CHEMICAL REACTIONS
AND EQUATIONS
-BY SWATI MA’AM
COMBINATION DECOMPOSITION DISPLACEMENT
REACTIONS REACTIONS REACTIONS
DOUBLE
REDOX
DISPLACEMENT
REACTIONS
REACTIONS
COMBINATION REACTIONS:
• Definition: Combination reactions, also known as synthesis reactions,
are chemical reactions in which two or more substances combine to
form a single product.
• General Form: A+B→AB
• Examples of Combination Reactions:
• Formation of Water: 2H2(g) + O2(g) → 2H2O(l)
Hydrogen gas combines with oxygen gas to form water.
• Burning of coal: C(s) + O2(g) → CO2(g)
Energy Change: Many combination reactions release
energy in the form of heat and/or light. So they are often
exothermic in nature.
COMBINATION REACTION B/W QUICK LIME AND WATER:
CaO(s) + H2O(l) → Ca(OH)2(aq) + HEAT
Solution of slaked lime produced above is used for white washing the walls
Ca(OH)2 + CO2 CaCO3 + H2O
IMPORTANT POINTS:
• Calcium carbonate (CaCO3) is also called as limestone.
• It is insoluble in water.
• It gives a shiny finish to the walls.
• It plays an important role in construction, be it as a building material
(marble) or as an ingredient in cement.
• It is used in medicinal industries which manufacture antacids, tablets
which are made of base materials etc.
• It is used as calcium supplement.
DECOMPOSITION REACTIONS:
• Definition: Decomposition reactions involve the breakdown of a single
compound into two or more simpler substances.
• Generally endothermic in nature.
• DIGESTION OF FOOD IN OUR BODY IS AN EXAMPLE OF
DECOMPOSITON REACTION.
THERMAL DECOMPOSITION
• HEATING OF LEAD NITRATE:
2Pb(NO3)2 (s) 2PbO (s) + 4NO2 (g) + O2 (g)
EVOLUTION OF PUNGENT SMELLING BROWN FUMES ARE DUE TO NO2
HEATING OF FERROUS SULPHATE CRYSTALS:
Colour of FeSO4.7H2O Colour of ferric oxide
OBSERVATIONS:
• Smell of burning Sulphur is observed.
• Drops of water are collected on the cooler parts of
test tube.
• SO2 and SO3 are pungent gases which can cause
chocking and suffocation.
ELECTROLYSIS OF WATER:
electricity
2H2O(l) 2H2(g) + O2(g)
OBSERVATIONS:
• When electric current is passed through acidified water, it gets
decomposed to hydrogen and oxygen gas.
• Formation of bubbles at both the electrodes.
• H2 gas is evolved at cathode and O2 gas is evolved at anode.
• Volume of H2 gas is double the volume of O2 gas, ratio of H2 : O2 = 2:1
• If we bring burning candles near these test tubes, we will observe that
the burning candle will continue burning in O2 TT.
• While it will extinguish in H2 TT making a popping sound.
PHOTOCHEMICAL DECOMPOSITION:
• It is observed that white crystals of silver chloride
will turn grey in sunlight.
• Above reactions are used in black and white
photography. Silver chloride turns grey in sunlight to
form silver metal.
DISPLACEMENT REACTIONS:
After 30 minutes we observe the following changes :
•Blue colour of CuSO4 has faded and it changes into light green due to
formation of iron sulphate (FeSO4).
•The iron nail is covered with a red brown layer of copper metal.
DOUBLE DISPLACEMENT REACTIONS:
“Those reactions in which two ionic compounds in the
solution react by exchange of their ions to form new
compounds are called double displacement reactions”
