Biology 12 - Academic Course

Parent Category: Chemistry 12

Balancing Chemical Equations

Rules for balancing chemical equations:

  • Always look at the number of atoms of oxygen and hydrogen first
  • Recall that diatomic molecules are double atoms of the same element (Oxygen, Hydrogen....)
  • Combustion Reactactions imply that elements or compounds are "burned" in oxygen
  • The products of complete combustions reactions are always Carbon Dioxide (CO2) and Water (H2O).  This is particularly applicable to combustion of hydrocarbons

Example:

The combustion of methane gas is CH4 + O2 --> CO2  + H2O. 

This equation is NOT balanced.  Why Not?

These are the questions you will need to ask yourself to find out if the equation is balanced or not:

  • How many atoms of reactantants and products do we have?
  • Are the number of atoms of reactants = the number of atoms of products?
  • Are the number of atoms of each reactant = the number of atoms of each products?

The balanced equation for the above combustion reaction is:

CH4 + 2O2 --> CO2  + 2H2O.  

Now we have 9 atoms of reactants and 9 atoms of products

-- more importantly there are

  • 1 atom of C on both sides  of the equation
  • 4 atoms of H on both sides  of the equation
  • and 4 atoms of O on both sides of the equation

Activity:

Balance the following Chemical Equations and name the type of reaction, the reactants and the products

  1. Fe + O2 --> Fe2O3
  2. HgO --> Hg + O2
  3. AgNO3  + MgCl2 --> AgCl + Mg(NO3)2

Answer

1. This is the oxidation reaction of Iron in Oxygen, more commonly called "rusting" when it occurs naturally in air.  The product is called Iron Oxide

  • The balanced equation is  4Fe + 3O2 --> 2Fe2O3

2. This is the decomposition of Mercury Oxide into its elements Mercury and Oxygen

  • The balanced equation is 2HgO --> 2Hg + O2

3. This is double displacemnt or double recombination reaction of Silver Nitrate in solution when Magnesium Chloride is added to it.  A white precipitate called Silver Chloride forms along with Magnesium Nitrate

  • The balanced equation is 2AgNO3  + MgCl2 --> 2AgCl + Mg(NO3)2

Balancing equations can be achieved by several methods.  The two most common methods are:

  • Trial and error - as we have done in the above examples
    • Using the law of Conservation of Mass 
    • Number of atoms on either side of the equation must be conserved for each reactant and each product
  • Redox technique (Reduction and Oxidation)
    • The principle behind this method of balancing equations is that the number of electrons gained (Reduction) by a chemical substance must equal the number of electrons lost (Oxidation) by another substance, in the same closed system.