What is a chemical reaction?

Definition: In a chemical reaction, the process in which molecules break down into atoms, which then recombine to form new substances, is called a chemical reaction.

There are five main chemical reactions as follows: Isomerization: (A → B): The compound undergoes a structural rearrangement without any change in its chemical composition. 　　 Chemical synthesis: combination reaction. Abbreviated as: A + B = C: Two or more elements or compounds combine to form a complex product. (That is, a reaction in which one new substance is formed from two or more substances.) ) Chemical decomposition: a decomposition reaction, abbreviated as: A = B + C: a compound breaks down into its constituent elements or smaller molecules. (That is, the reverse reaction of a combination reaction.) It refers to a reaction in which a compound breaks down under specific conditions into two or more simpler elements or compounds. ) Substitution reaction (monosubstitution reaction) Briefly expressed as: A + BC = B + AC: This indicates that an additional reacting element replaces one of the elements in a compound. (That is, a reaction in which one element and one compound produce another element and another compound.) ) A substitution relationship refers to the replacement of an element that makes up a compound by an element that constitutes a simple substance. A displacement reaction must be a redox reaction, but a redox reaction is not necessarily a displacement reaction. ) Chemical reaction formulas: Based on the types of elements present in the reactants and products, displacement reactions can be categorized into 4 types: ① A more reactive metal displaces a less reactive metal or hydrogen; ② A more reactive non-metal displaces a less reactive non-metal; ③ A non-metal displaces a metal; ④ A metal displaces a non-metal. (For more details, please refer to the entry on displacement reactions…) Double-displacement reactions: Simplified as AB + CD = AD + CB: In an aqueous solution (also known as an ionic environment), two compounds exchange elements or ions to form different compounds. (That is, a reaction in which the components of two compounds exchange with each other to form two other compounds.) ) The essence of a double-displacement reaction is that the ions in the solution combine to form substances that are difficult to ionize (such as water), insoluble substances, or volatile gases, thereby driving the reaction to completion. Reactions that occur between acid, base, and salt solutions generally involve the exchange of components between two compounds. That is, the compounds involved in the reaction ionize in an aqueous solution to form freely moving ions, which then recombine to form new compounds; therefore, reactions between acid, base, and salt solutions are typically double-displacement reactions. Double-displacement reactions involve the recombination of ions or ion groups. Since the oxidation states of all elements remain unchanged before and after such reactions, double-displacement reactions are not redox reactions. 　　Of course, there are many more complex scenarios, but they can still be simplified step by step to be regarded as sequential reactions of the aforementioned reaction categories. Chemical reactions are diverse, making it difficult to establish simple classification criteria. However, some similar chemical reactions can still be classified. For example: Disproportionation reaction: This refers to an oxidation-reduction reaction that occurs between atoms of the same element with the same valence state within the molecules of the same substance. Elements in the same oxidation state experience \"differences in changes in oxidation state\" during redox reactions; some have their oxidation states increase while others decrease. Elements that undergo disproportionation reactions must have corresponding compounds in higher and lower oxidation states; disproportionation reactions occur only in elements with intermediate oxidation states. Fluorine (F2) does not undergo disproportionation, because fluorine has the highest electronegativity; it has no positive oxidation states and only negative ones. 　　Both auto-redox reactions and disproportionation reactions are types of redox reactions that occur between the same substance; disproportionation reaction is a type of auto-redox reaction, but not all auto-redox reactions are disproportionation reactions. 　　Reduction reaction (disproportionation reaction): Refers to an oxidation-reduction reaction that occurs between atoms of the same element in different oxidation states within a substance. That is, the valence state of the same element changes from its higher and lower valence states before the reaction to an intermediate valence state after the reaction. In chemical reactions, there is a rule regarding the change in an element’s valence state: it only moves closer, without crossing over. Therefore, both the high and low valences of an element can only move toward the middle. The reduction reaction and the disproportionation reaction are two ‘opposite’ processes, and both of these reactions are necessarily redox reactions. 　　Organic reactions: refer to various reactions that primarily involve carbon-based compounds. 　　Redox reaction: Refers to the transfer of electrons between two compounds (such as single-substitution reactions and combustion reactions). Combustion reaction (also called an oxidation reaction in junior high school chemistry textbooks): Refers to the reaction between a substance and oxygen.

Definitions and explanations of common chemical reactions: 1. Substitution reaction: A reaction in which certain atoms or atomic groups within an organic molecule are replaced by other atoms or atomic groups. ⑴Nitration reaction: The reaction in which the hydrogen atoms in a benzene molecule are replaced by —NO2. ⑵Sulfonation reaction: A reaction in which the hydrogen atoms in a benzene molecule are replaced by the sulfonic acid group (—SO3H) from sulfuric acid molecules. ⑶Esterification reaction: A reaction in which an acid and an alcohol react to form an ester and water. ⑷Hydrolysis reaction: Generally refers to a chemical reaction in which an organic compound reacts with water under certain conditions to produce two or more substances. (The saponification reaction also belongs to hydrolysis reactions.) Saponification reaction: The reaction in which fats and oils are hydrolyzed in the presence of a base to produce sodium higher fatty acids and glycerol. ) Hydrolysis reactions include hydrolysis of halohydrocarbons, esters, sugars, and proteins. Additionally: the polycondensation reaction also belongs to substitution reactions. 2. Addition reaction: A reaction in which the carbon atoms at either end of a double bond (or triple bond) in an organic molecule combine directly with other atoms or atomic groups to form new compounds. ⑴The hydrogenation of fats is an addition reaction. Hydrogenation of fats: It is a reaction in which liquid oils undergo an addition reaction with hydrogen gas in the presence of a catalyst such as Ni, under conditions of heat and pressure, thereby increasing the saturation level of the fats; this process is also known as fat hardening. ⑵Hydration reaction: In organic chemistry, it refers to the reaction in which unsaturated bonds (double or triple bonds) or carbonyl groups in a molecule combine with water molecules, in the presence or absence of a catalyst. 3. Elimination reaction: A reaction in which, under certain conditions, an organic compound loses a small molecule (such as H2O, HBr, etc.) from one of its molecules, resulting in the formation of an unsaturated compound containing double or triple bonds. 4. Polymerization reaction: A reaction in which molecules of compounds with relatively low molecular weights combine to form high-molecular-weight polymers. ⑴Polymerization reaction: A reaction in which unsaturated compounds with low molecular weight combine to form high-molecular-weight polymers. Such a polymerization reaction is also an addition reaction, which is why it is called addition polymerization. ⑵Polycondensation reaction: A reaction in which molecules with relatively low molecular weights combine to form high-molecular-weight polymers, while small molecules such as H2O, NH3, HX, etc., are produced. 5. Oxidation reaction: In organic reactions, a reaction in which oxygen atoms are added to organic molecule or hydrogen atoms are lost is generally referred to as an oxidation reaction. 6. Reduction reaction: In organic reactions, a reaction in which hydrogen atoms are added to an organic molecule or oxygen atoms are removed is usually called an oxidation reaction. 7. Denaturation: The process by which proteins undergo changes in their internal molecular structure and properties as a result of physical or chemical factors. 8. Cracking: The process of breaking down hydrocarbons with relatively high molecular weights and boiling points into hydrocarbons with lower molecular weights and boiling points under certain conditions. 9. Pyrolysis: In the petrochemical production process, it is a treatment method that uses higher temperatures than cracking (700°C–800°C, and sometimes even above 1000°C) to break down the long-chain hydrocarbons in petroleum distillation products (including petroleum gas) into short-chain hydrocarbons such as ethylene and propylene.