Download File

SCH4U
5.2 Oxidation Numbers
Oxidation number the apparent charge an atom would have if it gained or lost
its bonding electrons
Consider the example of sulfur dioxide, SO2. In sulfur dioxide, oxygen is more
electronegative than sulfur. Since oxygen gains two electrons to form the oxide
ion, O-2, in ionic compounds, the oxygen atoms in sulfur dioxide are assigned an
oxidation number of -2. The sum of all the oxidation numbers in a neutral
molecule must be zero, since the molecule has no overall charge. You can use
this information to calculate the oxidation number of sulfur. In the calculation
below, the symbol S represents the oxidation number of sulfur in sulfur dioxide.
*Rules for Assigning Oxidation Numbers
Oxidation numbers:
S + 2 ( -2 )
=0
S = +4
Therefore, sulfur has an oxidation number of +4.
Sample Problem: Assigning Oxidation Numbers
a) Determine the oxidation number of the nitrogen atom in potassium nitrate,
KNO3.
Step 1: Write All Known Oxidation Numbers
Use the symbol N to represent the oxidation number of nitrogen in potassium
nitrate.
K
N
O3
+1
N
-2
Step 2: Multiply Oxidation Numbers by Number of Each Type of Atom
K
N
O3
+1
N
-2
Step 3: Solve for Unknown Oxidation Number
(+1) + N + 3(-2) = 0
N–5=0
N = +5
Therefore, the oxidation number of the nitrogen atom in potassium nitrate is +5.
SCH4U
b) Determine the oxidation number of the sulfur in the sulfate ion, SO4-2.
Remember that the sum of the oxidation numbers must equal the charge of
the ion.
Step 1: Write All Known Oxidation Numbers
Use the symbol N to represent the oxidation number of sulfur in the sulfate.
S
O4-2
N
-2
Step 2: Multiply Oxidation Numbers by Number of Each Type of Atom
S
O4-2
N
4(-2)
Step 3: Solve for Unknown Oxidation Number
N + 4(-2) = -2
N – 8 = -2
N = +6
Therefore, the oxidation number of the sulfur in the sulfate ion is +6.
Identifying Redox Reactions Using Oxidation Numbers
All redox reactions, by definition, involve a loss or gain of electrons. Therefore,
oxidation numbers must change.


In reduction, oxidation numbers decrease.
In oxidation, oxidation numbers increase.
Sample 2: Using Oxidation Numbers to Identify a Redox Reaction
a) Use oxidation numbers to show that the reaction of zinc metal with sulfur is a
redox reaction. The chemical equation for this reaction is
Zn(s) + S(s) → ZnS(s)
Step 1: Write all Known Oxidation Numbers
Zn(s) + S(s) → ZnS(s)
0
0
+2 -2
The oxidation number of zinc changes from 0 to +2.
The oxidation number of sulfur changes from 0 to -2.
Since there is a change in oxidation numbers, the reaction of zinc with sulfur is a
redox reaction.
SCH4U
b) Is the following reaction of sulfur trioxide with water a redox reaction?
SO3(g) + H2O(l) → H2SO4(aq)
Step 1: Write All Known Oxidation Numbers
SO3(g) + H2O(l) → H2SO4(aq)
S -2
+1 -2
+1 S -2
Step 2: Multiply Oxidation Numbers by Number of Each Type of Atom
SO3(g) +
S + 3(-2)
H2O(l) →
H2SO4(aq)
2(+1) -2
2(+1) + S +4( -2)
Step 3: Solve for Unknown Oxidation Number
S + 3(-2) = 0
S–6=0
S=6
2(+1) -2 = 0
2(+1) + S + 4(-2) = 0
2 + S -8 = 0
S=6
Since there is no change in oxidation numbers for any of the atoms, the reaction
of sulfur trioxide with water is not a redox reaction.
SCH4U
Worksheet 5.2: Oxidation-Reduction (Redox) Reactions
1. Determine the oxidation number of each element in each of the following
a) Cl2
b) S8
c) H2S
d) F–
e) Na2O
f) CO2
g) FeO
h) NH3
2. Determine the oxidation number of the underlined element in each of the
following:
a)
b)
c)
d)
e)
f)
g)
h)
HClO3
KNO3
SO32- (careful!!)
Na2CO3
ClO4–
Fe(NO3)2
PbSO4
MnO4–
3. Which of the following equations represent a redox reaction? Justify your
answers using oxidation numbers.
a)
b)
c)
d)
e)
f)
g)
H2 (g) + Cl2 (g) → 2 HCl (g)
2 K (s) + I2 (g) → 2 KI (s)
CaCO3 (s) → CaO (s) + CO2 (g)
2 H2O (l) → 2 H2 (g) + O2 (g)
2 Li (s) + 2 H2O (l) → 2 LiOH (aq) + H2 (aq)
Fe2O3 (s) + 3 CO (g) → 2 Fe (s) + 3 CO2 (g
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (g)