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Types of Chemical Reactions - topmarksapp.com

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Types of chemical reaction

Combination Reaction

Combination Reaction

A chemical reaction in which two or more compounds react to give a new product.

Let’s see some everyday reactions to understand this-

  1. Burning of coal

    C(s) + O2(g) →CO2(g)

  1. Formation of water from H2(g) and O2(g)

    2H2O + O2(g) →2H2O(l)

  1. Formation of calcium hydroxide (slacked lime)

         CaO(s) + H2O(l) →Ca(OH)2(aq)  + Heat

Here we can see that Calcium oxide reacts with water to form a single product Calcium Hydroxide. In some way, we can say that a combination reaction is a reaction in which two or more substances(compound or elements) combine to give a single product. Here you can observe that a large amount of heat is evolved.

Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

But there are many combination reactions in which more than one compound is formed.

Example 2CO + O2 → 2CO2

Do you know?

Respiration is a combination reaction.

We, humans, require energy to survive and we receive it from the food we consume. During digestion, food is broken down into simpler substances. Food items like rice, potatoes contain carbohydrates. These are broken down into glucose. This glucose combines with oxygen in the cells of our body and provides energy. This process is called respiration and can be represented as:

 C6H12O6(aq)  +  6O2(aq)→6CO2(aq) + 6H2O(l) + energy

Burning of natural gas

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Some other examples of combination reaction

  1.  2SO2(g) + O2(g) → 2SO3(g)       
  2.  2NO(g) + O2(g) → 2NO2(g)

Decomposition reaction

A chemical reaction that breaks down a single compound into two or more elements or creates a new compound.

Let’s see a reaction to understand it more clearly

The Ferrous sulphate crystals (FeSO4.7H2O) on heating lose water. Thus, it decomposes to give ferric oxide (Fe2O3), Sulphur oxide (SO2), and sulphur trioxide (SO3). The colour of the crystals changes on heating.

  1. Decomposition of Calcium carbonate to calcium oxide and carbon dioxide.

This reaction is used in various industries. The calcium oxide (quick lime) formed is extensively used in the manufacture of cement. Since the decomposition reaction is carried out by heating, it is called thermal decomposition.

  1. Decomposition of silver chloride into silver and chlorine.

Here the white silver chloride (AgCl) turns grey in the presence of sunlight. This is due to the decomposition of AgCl to Silver (Ag) and chlorine (Cl2) by sunlight.

  1. Decomposition of silver bromide to silver and bromine

Silver bromide also reacts in the same way as silver chloride reacts with sunlight.

This reaction is used in black and white photography.

In all these reactions we can see that some form of energy either heat, light, or electricity is required for breaking the reactants. A reaction in which energy is absorbed is called an endothermic reaction.

Examples of decomposition reaction

Displacement reaction

A displacement reaction refers to a reaction in which a part of one reactant is displaced by another reactant in a compound.

In the above reaction, iron has displaced copper from copper sulphate solution. Here, the more reactive element iron (Fe) displaces a less reactive element copper from its copper sulphate solution.

Some other examples of displacement reactions:

When compared to copper, zinc and lead are more reactive elements. Hence, they displace copper from its compounds. Thus, we can conclude that in a displacement reaction, a more reactive element displaces a less reactive element from its compound. 

Examples of displacement reaction

The copper displaces silver from silver nitrate giving copper nitrate.

The magnesium displaces copper from copper sulphate giving magnesium sulphate.

Double Displacement Reaction

A double displacement reaction takes place when two chemicals react and the positive ions (cation) and negative ions (anion) of the two reactants exchange positions.

When the solution of sodium sulphate is mixed with barium chloride, a white substance is formed. This white substance which is insoluble in water is called a precipitate.

Here, the reaction is ionic. When the reactants are dissolved in water, they become ions and the ions get exchanged in the solution. The Ba2+ and SO42- ions react to give the white precipitate of BaSO4. The NaCl is formed by the reaction of Na2+ and Cl– ions remain in the solution. Thus, it gives a precipitate of barium sulphate in an aqueous solution of sodium chloride.

Examples of double displacement reaction:

Oxidation and Reduction

Let’s consider a reaction in which copper powder is heated in a China dish.

During this reaction, the oxygen reacts with copper to give a black coating of copper (II) oxide. This black colour is due to the reaction of oxygen.

When hydrogen is passed over this copper oxide (CuO), the black coating turns brown. Thus, the reverse reaction takes place and copper is formed again.

In the first reaction, the substance copper gains oxygen i.e., copper is oxidised. In the second reaction, the copper (II) oxide on reaction with hydrogen loses oxygen i.e., copper oxide is reduced. The hydrogen is gaining oxygen and is oxidised. Such a kind of reaction in which one reactant is oxidised while the other gets reduced is called oxidation-reduction reaction or redox reaction.

Other examples of redox reaction are:

In this reaction, carbon is oxidised to carbon monoxide (CO) and zinc oxide (ZnO) is reduced to zinc (Zn).