Balancing Chemical Equations
In a chemical reaction reactants react to form products.
Reactions are described by word equations and symbol equations.
In a symbol equation the number of atoms of each element on the reactant side must be equal to the number of atoms of
each element on the product side. (a balanced symbol equation)
SO ........ the reaction between hydrogen gas and oxygen gas to make water can be written as:
Word equation
Hydrogen + Oxygen  Water
Unbalanced symbol equation
H2 + O2  H2O
Balanced symbol equation
2H2 + O2  2H2O
(There are 2 atoms of hydrogen on each side but 2 of
oxygen on the left and only 1 on the right. Putting a 2 in front of hydrogen and
water resolves this.)
(Note: you can only balance equations by putting a
number in front of a formula NOT by changing the formula of a substance.)
AND ........ the reaction between sodium metal and water to produce sodium hydroxide solution and hydrogen gas.
Word equation
Sodium + Water  Sodium hydroxide + Hydrogen
Unbalanced symbol equation
Na + H2O  NaOH + H2 (There is 1 atom of sodium and 1 atom of oxygen on
each side but 2 of hydrogen on the left and 3 on the right. Putting a 2 in front of
sodium, water and sodium hydroxide resolves this.)
Balanced symbol equation
2Na + 2H2O  2NaOH + H2
(Note: you can only balance equations by
putting a number in front of a formula NOT by changing the formula of a
substance.)
Balance these equations by putting the numbers of moles in the relevant boxes. (The first 2 have been done as examples.)
2
CuCO3
K
CH4
Na
Mg
+
+
+
+
+
Na2CO3
Fe
C2H5OH
Al
CuO
Al
+
+
+
+
+
+
2
2
HNO3
H2O
O2
Cl2
HCl
CaCO3
HCl
O2
O2
Cl2
CH4
H2SO4












2
Cu(NO3)2
NaOH
CO2
NaCl
MgCl2
CaO
NaCl
Fe2O3
CO2
AlCl3
Cu
Al2(SO4)3
+
+
+
H2O
H2
H2O
+
+
+
H2
CO2
CO2
+
H2O
+
+
CO2
H2
+
CO2
+
H2O
+
H2O
Write balanced equations from the following word equations. (You’ll need to work out the formula for some of the substances or
look them up on the sheet you’ve been given.)

Potassium + Oxygen  Potassium oxide

Sodium hydroxide + Sulfuric acid  Sodium sulfate + water

Copper oxide + Nitric acid  Copper nitrate + water