There must be the same number of atoms belonging to each element on each side of the equation. For example,
C+O2 → CO2
On the left there is one atom of carbon (C) and two atoms in a molecule of oxygen (O2). This must balance with the right: CO2 is one atom of carbon combined with two atoms of oxygen. If the equation is not balanced, only big numbers in front of various chemicals (ie in the spaces) to balance the equation. For example,
__Fe2O3 +__C →__Fe + __CO2
Numbers in subscript (ie 2 and 3 in Fe2O3) cannot be changed because during so will change the identity of the chemical (ie Fe2O3 is different from FeO).
3.1.2.5 Balanced equations and associated calculations
Equations (full and ionic).
Students should be able to:
• write balanced equations for reactions studied
• balance equations for unfamiliar reactions when reactants and products are specified.
Oxford Textbook Pages : 35 - 37
CGP Revision Guide Pages : 16