Types of Chemical Reactions

A chemical reaction is a transformation of one type of substances into new substances. All chemical reactions do not occur or behave in a similar way. Accordingly, we have different types of chemical reactions, which are described below;

1) Combination Reactions

It refers to a chemical reaction in which two or more substances combine or react chemically to form a single new substance. Some examples are given below;

3 Mg + N₂ → Mg₃N₂

2H₂ + O₂ → 2H₂O

2Mg + O₂ → 2MgO

In the first reaction, two types of atoms (Magnesium and Nitrogen) are combining to form a single product, magnesium nitrite. Similarly, in the second reaction, the atoms of hydrogen and oxygen combine and forms a single product (H₂O). In the third reaction, only magnesium oxide is formed when magnesium and oxygen react.

So, it is a characteristic feature of a combination reaction that there may be two or more reactants but only a single product is formed when the reactants react or combine with each other.

2) Decomposition reactions

It refers to a chemical reaction in which a single substance or reactant breaks down into two or more new products. So, it is the opposite of the combination reaction. For example,

a) In the below reaction, calcium carbonate when heated changes into two products Calcium oxide and Carbon dioxide.

C_aCO₃ (s) → CaO (s) + CO₂ (g)

b) During electrolysis of water, the water breaks down into hydrogen and oxygen which are two new products with different properties than the reactant, water.

2H₂O + electricity → 2H₂ + O₂

c) Iron sulphate decomposes into three different products as shown below.

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃

So, it is the characteristic feature of decomposition reactions that there will be only one substance that breaks down into two or more substances.

Types of decomposition:

Thermal decomposition: It refers to the decomposition of reactants by providing energy in the form of heat. So, these reactions are also called thermal decomposition reactions.

Electrical decomposition: The decomposition of a reactant is also carried out through electrolysis that involves the use of electrical energy in the form of electricity such as the electrolysis of water.

Photolysis or photochemical decomposition: In this type of decomposition of a substance, light energy is used to carry out the decomposition. Silver chloride breaks down into two products, silver and chlorine gas in the presence of sunlight.

2AgCl → 2Ag (solid) + Cl₂ (gas)

3) Neutralization reaction

In this type of chemical reaction, an acid reacts with the base and produces salt and water as the products. The water is formed by the combination of H⁺ and OH^- ions. When a strong acid reacts with a strong base during a neutralization reaction, the pH of the solution remains 7.

HCl + NaOH → NaCl + H₂O

In the above neutralization reaction, hydrochloric acid and sodium hydroxide react or combine to produce Sodium Chloride (common salt).

4) Redox Reaction

A chemical reaction in which oxidation and reduction occur simultaneously is called a redox reaction. Neither oxidation nor reduction can occur alone, so, all reaction that involve oxidation or reduction are redox reaction. The transfer of an electron between chemical species in a redox reaction is shown below;

Zn + 2H⁺ → Zn²⁺ + H₂

In the above reaction, one atom of Zinc (Zn) reacts with 2 hydrogen ions with a positive charge (H⁺). The electrons are transferred from Zinc to hydrogen, so, hydrogen is reduced and changes into a stable molecule, whereas, after losing electrons Zinc gets oxidised and becomes an ion with a positive charge (cation). So, oxidation of Zn and reduction of H occurs simultaneously.

5) Double-Displacement Reaction

It refers to a displacement reaction, where two compounds react and their cations and anions are exchanged to form new products. As the name suggests, in these reactions, two different atoms or molecules are displaced by other two atoms or molecules, such AB + CD → CB + AD

For example, silver nitrate and sodium chloride react to form silver chloride and sodium nitrate after undergoing a double-decomposition reaction.

AgNO₃ + NaCl → AgCl + NaNO₃

In this reaction, silver replaces sodium in sodium chloride to form Silver chloride and sodium combines with nitrate to form Sodium nitrate.

Similarly, in the below reaction, Cu is displacing Na and joining with hydroxide and Na takes the position of copper and forms sodium sulphate.

2NaOH + CuSO₄ → Cu (OH)₂ + NaSO₄

6) Synthesis Reaction

As the name suggests, in this type of chemical reaction, multiple simple compounds react under particular physical conditions to form a complex product. The product formed in this reaction is always a compound. For example,

2Na(s) + Cl(g) → 2NaCl (s)

In this reaction, 2 atoms of solid sodium react with chlorine gas to form sodium chloride (common salt).

7) Combustion reaction

It is an exothermic reaction as energy is released in the form of heat. This reaction generally occurs between fuel and an oxidant such as oxygen present in the atmosphere. It generally produces smoke, water and heat. For example, when methane burns in the presence of oxygen it produces carbon dioxide and water as shown below;

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Exothermic reaction: It refers to chemical reactions, in which the formation of products is accompanied by the evolution of heat. The new products are formed along with the evolution of heat. For example, A + B → C + D + heat

For example;

CH4 + O2 → CO2 +H2O + heat energy

The above reaction is an exothermic reaction in which methane reacts with oxygen to produce carbon dioxide and water along with the production of heat.

Similarly, when magnesium burns in the presence of oxygen in presence of oxygen, it forms magnesium oxide along with the production of heat.

Mg + O2 → MgO + heat

Endothermic reaction: It is a chemical reaction in which heat is absorbed by reactants to react with each other. For example, A + B + hear → heat. The making of tea is also an endothermic reaction, as heat is needed or absorbed by the reactants to produce tea.

Some decomposition reactions that absorb heat to decompose can also be considered endothermic reactions. For example, calcium carbonate when heated decomposes into calcium oxide and carbon dioxide;

CaCO3 + heat → CaO + CO2

So, one chemical reaction can be of two or more types.

8) Precipitation reaction

In this type of chemical reaction, two soluble salts in an aqueous solution combine to form new products out of which one product is an insoluble salt, which is known as a precipitate.

Na₂SO₄ + BaCl₂ → BaSO₄ + NaCl

In the above reaction, sodium sulphate reacts with barium chloride and double displacement occurs which results in the formation of Barium sulphate and sodium chloride (common salt). Now, this is a precipitation reaction, as one of the products barium sulphate settle down at the bottom of the solution in the form of a precipitate, whereas, sodium chloride gets dissolved in the solution. Earlier, both sodium sulphate and barium chloride were soluble in water but after they combine and form two products only one product is soluble in water.

9) Single Displacement reaction

It is a chemical reaction in which a more reactive element of a reactant displaces the less active element of another reactant to form a new compound.

CuSO₄ + Fe → FeSO₄ + Cu

In this reaction, iron is more reactive than copper, so it displaces copper to form iron sulphate. It is a single displacement reaction, as one element displaces another element in the compound.

Similarly, in the below reaction, Zn atoms are more reactive than hydrogen atoms. So, Zn replaces hydrogen.

2HCl + Zn → ZnCl₂ + H₂