OXIDATION and REDUCTION
Chapter 14
Oxidation
 The addition of oxygen to a substance
Carbon + Oxygen
C
+
O2
Carbon dioxide
CO2
Carbon gains oxygen and is oxidised
Reduction
 Removal of Oxygen from a substance
Copper(II)oxide + Hydrogen
CuO
+
H2
Copper + Water
Cu + H2O
Copper(II)oxide loses oxygen and is reduced
Oxidation
 Loss of electrons
2Mg
+ O2
2Mg2+O2-
Magnesium loses 2 electrons
and is oxidised
Reduction
 Gain of electrons
Cu2+O2-
+
H2
Cu
+
H2O
Cu2+ gains 2 electrons and is reduced
OIL RIG
 Oxidation is Loss of electrons
 Reduction is Gain of electrons
Redox Reactions
If one substance loses electrons (oxidation),
there must be some other substance to
accept these electrons (reduction).
oxidation
2Na
+
Cl2
2Na+Cl-
reduction
Oxidation and Reduction always occur together
Oxidising Agent
Each Zn atom loses 2 electrons
oxidation
Zn
+
Cu2+
Zn2+
+
Cu
reduction
Each Cu2+ ion gains 2 electrons
Zn atom could not lose electrons (oxidation)
without the Cu2+ ion accepting them
Copper is an oxidising agent (brings about
oxidation)
Oxidising Agent
 Is a substance that brings about oxidation in
other substances
 Is always reduced
oxidation
Zn
+
Cu2+
Zn2+
reduction
+
Cu
Copper is the oxidising agent
the oxidising agent is always reduced
Reducing Agent
Each Zn atom loses 2 electrons
oxidation
Zn
+
Cu2+
Zn2+ +
Cu
reduction
Each Cu2+ ion gains 2 electrons
Cu2+ ion could not gain electrons (reduction)
without the Zn atom losing them
Zn is a reducing agent (brings about
reduction)
Reducing Agent
 Is a substance that brings about reduction
in other substances
 Is always oxidised
oxidation
Zn
+
Cu2+
Zn2+
+
Cu
reduction
Zinc is the reducing agent
the reducing agent is always oxidised
 Now try question 14.2 page 190
Oxidation Numbers
Oxidation Number is the charge that an
atom has or appears to have when
electrons are distributed according
to certain rules
Rules for Assigning Oxidation
Numbers
1. The oxidation number of any uncombined
element i.e. an element that is not bonded to
another element is zero.
e.g.
Na, Zn, Fe, O2, Cl2, H2 etc.
2. The oxidation number of an ion of an
element is the same as its charge
Fe2+
oxidation number is +2
NaCl oxidation number of Na+ is +1 and
oxidation number of Cl- is -1
3. The sum of the oxidation numbers of all
the elements in a compound must add up
to zero
NaCl
(+1)(-1)
+1-1=0
H 2O
2(+1)(-2)
+2-2=0
4.
Oxygen always has an oxidation number
of -2
except
in peroxides
e.g. H2O2
O.N. of -1
in the compound OF2…….. O.N. of +2
5.
Hydrogen is always assigned an
oxidation number of +1
except in a metal hydride
e.g. Na+H- ………..O.N. of -1
6. The Halogens have an oxidation number
of -1 in their binary compounds unless
bonded to a more electronegative atom
e.g. in NaCl …………
(+1)(-1)
O.N. of Cl is -1
in Cl2O …………
2(+1)(-2)
O.N. of Cl is +1
in Cl2O7 …………
2(+7)7(-2)
O.N. of Cl is +7
7. The sum of the oxidation numbers of all
the elements in a complex ion
e.g.
NO3(+5) 3(-2)
SO42(+6) 4(-2)
must equal the charge on the ion
Using oxidation numbers to indicate the
species oxidised and reduced in a reaction
inc. in oxidation number
oxidation
2H2
(0)
+
O2
2H2O
(0)
(+1) (-2)
reduction
dec. in oxidation number
 Look at examples 14.2 – 14.7 pages 180 –
183
 Try questions 14.3 – 14.7 page 190
Oxidation Numbers
 Oxidation is an increase in oxidation
numbers
 Reduction is a decrease in oxidation
numbers