Chemical Reactions and Equations

A chemical equation is a "balanced account of a chemical transaction." In any chemical transaction or reaction, the number of atoms of all the participating elements will remain proportionately constant before and after the reaction (The Law of Conservation of Mass).

The substance(s) that undergo the chemical reaction are known as "reactants". The "products" are the substances produced during the chemical reaction. In a chemical equation, the formulae of the reactants and products are used. The chemical formula of a substance is the symbolic representation of the actual number of atoms present in one molecule of that substance.

Example 1: Formula of one molecule of oxygen is O₂. It shows that one molecule of oxygen is made up of 2 atoms of it.

Example 2: Formula of one molecule of water is H₂O. It shows that one molecule of water is made up of 2 atoms of hydrogen and one atom of oxygen.

Example 3: Formula of one molecule of sulphuric acid is H₂SO₄. It shows that one molecule of sulphuric acid is made up of 2 atoms of hydrogen, 1 atom of sulphur and 4 atoms of oxygen.

A word-equation shows change of reactants to products through an arrow placed between them (). The arrow may be read as "to yield" or "to form" or "to give" and shows the direction of the reaction. The reactants are placed on the left hand side (LHS) of the arrow and the products on the right hand side (RHS). The different reactants as well as products are connected by a plus sign (+).

Writing a Chemical Equation
To be able to write a chemical equation, you must know the reactants, products, and their chemical formulae.

Initially, write the word equation, and below it, write the molecular equation.

Example 1:

Now count and compare the number of atoms of each element on the LHS and RHS. Do they match? If they do not, then it is an unbalanced equation. Such an equation is called a skeletal chemical equation. In this example, in the reactant side, there are 2 atoms of oxygen, but on the product side, there is only one atom of oxygen.

Balanced Chemical Equations
According to the Law of Conservation of Mass the number of atoms of all the participating elements should remain proportionately constant before and after the reaction. Therefore add 2 in front of H₂O, to make the number of oxygen atoms equal to 2.

Now there are 4 hydrogen on the right side, but only 2 on the left side. So add 2, in front of H₂, to make it equal to 4 i.e.,

Now it has been balanced.

Example 2:

+ + Water +

+ HCl CaCl₂ + H₂O + CO₂

On the right side, there are 2 atoms of chlorine and 2 atoms of hydrogen. So add 2 in front of HCl.

CaCO₃ + 2 HCl CaCl₂ + H₂O + CO₂

To Check

Ca = 1              Ca = 1

C = 1                C = 1

O = 3                O = 3

H = 2                H = 2

Cl = 2                Cl = 2

Example 3:

Lead nitrate Lead monoxide + Nitrogen dioxide + Oxygen

PbO + NO₂ + O₂

The number of oxygen atoms on the reactant side is (3 x 2 = 6) an even number, where as, on the product side (1+ 2 + 2 = 5) it is an odd number. Make it into an even number, by adding 2 in front of PbO. (Remember, we cannot place 2 between Pb and O, such as Pb₂O, or 2 after PbO, such as PbO₂). If you want to double the oxygen, double the whole molecule.

So if you add 2 in front of PbO, you must add 2 in front of Pb(NO₃)₂, also. So it will be

Now on the left side, there are 4 atoms of N. So add 4 in front of NO₂. So it will be

There are 12 atoms of O on both the sides.

The equation is now balanced.

Example 4:

+ + + +

Whenever H₂O is present on any side, the number of hydrogen on both the sides should be an even number (2 atoms of hydrogen in water). If there are 4H₂O, then 4 x 2 = 8 hydrogen atoms. If there are 7H₂O, then 7 x 2 = 14 hydrogen atoms i.e., all are even numbers.

On the reactant side, there must be an even number in front of HCl. (What that number is, we will find out later). As a result, the number of chlorine atoms will also be even. But on the product side, the number of chlorine atoms is odd (i.e., KCl = 1, MnCl₂= 2, Cl₂ = 2. i.e., 1 + 2 + 2 = 5). The only odd number of chlorine atoms is in KCl. Let us change it into the simplest even number possible i.e., 2.

Since number of K atoms in 2 KCl = 2, place 2 in front of KMnO₄ to balance K atom.

In 2 KMnO₄, there are 2 K, 2 Mn, and 8 O. So add these numbers in front of K, Mn and O, (K is already done).

If there are 8 H₂O on the product side, there should be 16 H (8 x 2) on the reactants side as well.

Now the only unbalanced one is ® Chlorine. On the left hand side, there are 16 Cl. On the right hand side, firstly, there are 2 Cl in 2 KCl + 4 Cl in 2 MnCl₂, making total of 6(2 + 4). Subsequently 10 more Cl atoms are to be accounted for. So place 5 in front of Cl₂ to make it 10 (5 x 2).

This type of balancing the chemical equations is known as the Hit and trial method, where we make trials to balance the equation using small whole numbers as coefficients. It is not possible to discuss all equations here. You will achieve mastery over it by repeated practice, with the help of your teacher.

Remember

Limitation of a chemical equation It does not mention the state of the substances. Accordingly, the following symbols should be added to make it informative: (s) for solid, (l) for liquid, (aq) if the reactant or product is present as a solution in water, (g) for gas and (vap) for vapour.            Example: CaCO3 (s)+ HCl (aq) CaCl2 (s) + H2O (l) + CO2(g) The reaction may or may not be complete. An equation does not reveal this. It does not give any information regarding the speed of the reaction. It does not give the concentration of the substances. In some cases, terms like diluted (dil) and concentrated (conc) may be added. It does not give the conditions of temperature, pressure, catalyst, etc. This is overcome by mentioning these above or below the arrow e.g. It does not give any idea about colour changes, which has to be mentioned separately.   It does not give any indication regarding the production or absorption of heat. This is mentioned separately.           C + O2 CO2 + Heat          C + CO2 2CO - Heat Some reactions are reversible. They are represented by or