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Chemical Equation

A chemical equation is a written description or symbolic representation of a chemical reaction. It represents the reactant and products by their chemical formulae and tells what happens in a chemical reaction. The reactants are written on the left side and the products formed are written on the right hand of the chemical equation. The arrow located between them shows the direction of the reaction. For example, the chemical equation of a reaction between hydrogen gas and oxygen gas is shown below;

2H2 + O2 → 2H2O

The above chemical equation clearly shows the reaction between hydrogen and water to form water.

Atoms can neither be destroyed nor created in a chemical reaction as a chemical reaction follows the law of conservation of mass. So, the number of atoms of each type remains the same on the left side and right side of the equation or before the start of a chemical reaction and after its completion. So, a balanced chemical equation satisfies the law of conservation of mass as it contains an equal number of atoms of an element in reactants and products.

How to balance a chemical equation

The easy steps to balance a chemical equation are described below;

  • First, write the unbalanced chemical equation.
  • Write down the number of atoms of all elements present on each side of the arrow
  • Add coefficients or numbers in front of the molecular formulas so that number of atoms of each element are same on both sides of the equation. Balance the oxygen and hydrogen atoms at the end as it makes the balancing easy.
  • Indicate the physical state of reactants and products at the end such as Fe (s).

Let us take a chemical reaction and balance its chemical equation.

The chemical reaction, which occurs during the rusting of iron is written below. Let us balance it by following the steps mentioned above.


The unbalanced chemical equation is written below;

Fe + O2 → Fe2O3


Find and write down the number of atoms of all elements in the equation. The subscript at each element indicates the number of atoms. The absence of subscript indicates a single atom of that element.

On the left side (reactant) side, we have one atom of iron (Fe) and two atoms of oxygen (O2), accordingly, we have put the number before its chemical symbol as shown below;

1 Fe
2 O

On the right (product) side, it contains 2 atoms of iron and 3 atoms of oxygen in one molecule of Fe2O3. So we have;

2 Fe
3 O

As it is an unbalanced equation, the number of atoms is not the same on both sides at this step.

Step 3:

It involves the addition of Coefficients (number) in front of the molecules. We cannot change the subscripts. We can add coefficients, which are whole number multipliers. For example, if we write 2CO2, it shows we have 2 times the number of atoms in one carbon dioxide molecule, which would be 2 carbon and 4 oxygen atoms. If the coefficient is absent, it shows only one molecule.

First, balance the Fe iron atom then hydrogen and oxygen atoms at the end. In this chemical equation, balance the iron atoms first as it is present in one reactant and one product. The left side has one iron atom and the right side has two, so, if we may try to add 2 in front of Fe on left side, the right side Fe atoms cannot be balanced by adding any coefficient.

Similarly, if we add 3 in front of Fe on the left side, the right side of Fe atom cannot be balanced. But, if we add 4 in front of Fe on the left side, the right side of Fe atoms can be balanced by adding 2 in front of the iron oxide molecule on the right side, as shown below;

4Fe + O2 → 2Fe2O3

Now, Fe atoms are balanced as there are 4 atoms of Fe on both sides of the equation.

Now, we have to balance the hydrogen and oxygen atoms by adding appropriate coefficients to them. These atoms are balanced last as they are generally present in many reactants and products. If they are balanced first, it may take more time to balance a chemical equation.

Let us see, which coefficient works for oxygen. If we add 2 in from of the oxygen molecule, it will give 4 atoms of oxygen on the left side, but, on the right side, you have 6 atoms (2x3). So, 2 will not work.

If we add 3, oxygen atoms of both sides of the equation become 6 or equal in number. Thus, coefficient 3 will balance the oxygen atoms as shown below;

4 Fe + 3O2 → 2Fe2O3

After following all the steps, you can check the number of atoms of each element on each side. If they are equal, the equation is balanced.

Let us balance one more unbalanced chemical equation given below;

Al + O2 → Al2O3

Step 1: write the unbalanced eq.

Al + O2 → Al2O3

In the above unbalance equation, we have 1 aluminium atom and 2 oxygen atoms on the left side. Whereas, there 2 aluminium atoms and 3 oxygen atoms on the other side.

Step 2: write down the number of atoms

On the left side;

1 Al

2 O

On the right side;

2 Al

3 O

Step 3: Add coefficient

First balance the atoms of Aluminium by adding coefficient 2 in the front of the Aluminium (Al), as shown below;

2AI + O2 → Al2O3

Now, in the end, we will balance the oxygen atoms. The equation has 2 oxygen atoms on the reactant (left) side and 3 oxygen atoms on the product or right side. If we add 2 in front of O2, we cannot balance oxygen. But if we add 3 we can balance oxygen by adding 2 in front of the single molecule of aluminium oxide (Al2O3) on the right side.

By adding 3 on the left side and 2 on the right side, we have an equal number of oxygen atoms, 6 on both sides. But, the Al atoms on the right side increases by 2. So, we replace the coefficient 2 with 4 in front of the Al atom on the right side as shown below;

4Al + 3O2 → 2Al2O3

After checking the number of atoms on each side, we can see that the above equation is balanced now.

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