Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.
In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.

Following are the signs of chemical reaction:
    1.Change in state
    2.Change in color
    3.Evolution of gases  

Reactant and Product:

Reactant: Substances which take part in a chemical reaction are called reactants.

Product: New substance formed after a chemical reaction is called product.

Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.
Example: A + B  C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.

Hydrogen + Oxygen  Water
H2 + O2  H2O

In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations.
Chemical Equation is a way to represent the chemical reaction in concise and informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides.
Example: Zn + H2SO4  ZnSO4 + H2
In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a balanced chemical equation.
According to Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. To obey this law, the total mass of elements present in reactants must be equal to the total mass of elements present in products.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H2O  Fe3O4 + H2
In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

Balancing a chemical equation:
To balance the given or any chemical equation, follow these steps:
Fe + H2O  Fe3O4 + H2
Write the number of atoms of elements present in reactants and in products in a table; as shown here.

Name of atom
No. of atoms in reactant
No. of atoms in product

Balance the atom which is the maximum in number; on either side of chemical equation.
In this equation, the number of oxygen atom is the maximum on the RHS.

To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H2O  Fe3O4 + H2

Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.
Fe + 4 x H2O  Fe3O4 + 4 x H2

After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.
3 x Fe + 4 x H2O  Fe3O4 + 4 x H2

Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.

Name of atom
No. of atoms in reactant
No. of atoms in product

After balancing, the above equation can be written as follows.
3Fe + 4H2O  Fe3O4 + 4H2
Balance the given equations.
     a.    H2 + O2  H2O
Answer: 2H2 + O2 
     b.    Fe + H2O  Fe2O3 + H2
Answer: 2Fe + 3H2O 
 Fe2O3 + 3H2
     c.    CO2 + H2O  C6H12O6 + O2
Answer: 6CO2 + 6H2O 
 C6H12O6 + 6O2
     d.    Fe2O3 + C  Fe + CO2
Answer: 2Fe2O3 + 3C 
 4Fe + 3CO2

Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
    a.    Gaseous state is represented by symbol ‘g’
    b.    Liquid state is represented by symbol ‘l’
    c.    Solid state is written by symbol ‘s’
    d.    Aqueous solution is written by symbol ‘aq’

Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

Types of Chemical Reaction

Chemical reactions can be classified in following types:
     a.    Combination Reaction
     b.    Decomposition Reaction
     c.    Displacement Reaction
     d.    Double Displacement Reaction
     e.    Oxidation and Reduction Reaction

Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.

A + B  AB

Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.

Mg + O2  2MgO
Magnesium + Oxygen  Magnesium oxide

When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.

C + O2  CO2
Carbon + Oxygen  Carbon dioxide

When hydrogen reacts with chlorine, hydrogen chloride is formed.

H2 + Cl2  2HCl
Hydrogen + Chlorine  Hydrogen chloride

When calcium oxide reacts with water, calcium hydroxide is formed

CaO + H2O  Ca(OH)2
Calcium oxide + Water  Calcium hydroxide

When carbon monoxide reacts with oxygen, carbon dioxide is formed.

2CO + O2  2CO2
Carbon monoxide + Oxygen  Carbon dioxide

Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:

AB  A + B

Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide

CaCO3  CaO + CO2
Calcium carbonate  Calcium oxide + Carbon dioxide

When ferric hydroxide is heated, it decomposes into ferric oxide and water

2Fe(OH)3  Fe2O3 + 3H2O
Ferric hydroxide  Ferric oxide + Water

When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.

2Pb(NO3)2  2PbO + 4NO2 + O2
Lead nitrate  Lead oxide + Nitrogen oxide + Oxygen

In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.

Electrolytic Decomposition:
Reactions in which compounds decompose into simpler compounds because of passing of electricity, are known as ELECTROLYTIC DECOMPOSITION. This is also known as ELECTROLYSIS.

Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.

2H2O  2H2 + O2

Photolysis or Photo Decomposition Reaction:
Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION.

Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.

2AgCl  2Ag + Cl2

Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.

2AgBr  2Ag + Br2

Photographic paper has coat of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal.

Displacement Reaction
Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.
A general displacement reaction can be represented using chemical equation as follows:

A + BC  AC + B

Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taken place.

Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

Zn + 2HCl  ZnCl2 + H2

When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.

Zn + CuSO4  ZnSO4 + Cu

When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper.

Ag + Cu(NO3 )2  No reaction takes place

Double Displacement Reaction:
Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

AB + CD  AC + BD

Example: When solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

BaCl2 + Na2SO4  BaSO4 + 2NaCl

When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.

NaOH + HCl  NaCl + H2O

Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.

Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quick lime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.

Oxidation and Reduction Reaction:

Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.

Oxidizing agent: Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.

Reducing agent: Compounds or elements which can cause reduction are called reducing agents.
In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.

Example: When iron reacts with air, it forms iron oxide (rust)

4Fe + 3O2  2Fe2O3

In this reaction, oxygen is added to iron, thus, iron is oxidized. Here oxygen is oxidizing agent.
When cupric oxide reacts with hydrogen, it gives copper and water.

CuO + H2  Cu + H2O

In this reaction, oxygen is removed from copper and oxygen is added to hydrogen. So, cupric oxide is reduced to copper and hydrogen is oxidized to water. Cupric oxide is oxidizing agent and hydrogen is reducing agent.
When sodium hydroxide reacts with hydrochloric acid, it gives sodium chloride and water.

NaOH + HCl  NaCl + H2O

In this reaction, sodium hydroxide is reduced to sodium chloride since hydrogen is removed from sodium hydroxide. Hydrochloric acid is oxidized to water, since oxygen is added to hydrogen chloride and non-metallic element chloride is removed. Sodium hydroxide is oxidising agent and hydrochloric acid is reducing agent.
In this reaction oxidation and reduction both takes place simultaneously, thus it is an example of redox reaction.

Significance of Oxidation Reduction in Everyday Life:
1.Respiration is oxidation reaction in which food is oxidized to produce energy.
2.Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.
3.Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.
4.Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.
5.Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.

6.We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.

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