We can say a chemical reaction has taken places if we observe-
• ︎ Change in state
• ︎ Change in color
• ︎ Evolution of a gas
• ︎ Change in temperature or release of energy
Whenever a chemical change occurs, we can say that a chemical reaction has taken place such as in fermentation of grapes, souring of milk, rust on iron, respiration etc.
Magnesium(Mg) ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide (MgO). It is formed due to the reaction between magnesium and oxygen present in the air.
The word-equation for the above reaction would be:
Magnesium + Oxygen ———————>Magnesium oxide
Such a chemical equation is a skeletal chemical equation for a reaction. This is
unbalanced and is a basic equation.
As per law of conservation of mass, the number of atoms of each element remains the
same, before and after a chemical reaction. Hence, we need to balance a skeletal
2Mg+O2 →2MgO is a balanced equation
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
The above word-equation may be represented by the following chemical equation
Zn + H2SO4 → ZnSO4 + H2 is a balanced equation
Fe + H2O → Fe3O4 + H2 is an unbalanced equation
To balance start with the compound that contains the maximum number of atoms. It may be a reactant or a product. In this case O needs to be balanced first as it contains 4 atoms.
Fe + 4H2O → Fe3O4 + H2
Then the next element to be balanced is H as it now contains 8 atoms.
Fe + 4H2O → Fe3O4 + 4H2
And now the third element to be balanced is Fe which contains 3 atoms.
3Fe + 4H2O → Fe3O4 + 4H2 is a balanced equation
To make a chemical equation more informative, the physical states of the reactants
and products are mentioned along with their chemical formulae. The gaseous, liquid,
aqueous and solid states of reactants and products are represented by the notations
(g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or
product is present as a solution in water.
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Types of Reactions
Exothermic Reaction and Endothermic Reaction
Double Displacement Reaction
RedOx Reaction (Oxidation + Reduction)
A reaction in which a single product is formed from two or more reactants is known as a combination reaction.
CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
(Calcium Oxide) (Calcium Hydroxide)
Cao - Quick Lime , Ca(OH)2 - Slaked Lime
A solution of slaked lime Ca(OH)2 produced in the reaction is used for whitewashing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of whitewashing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO3.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
In simple language we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.
- C(s) + O2(g) → CO2(g) (Burning of Coal)
- 2H2(g) + O2(g) → 2H2O(l) (Formation of Water)
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
1. Respiration is an exothermic process because of release of energy by breakdown of glucose in presence of oxygen.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
2. Burning of Natural Gas (Methane) is also an exothermic reaction as there is release of energy.
CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)
3. Decomposition of vegetable matter into compost is also an exothermic reaction. Reactions in which energy is absorbed are known as endothermic reactions.
When we supply energy or energy is absorbed in a reaction, its reactants breakdown into simpler products and such a reaction is called as decomposition reaction.
Most decomposition reactions are endothermic reactions.
2FeSO4 (s) —————HEAT—————-> FeO (s) + SO2 (g) + SO3 (g)
(Ferrous Sulphate) (Ferric Oxide)
Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3), Sulphur dioxide (SO2) and Sulphur trioxide (SO3). Ferric oxide is a solid, while SO2 and SO3 are gases.
CaCO3(s) ⎯ ⎯ ⎯ HEAT⎯ ⎯ ⎯→ CaO(s) + CO2(g)
(Calcium Carbonate) (Quick Lime)
When a decomposition reaction is carried out by heating, it is called thermal decomposition.
2Pb(NO3)2 (s) ———HEAT————> 2PbO(s) + 4NO2 (g) + O2 (g)
(Lead Nitrate) (Lead Oxide) (Nitrogen Dioxide)
NO2 is a brown colored gas
When a decomposition reaction is carried out by light, it is called photo decomposition.
2AgBr(s)—— Sunlight———-> 2Ag(s) + Br2(g)
2AgCl(s)—— Sunlight———-> 2Ag(s) + Cl2(g)
These reactions are used in development of black and white photographs from films. Silver chloride or bromide turns grey in color due to decomposition into silver.
When one element displaces the other from its solution, then the reaction is called a displacement reaction.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Zn(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)
Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Double Displacement Reaction
In reactions where there is exchange of ions between the reactants is called a double-displacement reaction.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
The insoluble substance formed in such a reaction is called a precipitate. Such a reaction is thus also called as precipitation reaction. The precipitate in the above reaction is BaSO4 and NaCl remains soluble in the solution.
In reactions where a substance gains oxygen or loses hydrogen are called oxidation reactions.
2Cu + O2 ——HEAT——-> 2CuO
Copper is getting oxidized.
In reactions where a substance loses oxygen or gains hydrogen are called reduction reactions.
CuO +H2 ⎯⎯HEAT⎯⎯→ Cu+H2O
Copper is getting reduced but Hydrogen is getting oxidized. (RedOx)
Reactions is which one reactant gets oxidized while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.
ZnO + C → Zn +CO
Zinc is getting reduced and Carbon is getting Oxidized.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Manganese is getting reduced and HCl is oxidized to Chlorine by losing hydrogen.
When a metal is exposed to air for a long time, its surface reacts with oxygen present in air and forms a layer of oxide. Similarly, if a metal is attacked by moisture, acids it develops a layer of another compound on its surface. This is referred to as corrosion.
- The reddish brown coating on iron called rust;
- Black coating on silver
- Green coating on copper are other examples of corrosion.
When oil or fats get oxidized they release a very foul smell, their taste changes and they are said to become rancid. To prevent rancidity, anti-oxidants are added in food products.
A bag of chips has nitrogen to slow down the process of oxidation of chips.