1
Oxidation
When methane burns in air, it oxidizes and
forms oxides of carbon and hydrogen.
CH4(g) + 2O2(g)
CO2(g) + 2H2O(g)
When elemental iron turns to rust, it slowly
oxidizes to compounds such as iron (III)
oxide.
4Fe(s) + 3O2(g)
2Fe2O3(s)
2
Reduction
Originally reduction meant a loss of oxygen
from a compound
2Fe2O3(s) + 3C(s)
iron oxide
carbon
4Fe(s) + 3CO2(g)
iron
carbon dioxide
Reduction of iron ore to metallic iron involves
the removal of oxygen from iron (III) oxide.
Involves heating the ore with carbon.
3
Question
What happens to magnesium and oxygen
when they react to form magnesium oxide?
+2
2Mg
magnesium
+
O2
oxygen
-2
2MgO
magnesium oxide
Magnesium loses electrons to form Mg2+
Oxygen gains electrons to form O2-
4
Redox Reactions
Redox reactions are currently understood to
involve any shift of electron between reactants.
Oxidation – a process that involves a loss of
electrons or a gain of oxygen.
• Results in an increase in the oxidation number of an atom
Reduction – a process that involves a gain of
electrons or the loss of oxygen.
• Results in a decrease in the oxidation number of an atom
5
Redox Reactions
Oxidation and reduction always occur
simultaneously.
The substance gaining oxygen or losing
electrons is oxidized
The substance losing oxygen or gaining
electrons is reduced.
6
Redox Reactions
During a reaction between a metal and a nonmetal,
electrons are transferred from atoms of the metal
to atoms of the nonmetal.
Mg +
magnesium
S
sulfur
Mg2+S2magnesium sulfide
2 electrons are transferred from a magnesium atom
to a sulfur atom.
Magnesium atoms become more stable by the loss
of electrons. Sulfur atoms become more stable
by the gain of electrons
7
Redox Reactions
Mg +
magnesium
Oxidation: Mg
S
sulfur
Mg2+S2magnesium sulfide
Mg2+ + 2e- (loss of electrons)
Reduction: S + 2e-
S2- (gain of electrons)
Magnesium atom is said to be oxidized to a
magnesium ion
Sulfur atom is said to be reduced to a sulfide ion.
8
Redox Reactions
Reducing agent
the substance that loses the electrons
Mg
+
S
MgS
reducing agent
oxidized
Oxidizing agent
the substance that accepts electrons
Mg + S
MgS
oxidizing agent
reduced
9
Questions
Define oxidation and reduction in terms of the
gain or loss of oxygen.
Oxidation is the gain of oxygen
Reduction is the loss of oxygen
Define oxidation and reduction in terms of the
gain or loss of electrons.
LEO the lion goes GER
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
10
End of Section 20.1
11
Oxidation Numbers
Oxidation number is a + or – number assigned to
an atom to indicate its degree of oxidation or
reduction.
General Rule
A bonded atom’s oxidation # is the charge that it
would have if the e- in the bond were assigned to
the atom of the more electronegative element.
12
Rules for Assigning Oxidation Numbers
1. The oxidation number of a monatomic ion is = in
magnitude and sign to its ionic charge.
Bromide (Br1-) is -1
Iron III (Fe3+) is +3
2. The oxidation number of hydrogen in a
compound is +1, except in metal hydrides, such
as NaH, where it is -1
3. The oxidation number of oxygen in a compound
is -2 except in peroxides, such as H2O2, where it
is -1 and in compounds with the more
electronegative fluorine, where it is positive.
13
Rules for Assigning Oxidation Numbers
4. The oxidation number of an atom in uncombined
(elemental) form is 0.
Potassium metal (K) is 0
Nitrogen Gas (N2) is 0
5. For any neutral compound, the sum of the
oxidation numbers of the atoms in the compound
must equal 0
6. For polyatomic ion, the sum of the oxidation
numbers must equal the ionic charge of the ion.
14
Some Thought
Determining oxidation numbers of elements in
compounds is a way for chemists to keep track of
electron transfer during redox reactions
What are other examples where items are
numbered to keep track of movement?
The numbers on a sports player’s jersey
The area codes assigned to telephone numbers in
different regions
15
Binary Ionic Compounds
In binary ionic compounds, such as NaCl and
CaCl2, the oxidation numbers of the atoms equal
their ionic charges.
Na1+ + Cl-1
NaCl
oxidation # +1 -1
neutral
Ca2+ + Cl-1
CaCl2
oxidation # +2 -1
neutral
Note the sign should be put before the oxidation #
16
Molecular Compounds
No ionic charges are associated with atoms of
molecular compounds.
However, oxygen is reduced in the formation of
water for example.
In water the two shared e- in the H – O bond are
shifted toward the O and away from the H.
Imagine the e- contributed by the two H atoms are
completely transferred to the O.
17
Molecular Compounds
The charge that would result from the transfer are
the oxidation numbers of the bonded elements.
The oxidation number of O is -2 and the oxidation
number of each hydrogen is +1
Oxidation numbers are often written above the
chemical symbols in a formula.
+1 -2
H2O
18
Multiple Oxidation Numbers
Many elements can have several different oxidation
numbers.
+1 +6 -2
K2CrO4 – Potassium Chromate
+1 +6 -2
K2Cr2O7 – Potassium Dichromate
19
Sample Problems
What is the oxidation number of each kind of atom
in the following ions and compounds?
+4 -2
SO2
+4 -2
CO32+1 +6 -2
Na2SO4
-3 +1
-2
(NH4)2S
20
Sample Problems
Determine the oxidation number of each element in
the following.
+3 -2
S2O3
+1
-1
Na2O2
+5
-2
P2O5
+5
-2
NO321
Sample Problems
Determine the oxidation number of chlorine in each
of the following substances.
+1 +5 -2
KClO3
0
Cl2
+2
+7
-2
Ca(ClO4)2
+1
-2
Cl2O
22
Sample Problems
What are the oxidation numbers of iodine in the
following?
+1 +7 -2
HIO4
+1 +5 -2
HIO3
+1 +1 -2
HIO
0
I2
+1 -1
HI
23
Oxidation Number Changes
When copper wire is placed in a solution of silver
nitrate the following reaction occurs
+1 +5 -2
0
2AgNO3(aq) + Cu(s)
+2
+5 -2
0
Cu(NO3)2(aq) + 2Ag(s)
Ag is reduced from +1 to 0
Cu is oxidized from 0 to +2
24
Oxidation Number Changes
An increase in the oxidation number of an atom or
ion indicates oxidation.
A decrease in the oxidation number of an atom or
ion indicates reduction.
+2 +6 -2
0
CuSO4(aq) + Fe (s)
0
+2 +6 -2
Cu(s) + FeSO4
Cu is reduced from +2 to 0
Fe is oxidized from 0 to +2
25
Sample Problem
Identify which atoms are oxidized and which are
reduced in the following reaction. Also identify
the oxidizing agent and the reducing agent.
+1 -1
0
2HBr(aq) + Cl2(g)
+1 -1
0
2HCl(aq) + Br2(l)
Cl is reduced from 0 to -1, so Cl2 is the oxidizing agent
Br is oxidized from -1 to 0, so Br1- is the reducing agent
26
Sample Problem
Identify which atoms are oxidized and which are
reduced in each reaction
0
0
O2(g) + 2H2(g)
+1 -2
2H2O(l)
O2 is reduced from 0 to -2
H2 is oxidized from 0 to +1
27
Sample Problem
Identify which atoms are oxidized and which are
reduced in each reaction
+1 +5 -2
+1 +3 -2
0
2KNO3(s)
2KNO2(s) + O2(g)
N is reduced from +5 to +3
O is oxidized from -2 to 0
28
Sample Problem
Identify the oxidizing agent and the reducing agent
in each equation.
0
0
O2(g) + 2H2(g)
+1 -2
2H2O(l)
O2 is reduced, thus O2 is the oxidizing agent
H2 is oxidized, thus H2 is the reducing agent
29
Sample Problem
Identify the oxidizing agent and the reducing agent
in each equation.
+1 +5 -2
+1 +3 -2
0
2KNO3(s)
2KNO2(s) + O2(g)
N is reduced, thus N is the oxidizing agent
O is oxidized, thus O is the reducing agent
30
Questions
What is the general rule for assigning oxidation
numbers?
The oxidation number is the charge a bonded atom
would have if the electrons in the bond were
assigned to the more electronegative element
How is a change in oxidation number related to the
process of oxidation and reduction?
An increase in oxidation number indicates oxidation; a
decrease in oxidation number indicates reduction.
31
Sample Problem
Identify which atoms are oxidized and which are
reduced in each reaction. Also identify the
oxidizing agent and the reducing agent.
0
0
2Na(s) + Cl2(g)
+1 -1
2NaCl(s)
Cl2 is reduced, thus Cl2 is the oxidizing agent
Na is oxidized, thus Na is the reducing agent
32
End of Section 20.2
33