Before defining Chemical Reaction, its’ important to understand what elements, substances and compounds are, in Chemistry.

In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their nuclei. Unlike chemical compounds, chemical elements cannot be broken down into simpler substances by any chemical reaction.

A chemical compound is a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds. A molecule consisting of atoms of only one element is therefore not a compound.

Chemical Reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

The basis for different types of reactions is the product formed, the changes that occur, the reactants involved and so on.

Types of Chemical Reactions

Combustion Reaction :

A combustion reaction is a type of chemical reaction in which a compound and an oxidant are reacted to produce heat and a new product. The general form of a combustion reaction can be represented by the reaction between a hydrocarbon and oxygen, which yields carbon dioxide and water:

hydrocarbon + O2 → CO2 + H2O

In addition to heat, it’s also common (although not necessary) for a combustion reaction to release light and produce a flame.

In general terms, combustion is one of the most important of chemical reactions and may be considered a culminating step in the oxidation of certain kinds of substances. Though oxidation was once considered to be simply the combination of oxygen with any compound or element, the meaning of the word has been expanded to include any reaction in which atoms lose electrons, thereby becoming oxidized. As has been pointed out, in any oxidation process the oxidizer takes electrons from the oxidizable substance, thereby itself becoming reduced (gaining electrons). Any substance at all can be an oxidizing agent. But these definitions, clear enough when applied to atomic structure to explain chemical reactions, are not as clearly applicable to combustion, which remains, generally speaking, a type of chemical reaction involving oxygen as the oxidizing agent but complicated by the fact that the process includes other kinds of reactions as well and by the fact that it proceeds at an unusually fast pace. Furthermore, most flames have a section in their structure in which, instead of oxidations,  reduction  reactions occur. Nevertheless, the main event in combustion is often the combining of combustible material with oxygen.

Decomposition Reaction :

Chemical decomposition, or chemical breakdown, is the process or effect of simplifying a single chemical entity (normal molecule, reaction intermediate etc.) into two or more fragments. Chemical decomposition is usually regarded and defined as the exact opposite of chemical synthesis. In short, the chemical reaction in which two or more products are formed from a single reactant is called a decomposition reaction.

A reaction is also considered to be a decomposition reaction even when one or more of the products are still compounds. A metal carbonate decomposes into a metal oxide and carbon dioxide gas.

For example, calcium carbonate decomposes into calcium oxide and carbon dioxide:

CaCO3(s)→CaO(s)+CO2(g)

Metal hydroxides decompose on heating to yield metal oxides and water. Sodium hydroxide decomposes to produce sodium oxide and water:

2NaOH(s)→Na2O(s)+H2O(g)

Some unstable acids decompose to produce nonmetal oxides and water. Carbonic acid decomposes easily at room temperature into carbon dioxide and water:

H2CO3(aq)→CO2(g)+H2O(l)

Neutralization Reaction :

In chemistry, neutralization is a chemical reaction in which acid and a base react quantitatively with each other. In a reaction in water, neutralization results in there being no excess of hydrogen or hydroxide ions present in the solution. The pH of the neutralized solution depends on the acid strength of the reactants.

A neutralization reaction is when and acid and a base react to form water and a salt and involves the combination of H+ ions and OH- ions to generate water. The neutralization of a strong acid and strong base has a pH equal to 7. The neutralization of a strong acid and weak base will have a pH of less than 7, and conversely, the resulting pH when a strong base neutralizes a weak acid will be greater than 7.

When a solution is neutralized, it means that salts are formed from equal weights of acid and base. The amount of acid needed is the amount that would give one mole of protons (H+) and the amount of base needed is the amount that would give one mole of (OH-). Because salts are formed from neutralization reactions with equivalent concentrations of weights of acids and bases: N parts of acid will always neutralize N parts of base.

Neutralization Reaction Equation

acid + base(alkali) → salt + water

Neutralization reaction equation

Neutralization Reaction Example – Formation of Sodium Chloride (Common Salt):
HCl + NaOH → NaCl + H2O

Redox Reaction :

Redox (reduction–oxidation) is a chemical reaction in which the oxidation states  of atoms are changed. Redox reactions are characterized by the actual or formal transfer of electrons between chemical species, most often with one species (the reducing agent) undergoing oxidation (losing electrons) while another species (the oxidizing agent) undergoes reduction (gains electrons). The chemical species from which the electron is removed is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. 

Example of Redox Reactions – Reaction Between Hydrogen and Fluorine

In the reaction between hydrogen and fluorine, the hydrogen is oxidized whereas the fluorine is reduced. The reaction can be written as follows.

H2 + F2 → 2HF

The oxidation half-reaction is:  H2 → 2H+ + 2e–

The reduction half-reaction is:  F2 + 2e– → 2F–

The hydrogen and fluorine ions go on to combine in order to form hydrogen fluoride.

Precipitation or Double-Displacement Reaction :

Double displacement reactions may be defined as the chemical reactions in which one component each of both the reacting molecules is exchanged to form the products. During this reaction, the cations and anions of two different compounds switch places, forming two entirely different compounds.

The general equation which represents a double displacement reaction can be written as:

AB + CD  –>  AD + CB

The easiest way to identify a double displacement reaction is to check to see whether or not the cations exchanged anions with each other. Another clue, if the states of matter are cited, is to look for aqueous reactants and the formation of one solid product (since the reaction typically generates a precipitate).

Double displacement reactions generally take place in aqueous solutions in which the ions precipitate and there is an exchange of ions.

For example, on mixing a solution of barium chloride with sodium sulphate, a white precipitate of barium sulphate is immediately formed. These reactions are ionic in nature. The reactants changes into ions when dissolved in water and there is an exchange of ions in solution. This results in the formation of product molecule.

    
Double displacement reactions can be further classified as neutralization, precipitation and gas formation reactions.

Neutralization reactions are a specific kind of double displacement reaction. An acid-base reaction occurs, when an acid reacts with equal quantity of base. The acid base reaction results in the formation of salt (neutral in nature) and water.

Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction. This process usually takes place when the concentration of dissolved ions in the solution exceeds the solubility product.

Synthesis Reaction:

Synthesis reactions are reactions that occur when two different atoms or molecules interact to form a different molecule or compound. Most of the time, when a synthesis reaction occurs, energy is released and the reaction is exothermic. However, an endothermic outcome is also possible. Synthesis reactions are one of the major classes of chemical reactions, which include single displacement, double displacement, and combustion reactions.

The general form of a synthesis reaction is:

A + B → AB

Examples of synthesis reactions:

Water:
2 H2(g) + O2(g) → 2 H2O(g)

Carbon dioxide:
2 CO(g) + O2(g) → 2CO2(g)

Ammonia:
3 H2(g) + N2(g) → 2 NH3(g)

Aluminum oxide:
4 Al(s) + 3 O2(g) → 2 Al2O3(s)

Iron sulfide:
8 Fe + S8 → 8 FeS

Potassium chloride:
2 K(s) + Cl2(g) → 2 KCl(s)