DECOMPOSITION REACTION
Decomposition reaction is nothing but the reaction in which a single reactant splitted into simpler products. As heat is required to decompose a molecule, This reaction is also called as endothermic reaction. Decomposition reaction is carried out by three ways:
1. By applying direct heat from sources like Bunsen Burner, etc.
Activity-1
In order to illustrate decomposition reaction, heat a few crystals of ferrous sulphate (green in colour) taken in a dry test tube over the flame of a burner. Intially, the crystals change to a dirty white solid. Actually, the crystals have the chemical formula FeSO4.7H2O and the hydrated salt is known as Green vitriol. Upon heating, they lose the molecules of water to form FeSO4. It is an anhydrous salt with dirty white colour.
(Heat)
FeSO4.7H2O (s) -> FeSO4(s) + 7H2O (g)
Upon further heating, the dirty white solid will decompose to give a brown residence. Along with this, the vapours of gases sulphur dioxide (SO2) and sulphur trioxide (SO3) accompanied with pungent and suffocating smell will be released. The decomposition reaction that has taken place is: (Heat)
2FeSO4 (s) -> Fe2O3 (s) + SO2 (g) +SO3 (g)
Since decomposition has been carried upon strong heating. It is also called thermal decomposition. A single substance has resulted in mixture of three substances in this particular reaction.
2. Electrolysis
Electrolysis is nothing but a process by which splitting of molecules take place to form simpler molecules by using electricity. It is basically used for the splitting of water molecules into hydrogen and oxygen.
Acitivity-2
The electrolysis of water resulting in hydrogen and oxygen gases is a common example of decomposition reaction carried by passing electric current. Water as such is a poor conductor of electricity but water containing a few drops of sulphuric acid (acidulated water) is a good conductor.
In a plastic mug, drill two holes at the base and insert graphite (carbon) electrodes into these holes. Connect these electrodes to a 6 volts battery. The plastic mug acts as voltameter. Fill the mug with water to nearly half and add a few drops of dilute sulphuric acid. Invert graduated test tubes containing water on both the electrodes as shown in the figure. Pass current slowly through the voltameter.
Bubbles of gases appear above the electrodes in both the tubes. As a result, water level will be pushed downwards. The gases are hydrogen and oxygen and are formed by the decomposition of water on passing electric current.
(Electrolysis)
2H2O (l) -> 2H2 (g) + O2 (g)
The volume of hydrogen collected is double than that of oxygen. This is quite expected as well according to the chemical equation. Actually hydrogen and oxygen are colourless and have been shown by the empty spaces in the tubes. These gases can be tasted later on.
3. Photolysis
Photoysis is nothing but the reaction in which splitting of molecules take place in the presence of sunlight. This is also called as photo-chemical reaction.
Acitivity-3
Let us now illustrate a decomposition reaction carried in the presence of sun light. Take a small amount of crystals of silver chloride which are white in colour on a watch glass. Place the watch glass under sun light for sometime. The crystals slowly acquire grey colour. On analysis, it is found that sunlight has caused the decomposition of silver chloride into silver and chlorine.
(Sunlight)
2AgCl (s) -> 2Ag (s) + Cl2 (g)
Actually, sunlight consists of photons which are the source of energy. They cause the decomposition of silver chloride. Similarly, silver bromide also undergoes decomposition in the presence of sunlight.
(Sunlight)
2AgBr (s) -> 2Ag (s) + Br2 (g)
If you have still any doubts, Then feel free to comment us.