The process in which substances (reactants) react to form new compounds (products), is known as a chemical reaction. This process involves the breaking of old bonds and formation of new bonds. If bond energies of reactants are greater than the bond energies of products, the reaction occurs with the evolution of energy in the form of heat. However, in an opposite condition, absorption of energy takes place.
Properties of a Chemical Reaction
A Chemical reaction can be observed with the help of any of the following observations.
- Change in state
- Change in color
- Evolution of a gas
- Change in temperature
- Formation of Precipitate
The short representation of a chemical reaction with the help of symbols of elements or formula of compounds is called chemical equation.
- The substances or compounds which take part in a reaction are called reactants. These are written on the left hand side (LHS) with a plus sign (+) in between them.
- The substances or compounds formed in the course of reaction are called products. These are written on the right hand side (RHS) with a plush sign (+) in between them.
- The arrow head (→) point towards the products which shows the direction of reaction. e.g., zinc reacts with sulphuric acid to form zinc sulphate and hydrogen gas.
Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen
Rules of Writing a balanced Chemical Reaction Equation
i) The number of atoms of reactants should be equal to the number of atoms of products. (According to the law of conservation of mass).
Fe + H2O → Fe3O4 + H2
As per rule, the above equation is incorrect and can be correctly written as
3Fe + 4H2O → Fe3O4 + 4H2
ii) The physical States of reactants and products should be mentioned along with their chemical formula in parenthesis.
The above equation can be written in accordance with to rule ii)
3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)
Types of Chemical Reactions
Different types of chemical reactions with examples of chemical reactions
A reaction in which a single new product is formed from two or more reactants, is called a combination reaction. Such reactions may occur in between the element or compounds.
For example, formation of slaked lime by the reaction of calcium oxide with water
CaO (s) + H2O (l)→ Ca(OH)2 (aq)
Other examples of combination reactions are
- Burning of coal C (s) + O2 (g) → CO2 (g)
- Formation fo water from H2(g) and O2 (g)
2H2 (g) + O2 (g) → 2H2O (l)
A chemical reaction in which a single reactant (compound) breaks down to give simpler products, is called a decomposition reaction. The decomposition reactions require energy in the form of heat, light or electricity. Therefore, decomposition reactions are of three types
When a decomposition is carried out by heating, it is called thermal decomposition.
For example, decomposition of calcium carbonate to calcium oxide and carbon dioxide upon heating
CaCO3 (s) → CaO (s) + CO2 (g)
Another example of thermal decomposition is the decomposition of lead nitrate to lead oxide, nitrogen dioxide (brown fumes) and oxygen.
2Pb(NO3)2 → 2PbO (s) + 4 NO2 (g) + O2
When a decomposition reaction is brought about by sunlight, it is called photolysis
For example, 2AgCl (s) → 2 Ag (s) + Cl2 (g)
- The above reaction is used in black and white photography since silver chloride or silver bromide turns grey in sunlight.
- When metal salts are heated, their ions emit various colors of light
- A decomposition reaction is the reverse of the combination reaction.
- Decomposition reaction of calcium carbonate is used in various industries. e.g., in the manufacturing of cement
When a decomposition reaction is brought about by electricity, it is called electrolysis
2 H2O (l) → 2H2 + O2
A reaction in which more reactive element displaces less reactive element from its compound present in the dissolved state, is called a displacement reaction.
For example, when an iron nail is suspended in an aqueous solution of copper sulfate for 20 minutes, it becomes brownish and the blue color of the solution is slightly faded. This indicates that iron has displaced copper from copper sulfate solution.
Fe (s) + CUSO4 (aq) → FeSO4 (aq) + Cu (s)
Zinc and lead are more reactive elements than copper, so they displace Cu from the aqueous solutions of its compounds.
Double Displacement Reaction
A chemical reaction in which there is an exchange of ions between the reactants to give new substances is called a double displacement reaction.
Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s)↓ + 2 NaCl (aq)
In the above reaction, precipitates are formed. So, this reaction is also known as precipitation reaction.
Acids and bases neutralize each other to form corresponding salts and water. This reaction is called neutralization reaction. If acid and base both are strong, 57.1 kJ heat is in released during the process.
HCl + NaOH → NaCl + H2O
Isomerisation or Rearrangement Reaction
A chemical reaction in which the atoms of the molecule of a compound undergo rearrangement is called an isomerization or rearrangement reaction. It is generally seen in case of organic compounds.
For Example, isomerization of ammonium cyanate into urea.
NH4CNO → 2NH3
Reversible and Irreversible Reaction
A chemical reaction which proceeds in both the directions is called a reversible reaction. For example, the formation of ammonia from nitrogen and hydrogen by Haber’s process.
N2 + H2 ↔ 2NH3
A chemical reaction which proceeds only in one direction is called irreversible reaction.
2NaOH + H2SO4
It is the reaction between salts of a weak acid or a weak base with water. Due to high dielectric constant, water has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some iconic compounds are hydrolyzed in water.
CH3COONa + H2O → CH3COOH + NaOH
These chemical reactions take place in the presence of sunlight.
6CO2 + 12H2O → C6H12O6 + 6H2O +6O2
The rate of a photochemical reaction is affected by the intensity of light.
The photosensitizer is a substance which brings about a reaction without undergoing any chemical change itself. In the process of photosynthesis, chlorophyll acts as a photosensitizer.
Exothermic and Endothermic Reactions
Reactions occurring with the evolution of energy are called exothermic reactions, e.g., respiration, decomposition, burning of natural gas, etc whereas reactions for the occurrence of which energy is absorbed, are called endothermic reactions. e.g., digestion
A+B → C + ΔE (exothermic)
A+B → C – ΔE (endothermic)
Oxidation and Reduction
It is defined as a chemical reaction in which a substance gains oxygen or any other electronegative element or loses hydrogen or electrons and shows an increase in oxidation number.
2Cu + O2 → 2CuO (Copper is oxidized to CuO)
CuO + H2 → Cu + H2O (Hydrogen is oxidized to H2O)
It is defined as a chemical reaction in which a substance gains hydrogen or electropositive element or electrons or loses oxygen or electronegative element and shows decrease in oxidation number
Oxidising Agent and Reducing Agent
The acceptor of electrons is an oxidizing agent (oxidant). The donor of electrons is reducing agent (reductant). In short, a substance which is oxidized or oxidation number of which is increased acts as a reducing agent while a substance which is reduced or oxidation number of which is decreased acts as an oxidizing agent.