Balanced Chemical Equation
These notes are based on the chapter Chemical Reactions and Equations from NCERT book based on CBSE syllabus.
- Example of reaction between magnesium and oxygen
- Example of reaction between zinc and sulfuric acid
- Example of reaction involving rusting of iron
Reaction between magnesium and oxygen
Mg + O2 → MgO
Let us closely observe the number of atoms of elements on LHS and RHS in above equation. The number of magnesium atoms is 1 on both sides of arrow. But the number of oxygen atoms is 2 on RHS and 1 on LHS.
Where did the remaining 1 atom of oxygen go?
As per Law of Conservation of Mass, we cannot create or destroy mass. In order to obey the law of conservation of mass, the number of atoms of elements on RHS needs to be equal to that of elements on LHS. When these numbers are equal on both sides of an equation, the equation is said to be a balanced equation.
Reaction between zinc and sulphuric acid
Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen
Zn + H2SO4 → ZnSO4 + H2
In this equation, let us compare the number of atoms of each element on both sides of equation. To make it easier, let us put the numbers in a table.
| Element | Number of atoms on LHS | Number of atoms on RHS |
|---|---|---|
| Zn | 1 | 1 |
| H | 2 | 2 |
| S | 1 | 1 |
| O | 4 | 4 |
You can notice that number of atoms of each element is equal on both sides of equation. This is a balanced chemical equation, as it obeys the law of conservation of mass.
Calculation:
In this example, mass of zinc is 65 g and mass of sulphuric acid is 98 g.
H2SO4 = 2 × 1 + 32 + 4 × 16 = 2 + 32 + 64 = 98
So, total mass of reactants is 163 g.
Now, mass of zinc sulphate is 161 g and that of hydrogen is 2 g.
So, total mass of products is 163 g. Thus, mass of reactants is equal to mass of products.
So, this equation obeys the Law of Conservation of mass.
More examples of balanced equation
Reaction between calcium and sulphuric acid
Ca + H2SO4 → CaSO4 + H2
Calcium + Sulphuric Acid → Calcium sulphate + Hydrogen
Reaction between magnesium and sulphuric acid
Mg + H2SO4 → MgSO4 + H2
Magnesium + Sulphuric Acid → Magnesium sulphate + Hydrogen
Unbalanced Chemical Equation
If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H2O → Fe3O4 + H2
In this example, number of atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.
Calculation:
In this example, mass of Fe is 56 g and that of water is 18 g. So, total mass of reactants is 74 g.
Now, mass of iron oxide is 231 g and that of hydrogen is 2 g.
Mass of Iron Oxide (Fe3O4) = 3 × 56 + 4 × 16 = 168 + 64 = 232 g
Now, total mass of reactants is 232 + 2 = 234 g. So, this equation does not obey the law of conservation of mass.
Note: While balancing an equation, you don't need to calculate atomic masses. These calculations are given to clear your fundamentals. You just need to count the number of atoms of various elements and match them on both sides of an equation.
More examples of unbalanced equation
Reaction of copper with sulphuric acid
Cu + H2SO4 → CuSO4 + SO2 + H2
Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Hydrogen
In this equation, number of copper atoms is 1 on both sides of equation. Similarly, number of hydrogen atoms is 2 on both sides of equation. But number of sulphur atoms is 1 on LHS and 2 on RHS. Similarly, number of oxygen atoms is 4 on LHS and 6 on RHS.