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 (Writing a Chemical Equation
To be able to write a chemical equation, you must know the reactants, products, and their chemical formulae.
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:
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On the right side, there are 2 atoms of chlorine and 2 atoms of hydrogen. So add 2 in front of HCl.
CaCO3 + 2 HClTo Check
Ca = 1 Ca = 1C = 1 C = 1
O = 3 O = 3H = 2 H = 2
Cl = 2 Cl = 2Example 3:
Lead nitrate
PbO + NO2 + O2
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The equation is now balanced.
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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.
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Limitation of a chemical equation
Example: CaCO3 (s)+ HCl (aq)
C + O2 C + CO2
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