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Chemistry-day-11 chemical-reactions-theory Target ssc cgl 2018

Chemistry-day-11 chemical-reactions-theory Target ssc cgl 2018

Chemistry-day-11 chemical-reactions-theory Target ssc cgl 2018

Chemical Reactions and Equations

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 CaCO2 .

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

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


Different types of reactions..

Combination reaction

The reaction in which two or more reactants combine and form a single product is called a combination reaction.

Example :  2Mg + O2  →  2MgO


Decomposition reaction

A reaction in which a simple compound breaks to form two or more simpler compounds is called a decomposition reaction.

Example :  2Pb(NO2)3   →  2PbO + 4NO2 + O2

Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important  decomposition reaction used in various industries. Calcium oxide is called lime or quick lime. It has many uses – one is in the manufacture of cement. When a decomposition reaction iscarried out by heating, it is called thermal decomposition.

Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.


Displacement reaction

The reaction in which a more reactive element displaces a less reactive element from its compound.

Example :  Pb + CuCl2  →  Pb Cl2 + Cu


Double Displacement reaction

A reaction in which  positive and  negative ions exchange their places is called a double displacement reaction.
Example :  BaCl2 + Na2SO4  →  BaSO4 + 2NaCl


Oxidation reaction

The reaction in which either addition of oxygen or removal of hydrogen takes place is called oxidation reaction.

Example :  2Cu + O2  →  2CuO

Reduction reaction

The reaction in which either removal of oxygen or addition of hydrogen takes place is called oxidation reaction.

Example :  MgO + H2  →  Mg + H2O

Oxidation and Reduction or Redox reaction

The surface of copper powder becomes coated with black copper(II) oxide. Why has this black substance formed? This is because oxygen is added to copper and copper oxide is formed.

If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

If a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced. During this reaction, the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidized while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.



You must have observed that iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.

Some other metals also get tarnished in this manner. Have you noticed the colour of the coating formed on copper and silver? When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. The black coating on silver and the green coating on copper are other examples of corrosion. Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron.


When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation. Do you know that chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised ?


What you have learnt

  • A complete chemical equation represents the reactants, products and their physical states symbolically.
  • A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation. Equations must always be balanced.
  • In a combination reaction two or more substances combine to form a new single substance.
  • Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.
  • Reactions in which heat is given out along with the products are called exothermic reactions.
  • Reactions in which energy is absorbed are known as endothermic reactions.
  • When an element displaces another element from its compound, a displacement reaction occurs.
  • Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
  • Precipitation reactions produce insoluble salts.
  • Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen.


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