In chemistry, chemical reactions are those reactions in which the different aspects of the chemicals involved in the reaction undergo significant changes. In chemical reactions, reactants are the species that change to form a new species in their orientation, bonding and composition under various conditions like temperature, pressure, catalysts, atmosphere and many other circumstances.

Types of chemical reactions:

On the basis of the process of formation of products from the reactants, chemical reactions can be classified into:

• Combination reactions: As the name suggests,  combination reactions may be explained as reactions when two or more than two species react with each other as reactants to form a single species called product. An example of a combination reaction is the combination of sodium (Na) metal with chlorine gas (Cl₂) for the formation of sodium chloride (NaCl) salt.

2Na  +  Cl₂   2NaCl

• Decomposition reaction: In this type of reaction, a whole single molecule breaks down into two or more than two molecules as a product. An example of a decomposition reaction is the decomposition of calcium carbonate (CaCO₃) solid molecules into calcium oxide (CaO) and carbon dioxide (CO₂) gas.

CaCO₃   CaO   +  CO₂

• Displacement reaction: In displacement reaction, the more reactive species displace the less reactive species from their reaction mixture. Displacement reaction is of two types:

• • Single displacement reaction: In the single displacement reaction, only one substrate of the given reactant is displaced by any other species and a new product is formed. An example of single displacement reaction is:

2HCl  + Zn    ZnCl₂ +  H₂

• • Double displacement reaction: In the double displacement reaction two reactants participate in the displacement of their constituent ions that lead to the formation of two new compounds as products. An example of a double displacement reaction is where potassium bromide and silver nitrate are present as reactants, and both ions of the given reactants show displacement as a result of which we obtain the mixture of potassium nitrate and silver bromide:

KBr  + AgNO₃   KNO₃ + AgBr

• Neutralisation reaction: In neutralisation reaction neutralisation of an acid and a base takes place and as a result of this we get a mixture of water and salt. An example of this reaction is the reaction between hydrochloric acid and sodium hydroxide base to produce sodium chloride salt and water. Chemically it is represented as:

HCl  +  NaOH    NaCl  +  H₂O

• Combustion reaction: In a combustion reaction, one of the reactants is an oxidant as it helps in the oxidation of another reactant, which is a fuel, and in a combustion reaction fuels are generally hydrocarbons (in which only hydrogen atoms and carbon atoms are present) for the formation of water (H2O), heat and carbon dioxide as a product. Chemically it is represented as:

CxHy  +  O₂     CO₂ +  H₂O  + heat

• Precipitation reaction: In this reaction, when a mixture of two solutions are taken into consideration in which soluble salts are present, it will form a precipitate salt as a product, which will settle at the bottom of the resultant solution. An example of precipitation reaction is described as:

AgNO₃ +  NaCl    NaNO₃ +  AgCl

• Redox reaction: Redox reactions are those reactions in which the oxidation process (i.e. the addition of oxygen or removal of hydrogen or electron) and the reduction process (i.e. the addition of hydrogen or electron or removal of oxygen) take place simultaneously. An example of a redox reaction is where iron (III) oxide shows reduction due to the loss of oxygen atom and carbon monoxide shows reduction due to the addition of oxygen atom. Chemically it is represented as:

Fe₂O₃ +  3CO    2Fe  +  3CO₂

On the basis of energy involved in any chemical reaction, chemical reactions can be classified into:

• Endothermic reaction: The reaction in which energy is absorbed by the reactant molecules for the completion of reaction and formation of product molecules, is known as endothermic reaction. In this reaction the value of enthalpy is always positive i.e. H=+ve.
• Exothermic reaction: The reaction in which energy is released for the formation of a product is known as exothermic reaction. In this reaction value of enthalpy change is always negative i.e. H=-ve

Conclusion

From the above description we can conclude that chemical reactions are those reactions in which we get a different species as the final product due to certain changes in the initial species. On the basis of the mechanism of the changes of chemical reactions it is classified  mainly into seven types as discussed above.