Balancing Chemical Equations
Signs that a chemical change or reaction has taken place:
 Color change
 Solid forms
 Gas forms (see bubbles)
 Heat and/or flame is produced.
 Heat is absorbed.
Chemical change always involves a rearrangement in the groupings of atoms.
We represent a chemical reaction by writing a chemical equation.
• Reactants are compounds present before reaction takes place and are written on the left of the equation.
• Products are the compounds present after the reaction takes place and are written on the right of the equation.
• Symbols for chemical equations, see table.
Symbol
(s) or (cr)
(l)
(g)
(aq)
Name
Solid, crystalline
Liquid
Gas
Aqueous
Meaning
Compound or element is it’s solid state
Compound or element is in a liquid state
Compound or element is in its gaseous state
Compound is dissolved in water.
Dalton’s theory and the law of conservation of mass say atoms are never created or destroyed simply
rearranged. Therefore, we must balance our chemical equation to show the law of conservation of mass.
Balancing equations – There must be the same number of each type of atom on the product side as there is on
the reaction side.
Steps to balancing:
1. Write formulas for all reactants and products. Remember how to write your formulas.
2. Write the unbalanced equation. Put reactants on left and products on right of arrow
3. Balance the equation.
a. You may never change a subscript in a compound’s formula. You may only change a coefficient
(number in front of compound).
b. Start with the most complicated compound.
c. Balance all atoms except hydrogen and oxygen.
d. Balance hydrogen.
e. Balance oxygen.
f. Make sure all coefficients are in the lowest possible ratio.
Example: Balance the following equation:
Currently there are 1 atom C, 4
atoms of H, and 2 atoms of O
on the left side of the arrow.
CH4(g) + O2(g)  H2O(g) + CO2(g)
# atoms on left
?
# atoms on right
1
C
1
4
H
2
Currently there are 1 atom C, 2
2
O
3
atoms of H, and 3 atoms of O
on the right side of the arrow.
The number of atoms of each element must be equal on both sides of the arrow in the equation.
Start with the carbon atoms - both are equal. There are no other elements left to balance except H and O.
Balance H first. There are 4 atoms of H on the left and only 2 atoms of H on the right.
Put a 2 as a coefficient for the H2O on the right side of the equation.
CH4(g) + O2(g)  2 H2O(g) + CO2(g)
# atoms on left
?
# atoms on right
1
C
1
4
H
4
Now there are 1 atom C, 4
Now there are 1 atom C, 4
2
O
4
atoms of H, and 4 atoms of O
atoms of H, and 2 atoms of O
on the left side of the arrow.
Now there are 1 atom C, 4
atoms of H, and 4 atoms of O
on the left side of the arrow.
on the right side of the arrow.
Now balance the Oxygen atoms.
CH4(g) + 2 O2(g)  2 H2O(g) + CO2(g)
# atoms on left
?
# atoms on right
1
C
1
Now there are 1 atom C, 4
4
H
4
atoms of H, and 4 atoms of O
4
O
4
on the right side of the arrow.
Both sides of the equation have the same number of each element and the coefficients (1,2,2,1) are in there
lowest possible ratio – THE EQUATION IS BALANCED.
CH4(g) + 2 O2(g)  2 H2O(g) + CO2(g)
Answers
Cl2(g) + 2 KBr(aq)  Br2(g) + 2 KCl(aq)
Mg(s) + 2 HCl  MgCl2(aq) + H2(g)
1. 2 K(s) + 2 H2O(l)  H2(g) + 2 KOH(aq)
2. 2 H2(g) + O2(g)  2 H2O (l)
3. 2 Li(s) + Cl2(g)  2 LiCl (s)
4 Fe(s) + 3 O2(g)  2 Fe2O3(s)
4.
5.
6.
Now try these:
1. K(s) + H2O(l)  H2(g) + KOH(aq)
2. H2(g) + O2(g)  H2O (l)
3. Li(s) + Cl2(g)  LiCl (s)
4. Fe(s) + O2(g)  Fe2O3(s) (hint balance O first this time)
5. Chlorine gas reacts with an aqueous solution of potassium bromide to produce liquid
bromine and an aqueous solution of potassium chloride.
6. Solid magnesium reacts with an aqueous solution of hydrochloric acid to produce an
aqueous solution of magnesium chloride and bubbles of hydrogen gas.