materials, which are commonly called reagents. As a result, other chemicals called products are formed. All interactions occur under certain conditions (temperature, pressure, light, radiation, the presence of catalysts, etc.). The nuclei of the atoms of the reagents of chemical reactions do not change. In nuclear transformations, new nuclei and particles are formed. There are several different signs by which types of chemical reactions are determined.
The classification can be based on the number of starting and forming substances. In this case, all types of chemical reactions are divided into five groups:
- Decompositions (several new ones are obtained from one substance), for example, decomposition by heating potassium chlorate into potassium chloride and oxygen: KCLO3 → 2KCL + 3O2.
- Compounds (two or more compounds form one new), interacting with water, calcium oxide turns into calcium hydroxide: H2O + CaO → Ca (OH) 2;
- Substitutions (the number of products is equal to the number of starting materials in which one component is replaced by another), iron in copper sulfate, replacing copper, forms ferrous sulfate: Fe + CuSO4 → FeSO4 + Cu.
- Double exchange (molecules of two substances exchange the parts leaving them), the metals in potassium iodide and silver nitrate exchange anions, forming precipitated silver iodide and cadmium nitrate: KI + AgNO3 → AgI ↓ + KNO3.
- Polymorphic transformation (a substance is transferred from one crystalline form to another), red mercury iodide, when heated, turns into yellow mercury iodide: HgI2 (red) ↔ HgI2 (yellow).
If chemical transformations are considered on the basis of a change in the degree of oxidation of elements in reacting substances, then the types of chemical reactions can be divided into groups:
- With a change in the degree of oxidation - redox reactions (OVR). As an example, we can consider the interaction of iron with hydrochloric acid: Fe + HCL → FeCl2 + H2, as a result, the degree of oxidation of iron (a reducing agent that gives electrons) changed from 0 to -2, and of hydrogen (an oxidizing agent that takes electrons) from +1 to 0 .
- No change in oxidation state (i.e., no OVR). For example, the reactions of acid-base interaction of hydrogen bromide with sodium hydroxide: HBr + NaOH → NaBr + H2O, as a result of such reactions, salt and water are formed, and the oxidation states of the chemical elements included in the starting materials do not change.
If we consider the chemical equilibrium and the rate of flow in the forward and reverse directions, then all types of chemical reactions can also be divided into two groups:
- Reversible - those that simultaneously flow in two directions. Most reactions are reversible. An example is the dissolution of carbon dioxide in water with the formation of unstable carbonic acid, which decomposes into the starting materials: H2O + CO2 ↔ H2CO3.
- Irreversible - they flow only in the forward direction, after the complete consumption of one of the starting materials is completed, after which there are only products and the starting material, taken in excess. Typically, one of the products is either a precipitated insoluble substance or a liberated gas. For example, during the interaction of sulfuric acid and barium chloride: H2SO4 + BaCl2 + → BaSO4 ↓ + 2HCl, insoluble barium sulfate precipitates .
The types of chemical reactions in organic chemistry can be divided into four groups:
- Substitution (one atom or group of atoms is replaced by another), for example, the interaction of chloroethane with sodium hydroxide produces ethanol and sodium chloride: C2H5Cl + NaOH → C2H5OH + NaCl, that is, the chlorine atom is replaced by a hydrogen atom.
- Attachment (two molecules react and form one), for example, bromine is attached at the site of double bond cleavage in the ethylene molecule: Br2 + CH2 = CH2 → BrCH2 – CH2Br.
- Cleavage (a molecule decomposes into two or more molecules), for example, under certain conditions, ethanol decomposes into ethylene and water: C2H5OH → CH2 = CH2 + H2O.
- Rearrangement (isomerization, when one molecule turns into another, but the qualitative and quantitative composition of atoms in it does not change), for example, 3-chlorutene-1 (C4H7CL) turns into 1 chlorobutene-2 (C4H7CL). Here, the chlorine atom passed from the third carbon atom in the hydrocarbon chain to the first, and the double bond connected the first and second carbon atoms, and then began to connect the second and third atoms.
Other types of chemical reactions are known:
- By the thermal effect: reactions proceeding with absorption (endothermic) or heat (exothermic).
- By type of interacting reagents or products formed. Interaction with water - hydrolysis, with hydrogen - hydrogenation, with oxygen - oxidation or combustion. Cleavage of water - dehydration, hydrogen - dehydrogenation, and so on.
- According to the conditions of interaction: in the presence of catalysts (catalytic), under the influence of low or high temperature, with a change in pressure, in the light, and so on.
- By the mechanism of the reaction: ionic, radical-chain or chain reactions.