Chapter 1 Chemical Reaction and Equations


1.1 What is a chemical reaction?

Technically speaking, a reaction that involves the rearrangement of the molecular or ionic structure of a substance that leads to a change in physical form or nuclear reaction is known as a chemical reaction.

In layman language, the chemical reaction is a process in which a one or more substance is converted to one or more different substance. The initial substance is known as a reactant. In other words, it’s an irreversible change.


1.2 What are the key concepts of a Chemical reaction?

Listed below are some of the concepts related to a chemical reaction.

  • A chemical change or reaction creates a new substance.
  • The atoms and molecules that interact with each other, in the reaction, is called reactants.
  • In a chemical reaction, the atoms and molecules produced by the reaction are called products.
  • In a chemical reaction, only the atoms present in the reactants can end up in the products. No new atoms are created, and no atoms are destroyed.
  • The Reactants contact each other, bonds between atoms in the reactants are broken, and atoms rearrange and form new bonds to make the products.
  • When heat or energy is released in the reaction it is known as an exothermic reaction. On the other hand, when heat is absorbed, it is known as an endothermic reaction.


1.3 Difference between physical and chemical reaction

A physical change, such as a state change or dissolving, does not create a new substance, but a chemical change does.

A physical change can be reversed; however, chemical reactions are irreversible.

No energy is released in a physical reaction, but some chemical reactions release heat or energy.


Example 1: When a candle is burnt following observations is made.

The candle melts to produce wax, ashes as well as heat.

It is an irreversible reaction, that is, you won’t get back the burnt part of a candle.

Hence, it’s an exothermic chemical reaction.


Example 2: Mixing of sugar in pure water.

This is a reversible reaction. If you leave the solution for evaporation, sugar will be left behind. The moisture can be condensed to make water.

No energy is consumed or released; hence, it is a physical reaction.


1.4 Types of chemical reaction

Atoms are neither created, nor is it destroyed in a reaction. It is just converted from one form to another. It actually involves breaking of original bonds and creating new ones. It can be classified into the following types on the basis of the bonds.

  1. Combination reaction
  2. Decomposition reaction
  3. Displacement reaction
  4. Double displacement reaction


Let us elaborate the above types.

1. Combination reaction

When two elements or compounds react to form a single compound, then, it is known as combination reaction.

H2 + Cl2 → 2HCl

2. Decomposition reaction

This is a type of chemical reaction in which a single compound breaks down or decomposes to create one or more element or compound. This reaction mostly involves energy that facilitates the decomposition of compound. The energy can be in the form of light, heat or even electricity.

CaCO3 → CaO + CO2

 3. Displacement reaction

It is a reaction in which an atom or a group of atom present in a molecule displaces to create a new molecule. For example,

Zn + CuSO4 → ZnSO4 + Cu

Here, zinc replaces copper from copper sulphate, thus, producing copper metal and zinc sulphate.


4. Double displacement reaction

In this reaction, the cations and anions of the reactant react with each other and displace to form new products.

Na2S + 2HCl → 2NaCl + H2S


1.5 Chemical Equation

A chemical equation is a symbolic representation of a reaction that takes between reactants to create the final product.

For example, H2+ O2 → H2O

The elements at the left side of equals to sign are reactants while you right at the chemical formula of the product at the right side.

The chemical equation represents a chemical reaction. The number of atoms at both the side of the equation should be same. That is, the number of atoms of hydrogen and oxygen on the left side should be equal to the number of atoms at the right side. This is known as a balanced chemical equation.


1.6 Balanced Chemical Equation

As mentioned above, when the number of atoms at both the side of the chemical equation is equal, then, it is known as a balanced chemical equation. Some examples of the same are mentioned below:

Zn + 2HCl → ZnCl2 + H2

                                            4Fe + 3O2 → 2Fe2O3                                                                                     

                                            2C2H6 + 7O2 → 4C02 + 6H2O  


1.7 How to balance a chemical equation?

Let us start with a simple equation.

Example 1:          H2 + O2 → H2O

STEP I: As discussed above, the number of atoms of hydrogen and oxygen in the above equation should be equal to both the side. So, count the number of atoms of the element.

STEP II: The trick is to balance the compound with a maximum number of the atom. In the above equation, H2O has got 3 atoms in total. So, we’ll balance it first. The number of hydrogen is same, i.e, 2 at both sides. However, the left side has got 2 oxygen atoms while the right has 1 atom. So, in order to equalize them, we can put 2 as a coefficient.

                                               H2 + O2 → 2H2O

STEP III: Now, we have equal numbers of oxygen atoms, but the hydrogen is to be balanced. The left side has 2 atoms of hydrogen while the right has 4. So, we will add 2 as a coefficient.

                                              2H2 + O2 → 2H2O

STEP IV: Cross-check the equation and count the number of atoms on both the side.


Example 2:                   KCl + O2 → KClO3

STEP I: Let us count the number of atoms.


Element LHS RHS
OXYGEN (02) 2 3

Hence, you need to balance oxygen first.

STEP II: Left side has got 2 oxygen atoms while the right has 3 atoms. So, in order to equalize them, making it 6 is the easiest way. We can put 2 as the coefficient of KClO3.

                                       KCl + O2 → 2KClO3

STEP III: Now, we have 6 atoms of oxygen at the right side, but 2 atoms in the left. So, we will add 3 as a coefficient.

                                      KCl + 3O2 → 2KClO3

STEP IV: Oxygen is balanced, but KCl has got to be balanced. Add 2 as a coefficient.

                                      2KCl + 3O2 → 2KClO3

STEP V: Count the number of atoms on both the side and check if it’s balanced.


1.8 How to interpret a balanced equation?

A chemical equation is a method of representing and interpreting the entire chemical reaction. Take the following example.

                                             Zn + 2HCl → ZnCl2 + H2

 The coefficient of the element and compound represents the number of molecules or moles. That is, it represents the number of moles of reactant required to create the products.

In the above reaction, 1 mole of zinc and 2 moles of HCl is required to make 1 mole each of zinc chloride and hydrogen.











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