According to the law of conservation of mass, matter can neither be created nor destroyed. Thus, mass of each element present in the products of a chemical reaction must be equal to its mass present in the reactants.
The number of atoms of each element remains the same before and after a chemical reaction. In a balanced chemical equation number of atoms of a particular element present in the reactants and products must be equal. If not, equation is said to be "not balanced".
Zn + H2SO4 → ZnSO4 + H2
This equation is balanced, as the number of Zn, H, S (sulphur) and O atoms are equal on both sides of the equation. Therefore, it is said to be a balanced chemical equation.
Mg + O2 → MgO
Number of atoms of magnesium in the reactant side is equal to the number of atoms of magnesium in the product side. However, the number of atoms of oxygen on the reactant side is two (in O2) but only one atom of oxygen is in the product side (in MgO). To make the same number of atoms of oxygen in the product side, you have to write 2MgO. Now, the equation becomes
Mg + O2 → 2MgO
In the above equation, there is a shortage of one atom of magnesium on the left hand side. For balancing the number of magnesium atoms, you need to put 2 before Mg and the equation becomes
2Mg + O2 → 2MgO
Now, the number of magnesium and oxygen atoms is equal on both sides of the arrow and the chemical equation is said to be balanced. This method of balancing of a chemical equation is called the Hit and Trial method.
Step I: Study the equation carefully and write the number of atoms of different elements in the imbalanced equation.
Step II: Start balancing with the compound that contains the maximum number of atoms. The compound may be a reactant or a product.
Use the simplest possible set of whole number coefficients to balance a chemical equation. Normally we do not write fractional coefficients in such equations as molecules are not available in fractions. We multiply the equation by an appropriate number to ensure the entire equation has whole number coefficients.
Do not change subscripts in formulae of reactants or of products during balancing, as that may change the identity of the substances.
Qualitatively, a chemical equation simply describes what the reactants and products are. However, a balanced chemical equation gives a lot of quantitative information about a chemical reaction.
The number of atoms and molecules taking part in the reaction and the corresponding masses in atomic mass unit (amu or u).
The number of moles taking part in the reaction, with the corresponding masses in grams or in other convenient units.
Relationship between the volume of the reactants and the products if all of them are in the gaseous state.
Chemical reactions are of the following types: