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
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
Balance the following Chemical Equations and name the type of reaction, the reactants and the products
- Fe + O2 --> Fe2O3
- HgO --> Hg + O2
- AgNO3 + MgCl2 --> AgCl + Mg(NO3)2
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.