Chemical Reactions and Equations:-
In our daily life, we can observe 2 types of changes:
Physical change which can be reversed.
Example: melting of wax , evaporation etc.
Chemical change which cannot be reversed.
Example: rusting of iron, burning of coal etc.
A reaction in which two or more substances react together to form a new substance have a different property.
Identification of Chemical reactions:-
If any of the following conditions are fulfilled, then it can be said that a chemical reaction has taken place:
1. New substance is formed
2. Change in temperature
3. Change in colour
4. Evolution of gas
5. change in state
It is a symbolic representation of a chemical reaction.
Example: Calcium + oxygen = calcium oxide
Reactants Vs Product
The substances that reacts are called reactants and the new product formed are called products.
Balanced Chemical Equations
A balanced chemical equation has the same no. of atoms of each element on both the side i.e on reactants and products side.
Example: 2Ca+O2 – 2CaO
We balance a chemical equation to satisfy the law of conservation of mass. According to this law, mass can neither be created nor be destroyed.
Points to remember while balancing a equation:-
- Start balancing the equation with a compound with the highest no. of atoms
- Don’t change the chemical formula of the compounds
- Always put coefficients to the compounds
- Try to use the lowest whole number
Na+ H2O = NaOH + H2
The above equation is an unbalanced equation, it can be balanced in the following way:-
2Na + 2H2O = 2NaOH + H2
To make the chemical equation more informative, physical states of reactants and products are mentioned e.g solids (s), liquids (l), gas (g), and aqueous (aq). Sometimes the reaction conditions are also mentioned like temperature, pressure, etc.
Types of Chemical Reactions
A reaction in which two or more substances combine to form a single product is called a combination reaction.
CaO+H2O = Ca(OH)2 + Heat
C + 02 = CO2
A reaction in which a single reactant breaks into two or more products is called a decomposition reaction.
Thermal decomposition: When decomposition reactions take place in presence of heat. Ex:
CaCO3 + Heat = CaO + CO2
Electrolytic decomposition: When decomposition takes place in presence of electricity. Ex:
2H2O = 2H2 + O2
Photolytic decomposition: When decomposition takes place in presence of sunlight. Ex:
2AgCl = 2Ag +Cl2
When a more reactive metal displaces a less reactive metal is called displacement reaction.
CuSO4 + Fe = FeSO4 + Cu
Double Displacement Reaction
A reaction in which exchange of ions takes place between reactants is called double displacement reaction.
Na2SO4 + BaCl2 = BaSO4 + 2NaCl
Here BaSO4 is an insoluble substance known as precipitate and such reaction in which precipitate is formed is called precipitation reaction.
A reaction in which heat is released is called exothermic reaction.
Burning of natural gas
Ch4 + O2 = CO2 + H20 + Heat
A reaction in which heat is absorbed is called endothermic reaction.
NH4Cl(s) + Heat = NH3(g) + HCl(g)
The process of addition of oxygen is called oxidation reaction.
2Cu + H2 = Cu + H2O
The brown colour copper burns in the presence of oxygen form a black coating od copper oxide
The process of removal of oxygen is called reduction reaction.
2KClO3 = 2KCl + 3O2
A reaction in which both oxidation and reduction reaction takes place Is called redox reaction.
CuO + H2 = Cu + H2O
Copper oxide gets reduced to copper and hydrogen gas get oxidized to water.
ZnO + C = Zn + CO
The process by which metal objects are attacked by water , air , acids etc. is called corrosion .
Rusting of iron and other metal oblects like copper and silver
When fats and oils containing food items are oxidized their taste and smell change and this phenomenon is called rancidity.
- It can be prevented by using antioxidant like BHA and BHT
- It can be prevented by keeping food items in an airtight container and in the refrigerator