Chemical Reactions| Types of chemical Reactions| chemical Reactions and Equations

The Chemical Reactions

What are Chemical Reactions?

Chemical reactions are concerned with breaking of old bonds to create new bonds. It is the interaction of two or more substances, resulting in chemical changes in them. Chemical reactions take place when some kind of energy is supplied to them. The supplied energy may be in the form of heat, light or mechanical shaking for proper contact between the reactant molecules or electricity. Chemical reactions involve rearrangement of the atoms and produced new substances in this process.

The chemical Reactions 

For example, in the above picture, just the mixing of two substances can cause

the chemical reaction to occur.

Chemical Reactions are everywhere in nature.

The process by which the plants make their own food is called photosynthesis. 

In the presence of sunlight , the chemical reactions take place on the surface of

the leaf. The green pigment inside the leaves called “chlorophyll” help in the

manufacturing of food by making use of CO2 , H2O and other soil nutrients. In 

this process Oxygen is released, which is our daily consumption.

Chemical Reactions take place in our body too. Our whole body depends on

chemical reactions for proper functioning. These chemical reactions keep on

happening in our body throughout our life. 

For example, the whole digestion process involves chemical reaction of acids

and the food. During digestion, the food is broken down in to smaller

molecules. The salivary glands in our mouth release digestive enzymes which 

help in the breakdown of food. Digestion continues in the stomach and small intestine with the help of digestive enzymes.

Different types of Chemical Reactions

A simple rearrangement of atoms is also a chemical reaction. For example

H4N2CO  H4N2CO

Ammonium cyanate Urea

The molecular formula is same but the bonds are rearranged. Such compounds

are called isomers. This reaction is called an isomerisation reaction.

A Decomposition Reaction- occurs when a molecule is broken down into

smaller parts. For example, Passing electric current through water produces H2

and O2. This electrolysis reaction – the breaking down of molecule by passing 

electricity is an example of decomposition reaction. 

 electricity

2H20  2H2 + O2

Decomposition reaction are used to obtain several metals from their oxides or

salts. The salt is electrolysed and the pure metal is obtained. Digestion in our

body is also an example of decomposition reactions. The starch decomposes in 

to sugar in the body and proteins get decomposed into smaller substances called 

amino acids.

Combination Reaction: The combination reaction is the opposite of

decomposition reaction. Water can be made by combining H2 and O2, 

2H2 + O2  2H20

Another example of a combination reaction is when the walls are white washed

with lime.

Lime (Calcium Oxide), when added to water, forms slaked lime (Calcium

Hydroxide). Calcium hydroxide reacts slowly with the carbon dioxide in the air

to form a thin layer of calcium carbonate on the walls.

CaO + H2O  Ca(OH)2

Ca(OH)2 + CO2  CaCO3 + H2O

Displacement Reaction- involves the displacement of one set of atoms in a

molecule by another. 

Example of displacement of Cu in a salt by another metal:

Mg + CuSO4  MgSO4 + Cu

Similarily for Zn and Fe:

Zn + CuSO4  ZnSO4 + Cu

Fe + CuSO4  FeSO4 +Cu

Oxidation Reaction – Oxidation reaction is a reaction with oxygen. Oxidation 

is used in a wide variety of industries such as in the production of cleaning

products. It involves the loss of an electron by a molecule, atom, or ion.

Reduction Reaction – Reduction is the opposite of oxidation, it is often 

reaction with hydrogen. It involves gain of an electron by a molecule, atom, or

ion. 

The oxidation and reduction always go together because one cannot occur

without the other. Electrons that are lost by one atom must be gained by 

another. 

H2 + F2  2HF

This reaction can be written as two half reactions. First , the oxidation reaction:

H2  2H+ + 2e-

And the reduction reaction 

F2 + 2e-

  2F-

In the formation of water from its elements, oxygen is reduced while hydrogen is oxidised.

 Types of Chemical Reactions

There are literally thousands of different chemical reactions. It would be impossible to memorize them all. However, most chemical reactions fit into five major categories. 

Understanding these categories of reactions can help you predict how compounds will react and what products will form.

1. Synthesis

A synthesis reaction is a type of chemical reaction in which two or more different substances (elements or compounds) combine and form one compound. Synthesis means “putting together”. You can recognize a synthesis reaction because two or more reactants form only one product.  The general format of a synthesis reaction is shown below

For example, hydrogen (H2) reacts with oxygen (O2) to form water (H2O):

2. Decomposition

In a decomposition reaction, one compound breaks down into two or more simpler substances. Notice that decomposition is the reverse of synthesis. You can recognize a decomposition reaction because one reactant forms two or more products. 

For example, water (H2O) decomposes into hydrogen (H2) and oxygen (O2) when exposed to electricity:

3. Single Replacement

In a single replacement (also called single displacement), one element replaces another element in a compound. In this type of reaction, an element and a compound react to form a different element and a different compound.

For example, the reaction between iron (Fe) and copper sulfate (CuSO4) is an example of a single replacement reaction:

4. Double Replacement

In a double replacement (also called double displacement), the positive ions in two compounds switch places, forming two new compounds. In this type of reaction, two compounds react and form two new compounds.

For example, the reaction between magnesium oxide (MgO) and calcium sulfide (CaS) is an example of a double replacement reaction:

5. Combustion

Combustion is a chemical reaction in which a substance reacts with oxygen and releases energy. This energy usually is released as thermal energy and light energy. For example, 

burning is a common combustion reaction. The general format of a combustion reaction of a hydrocarbon (compound made of hydrogen and carbon) is shown below. The products of the combustion of a hydrocarbon are always CO2 and H2O.

For example, the burning of fossil fuels, like methane (CH4) produces carbon dioxide (CO2) and water vapor (H2O):

Chemical Reactions and Equations

Topics in the Chapter

• Introduction

• Chemical reaction

• Chemical equation

• Balanced chemical equation

• Types of chemical reaction

• Corrosion

• Rancidity

Introduction

→ Most of the substance around us undergoes various changes. Some of these changes are temporary with no new substance being formed. They are called physical changes.

→ In certain other changes the new substance formed in which the reactant or the parent loses its identity to form new substance called product. These changes are permanent changes as we won’t get the reactant back.

Chemical Reaction

→ Chemical reaction is the process by which two or more substance react with each other to form new substance with different properties.

→ These are the following changes to determine that the chemical reaction has taken place:

(i) Change in state

(ii) Change in colour

(iii) Evolution of gas

(iv) Change in temperature

Chemical Equation

→ A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and formulae, wherein the reactant 

entities are given on the left-hand side and the product entities on the right-hand side.

 Magnesium + oxygen → magnesium oxide

 (Reactant) (Product)

Balanced chemical equation

→ The chemical equation that shows the chemical reaction needs to be balanced. A balanced chemical equation occurs when the number of the atoms involved in the reactants side is equal to the number of atoms in the products side.

Zn + H2SO4 → ZnSO4 + H2

3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)

Types of chemical reaction

→ Combination reaction: Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

CaO (s) + H2O (l) → Ca(OH)2 (aq)

Calcium oxide reacts vigorously with water to produce slaked lime 

(calcium hydroxide) releasing a large amount of heat.

A solution of slaked lime produced by the reaction is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

(slaked lime) (calcium carbonate)

Burning of coal: C (s) + O2 (g) → CO2 (aq)

Formation of water: H2 (g) + O2 (g) → H2O (aq)

Burning of natural gas (Methane): CH4 (g) + O2 (g) → CO2 (g) + H2O 

(l)

→ Exothermic reaction: An exothermic process releases heat, and causes the temperature of the immediate surroundings to rise The rice, 

potatoes and bread we eat contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration is an exothermic reaction.

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

(glucose)

→ Endothermic reaction: An endothermic process absorbs heat and cools the surroundings.

The decomposition of vegetable matter into compost is also an example of an endothermic reaction.

→ Decomposition reaction: When single reactant breaks down to give simpler products. This is a decomposition reaction.

White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

The above reactions are used in black and white photography.

Displacement reaction: Displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound. Both metals and non-metals take part in displacement reactions. Reaction of iron nails with copper sulphate solution.

Fe (s) + CuSO4 (aq) → FeSO4 (g) + Cu (s)

Pb (s) + CuCl2 (aq) → PbCl2 (aq) + Cu (s)

→ Double Displacement reaction: A double displacement reaction, also known as a double replacement reaction or metathesis, is a type of chemical reaction where two compounds react, and the positive ions 

(cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

Na2(SO)4 (aq) + BaCl2 (aq) → BaSO4 (s) + NaCl (aq)

→ Redox reaction: An oxidation-reduction (Redox) reaction is a type 

of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.

Oxidation: This process involves gain of oxygen or loss of hydrogen.

Reduction: This process involves gain of hydrogen or loss of oxygen.

→ Oxidizing Agent

It is the substance which gives oxygen or gains hydrogen.

Or it the substance which is reduced itself and oxidizes other.

→ Reducing Agent

It is the substance which gives hydrogen or gains oxygen.

Or it the substance which is oxidized itself and reduces other.

Oxidation is the process which involves loss of electrons but reduction is the process which involves gain of electrons.

Corrosion

The process of slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. 

present in the atmosphere is called corrosion.

Rusting – Iron when reacts with oxygen and moisture forms red substance called rust.

Rancidity

→ The taste and odour of food materials containing fat and oil changes 

when they are left exposed to air for long time. This is called rancidity. 

It is caused due to oxidation of fat and oil present in food material.

→ It can be prevented by using various methods such as by adding antioxidants to the food materials,Storing food in air tight container and by flushing out air with nitrogen.

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