Chemical Equation

A chemical equation is a shorthand way to represent the components of a chemical reaction. Several pieces of information are provided in a chemical equation for those working with the chemical equation or corresponding chemical reaction.

You've worked with equations in mathematics, where an equation represents information such as equalities or inequalities. Chemical equations are different from mathematical equations because the two parts of a chemical reaction represent the 'before' and 'after' of a chemical reaction. In mathematical equations, an equals sign separates the two parts of the equation. In chemical equations, equal signs are not used. Instead, an arrow is used to separate the two sides of the equation, and it points in the direction that a chemical reaction will proceed.

Writing a Chemical Equation: A chemical equation represents a chemical reaction. If you recall formulae of magnesium, oxygen and magnesium oxide, the above word equation can be written as

Mg + O₂ → MgO

Count and compare the number of atoms of each element on the LHS and RHS of the arrow. Is the number of atoms of each element the same on both sides? If yes, then the equation is balanced. If not, then the equation is unbalanced because the mass is not the same on both sides of the equation.

When a chemical reaction occurs, the mass of the products should equal the mass of the reactants. A balanced chemical equation occurs when the number of atoms involved on the reactants side equals the number of atoms on the product side.

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

The following chemical equation may represent the above word equation –

Zn + H₂SO₄ → ZnSO₄ + H₂

Let us examine the number of atoms of different elements on both sides of the arrow.

As the number of atoms of each element is the same on both sides of the arrow, it is a balanced chemical equation.

Let us try to balance the following chemical equation –

Fe + H₂O → Fe₃O₄ + H₂

Step I: To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

Fe + H₂O → Fe₃O₄ + H₂

Step II: List the number of atoms of different elements present in the unbalanced equation.

Step III: It is often convenient to start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element with the maximum number of atoms. Using these criteria, we select Fe₃O₄ and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS. To balance the oxygen atoms.

To equalise the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions. For example, to balance oxygen atoms, we can put coefficient 4’ as 4 H2O, not H₂O₄ or (H2O)₄. Now the partly balanced equation becomes–

Fe + 4H₂O → Fe₃O₄ + H₂

Step IV: Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms in the partly balanced equation. To equalise the number of H atoms, make the number of molecules of hydrogen four on the RHS.

Fe + 4H₂O → Fe₃O₄ + 4H₂

Step V: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.

To equalise Fe, we take three atoms of Fe on the LHS.

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Step VI: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.

3Fe + 4H2O → Fe3O4 + 4H2

The numbers of atoms of elements on both sides of the eq are equal. This equation is now balanced. This method of balancing chemical equations is called the hit-and-trial method, as we make trials to balance the equation by using the smallest whole-number coefficient.

To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae. The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. Aqueous (aq) is written if the reactant or product is present as a solution in water. The balanced Eq becomes

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)