CHEMICL EQUATION: A chemical equation is the symbolic representation of a chemical reaction in terms of chemical formulas. For example, the burning of sodium in chlorine to produce sodium chloride is written as
The formulas on the left side of the arrow represent the reactants; a reactant is a starting substance in a chemical reaction. The arrow mark indicates " react to form". The formulas on the right side of the arrow represent the products; a product is a substance that formed after the chemical reaction. The coefficient 2 in front of the formula NaCl, such coefficients give the relative number of molecules or formula units involved in the reaction. Coefficient 1 are usually are not written, but are understood.
In many cases, it is useful to indicate the states or phase of the substance in parenthesis following the formula for the substance. The phase labels commonly used are (g) = gas, (l) = liquid, (s) = solid, (aq) = aqueous solution. In the above equation, the reactant Na is a solid while the reactant Cl2 is a gas and the product of the reaction, NaCl is a solid substance.
BALANCING CHEMICAL EQUATION:
When the coefficients in a chemical equation are correctly given, the number of atoms of each element are equal on both sides of the arrow. The equation is then said to be balanced.
Following the atomic theory, any chemical equation is valid only if it is balanced. In a chemical reaction, the atoms are not destroyed or created. They are simply undergoing recombination. For example, consider the burning of natural gas, comprising mostly of methane, CH4. This reaction of burning can be described by considering just one molecule of methane, which combines with two molecules of oxygen, O2 to form one molecule of carbon dioxide, CO2 and two molecules of water, H2O as shown in the following equation.
Before one can write a balanced chemical reaction, one must be determine, by experiment, those substances that are reactants and those that are products. One must know the chemical formula for each substance.
As an example, consider the burning of propane gas. By experiment, it can be found that propane gas reacts with oxygen from the air to produce carbon dioxide and water. The chemical formulas for each of the substance are to be written; propane (C3H8), oxygen (O2), carbon dioxide (CO2) and water (H2O). Then one can write the chemical equation as follows,
This equation is not balanced because the coefficients that give the relative number of molecules involved have not been determined. In order to balance a chemical equation, one must select the coefficients that will make the number of atoms, for each substance, are equal on both sides of the equation.
It may be noted that there are 3 carbon atoms in propane, C3H8, on the left side of the equation, one must have 3 carbon atoms on the right side of the equation as well. This is achieved by putting a coefficient 3 in front of the substance (CO2) containing carbon on the right side of the equation. The equation now becomes
Similarly, there are 8 hydrogen atoms on the left side of the equation, so one must add a coefficient 4 in front of the substance containing hydrogen atoms on the right side, since the suffix 2 in the water molecule (H2O) represents the presence of two hydrogen atoms in each molecule of water. So considering 4 water molecules on the right makes the number of hydrogen atoms 8, same as on the left side. The equation now becomes,
In the above equation, the oxygen atoms are not balanced. On the left, there are only 2 atoms while there are 10 oxygen atoms on the right side of the equation, 6 from the 3 Co2 molecules and 4 from the 4 water molecules 4H2O. To balance the oxygen atoms, one can add a coefficient 5 in front of the O2 molecule on the left side of the equation,
Now the equation must be checked by counting the number of atoms of each element, on both sides of the equation, are correct or not. It can be seen that the number of carbon atoms, hydrogen atoms and oxygen atoms on both sides of the equation are equal. Hence, the above chemical equation representing the burning reaction of propane gas in oxygen is completely balanced.
It may be noted the combustion of propane can also be represented by the following balanced equation, by simply multiplying each coefficient by a factor 2 as shown below.
