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.
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.
- Physical state of the reactant and products are mentioned to make chemical reaction more informative. eg we use (g) for gas, (l) for liquid, (*) for solid and (aq) for aqueous.
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() + 4H2O(g) --> Fe2O3() +4H2(g)
Combination Reaction :– The reaction in which two or more substances combine to form a new single substance
eg *CaO() + 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.
Ca(OH)2(aq) + CO2(g) ---> CaCO3()+ H2O (l)
Calcium Calcium
hydroxide Carbonate
– Burning of Coal
C() + O2(g) ---> CO2(g) + heat + light
– Formation of water
2H2(g) + O2(g) --> 2H2O(l)
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.
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() + Br2(g)
– The above two reactions are used in **** 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() + CuSO4(aq) ---> FeSO4 + Cu()
Copper (aq)
Sulphate Iron Sulphate
Double Displacement Reaction : The reaction in which two different atoms or group of atoms are mutually exchanged
eg. Na2 SO4 + BaCl2 ---> BaSO4(*) + 2NaCl
(aq) (aq) (aq)
Sodium Barium Barium Sodium
Sulphate Chloride Sulphate Chloride
A white substance is formed due to above reaction. The insoluble substance
is called precipitate.
Precipitation Reaction – Any reaction that produces a precipitate is called a precipitation reaction.
eg. Pb(NO3)2 + 2KI ---> PbI2 (down arrow) +2KNO3
(aq) (aq) (aq)
Lead Nitrate Potassium Lead Potassium
Iodide Iodide Nitrate
– Oxidation : Oxidation is the gain of oxygen or loss of hydrogen
eg. 2Cu + O2 heat--> 2CuO
When Copper is heated a *** colour appears. If this CuO is reacted with
hydrogen gas then again Cu becomes brown as reverse reaction takes place
CuO + H2 heat--> Cu + H2O
– Reduction : Reduction is the loss of oxygen or gain of hydrogen.
– Redox Reaction :** The reaction in which one reactant gets oxidised while other gets reduced
eg. ZnO + C ---> Zn + CO
MnO2 + 4HCl ---> MnCl2 + 2H2O + Cl2
Corrosion : When a metal is attacked by substances around it such as
moisture, acids etc.
eg. Reddish brown coating on iron.
(ii) *** coating on Silver.
– Rancidity :** When fats and oils are oxidised they become rancid and their
smell and taste change.
– Antioxidants are added to foods containing fats and oil.
