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Chemical Reactions and Equations

 Chemical Reactions and Equations


q Chemical Reaction : – Whenever a chemical change occurs we can say that a chemical reaction has taken place

eg – Food gets digested in our body

– Rusting of iron.

q Chemical Equation :– A chemical reaction can be expressed symbolically by using chemical equation

eg magnesium is burnt into air to form magnesium oxide can be represented as

Mg + O2  MgO

– We can observe or recognise a chemical reaction by observing change in state, colour, by evolution of gas or by change in temperature.

q Physical state of the reactant and products are mentioned to make chemical reaction more informative. eg we use (g) for gas, (l) for liquid, (s) for solid and (aq) for aqueous.

q Balancing Equation :– We balance the chemical equation so that no. of atoms of each element involved in the reaction remain same at the reactant and product side.

eg Fe + H2O  Fe2O3 + H2 can be written as 3 Fe(s) + 4H2O(g)  Fe2O3(s) +4H2(g)

q Combination Reaction :– The reaction in which two or more substances combine to form a new single substance


eg CaO(s) + H2O(l)  Ca(OH)2 (aq) Calcium Water Calcium hydroxide oxide (slaked lime)

Quick lime

– Ca(OH)2 slaked lime is used for white washing walls. It reacts will CO2 to form CaCO3 and gives a shiny finish to the walls.

2 + 2 3 + 2

Ca(OH) CO  CaCO H O (l)

(aq) (g) (s)

Calcium Calcium

hydroxide Carbonate

  • Burning of Coal

    C(s) + O2(g)  CO2(g) + heat + light

  • Formation of water

2H2(g) + O2(g)  2H2O(l)

q Exothermic Reactions :– Reaction in which heat is released along with the formation of products.

eg. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

  • Respiration is also exothermic reaction.

  • De composition of vegetable matter into compost.

q De compositon Reactions :– The reaction in which a single substance decomposes to give two or more substances. De composition reactions can be of three types

Thermal Decompositon :– When a decompositon reaction is carried out by heating

  • Silver bromide behaves similarly

    2Ag Br Sunlight

    2Ag(s) + Br2(g)


  • The above two reactions are used in black and white photography.




    • Endothermic Reactions – The reactions which require energy in the form of heat, light or electricty are called Endothermic Reactions.

      2Ba(OH)2 + NH4Cl  2BaCl2 + NH4OH

    • Displacement Reaction : The chemical Reaction in which an element displaces another element from its solution

      Fe(s) + CuSO4(aq)  FeSO4 + Cu(s) Copper (aq)

      Sulphate Iron Sulphate


    • The nail becomes brownish in colour and the blue colour of Copper Sulphate solution fade.

    • Other examples Zn(s) + CuSO4  ZnSO4 + Cu(s)

      (aq) (aq)

      Copper Zinc

      Sulphate Sulphate

      Pb(s) + CuCl2  PbCl2 + Cu(s)

      (aq) (aq)

      Copper Lead

      Chloride Chloride

    • Zinc and lead are more reactive elements than copper. They displace copper from its compounds.