1. Chemical Equations
Section 3.5

2. Chemical Equations
• A chemical equation is a way of representing a chemical change.
• It shows reactants and products.
• To balance an equation means to change the numbers of each
molecule involved, so that the same number of atoms of each
element appear on the reactants side and on the products side.
• Chemical equations balance on an atomic level, not molecular.
• You cannot change the formula of a substance, i.e. if the equation
has NH3 you cannot change this you can only put a number in front
of it, 2NH3, increasing the number of N’s and the number of H’s.
• Never change the subscripts (small numbers).
• It is possible to write balanced equations for lots of reactions but
that does not mean that the reaction actually takes place.

3. Formation of Water
(Oxygen does not exist as single atoms)
(This reaction does not take place)

4. Balancing an Equation
Methane reacts with oxygen, forming carbon dioxide and water vapour only.
Write a balanced chemical equation for this reaction.
Solution
CH4 O2
CO2 H2 O
Examine each element in turn, to have the same no. of atoms of each on either side.
(Leave the O until last as it appears the most often)
Examining C: 1 on LHS and 1 on RHS, leave as they are.
Examining H: 4 on LHS and 2 on RHS, put a 2 in front of the H2 O, on the RHS.
CH4
O2
CO2
2H2 O
Examining O: 2 on LHS and 4 on RHS, put a 2 in front of the O2 , on the LHS.
CH4
2O2
CO2
2H2 O

5. Balance the following
1.
Fe HCl
FeCl2 H2
Fe 2HCl
2.
O2
H2
3.
H2
1
2
FeCl2 H2
H2 O
O2
H2 O
O2
C 4 H10
4.
C 4 H10
13
2
Al O2
2Al
3
2
CO2
O2
4CO2
Al2 O3
O2
H2 O
Al2 O3
5H2 O

6. Balancing Redox Equations
• In working out what is oxidised and what is
reduced in a reaction, it is important to
remember that oxidation numbers are not a
charge.
• Write the oxidation numbers below each atom
to which it applies, as shown in the examples.
H2 O
1
2
Cr2 O7 2
6
2
OH
2
1
• Oxidation is an increase in oxidation number.
• Reduction is a decrease in oxidation number.

7. Balance the following redox equation
Cr2 O72
1.
Fe2
H
Cr 3
Fe3
H2O
Cr2 O7 2
Fe2
H
Cr 3
Fe3
3
3
H2 O
6
2.
2
2
Cr2 6e
2Cr 3
Fe2
Fe3
6e
6e
2Cr 3
6Fe3
3
e
Cr2
2
Cr2 6Fe2
e
3
Balance the half equations.
2Cr 3
4. Cr2 O72
6Fe2
H
Cr2 O72
6Fe2
14H
Write down half equations of what is oxidised
and reduced.
Attach subscripts to atoms oxidised and reduced
and balance the half equations.
3
6
3
6Fe2
5.
1
Fe3
6
2
1
Cr 3
Cr 3e
Fe2
3.
2
Assign oxidation numbers to all
the atoms in the equation.
6Fe3
2Cr 3
6Fe3
2Cr 3
6Fe3
H2 O
7H2 O
Attach species that were attached to the oxidised
and reduced atoms.
Include all the original species and complete the
balancing by inspection.

8. Calculations using Chemical Equations
• A balanced equation for a chemical reaction
gives the relative amounts of each reactant
and each product involved in the reaction.
• If the amount of one substance is known,
based on the molar ratios in the equation, the
amounts (masses, particles or volume)of other
substances can be calculated.
• Make sure to always work in moles.

9. A dry - cleaning fluid composed of carbon and chlorine was found to have the
composition 14.5% C, 85.5% Cl. What is the empirical formula of this compound?
Element Percentage
C
14.5%
Cl
85.5%
CCl2
Percentage
Ar
14.5
1.208
12
85.5
2.408
35.5
Simplest
Ratio
1
2