Oxidation Numbers
Oxidation numbers
• Know what oxidation numbers are
• Understand oxidation and reduction in terms of
electron transfer and changes in oxidation number
• Calculate oxidation numbers of elements in
compounds and ions
• Know redox reactions are
• Use Roman numerals to indicate the oxidation
number of an elements in a compound or ion
• To be able to write ionic half equations
• Combine ionic half equations to construct full ionic
equations
Oxidation numbers
• Also known as oxidation state, we use
these numbers to measure the number of
electrons that an atom is using to bond to
another atom.
• We use a set of rules to calculate the
oxidation numbers of elements in
compounds and ions.
Oxidation numbers
Species
Oxidation number
Examples
Uncombined element
0
C, Na, O2
Combined oxygen (except
in peroxides O22- and
compounds with fluorine,
where it is +2)
-2
H2O, CaO
Combined hydrogen (except
metal hydrides, H-)
+1
NH3, H3S
Simple ion
Charge on ion
Na+ = 1
Mg2+ = 2
Chlorine (except in
compounds with oxygen
and fluorine)
-1
HCl
Combined fluorine
-1
NaF, CaF, AlF3
Oxidation States
Deduce the ox state of nitrogen in HNO3
HNO3
+1
x
3(-2)
Sum = (+1) + x + 3(-2) = 0 (neutral
compound)
+1 + x
- 6
= 0
x = 5
nitric(V) acid
Oxidation numbers
• The overall charge on a compound such
as SO2 is 0.
• The sum of the oxidation numbers in the
compound must equal the charge.
• Look at the table from before, which
element do we know the O.N. of?
• Oxygen = -2
• There are two of them.
Oxidation numbers
Oxygen = -2
There are two of them.
Total contribution of oxygen is:
-4
Overall the O.N. must sum to 0, so what is
the O.N. of sulphur?
• +4
•
•
•
•
•
Oxidation numbers
Molecular ions
• In an ion such as CO32-, the overall sum of
the O.N.s must equal the charge.
• Work out the O.N.s of oxygen and carbon
in CO32• O = -2
• C = +4
Oxidation States
Deduce the ox state of manganese in MnO42–
MnO42–
4(-2)
x
Sum = x + 4(-2) = -2
x
- 8
(overall charge on ion = -2 )
= -2
x = + 6
manganate(VI)
this is a green ion
different from the purple MnO4– manganate(VII)
Oxidation States
Deduce the ox state of chromium in Cr2O72–
Cr2O72–
2x
7(-2)
Sum = 2x + 7(-2) = -2 (overall -2 ion)
2x - 14 = -2
2x = +12
x = + 6
dichromate(VI)
orange
Oxidation numbers
Chemical names:
• In a compound such as Iron(III) chloride,
the Roman numeral tells you the oxidation
state of the iron.
E.g.
• FeCl2 Iron(II) chloride Fe: O.N. = +2
• FeCl3 Iron(III) chloride Fe: O.N. = +3
Oxidation numbers
Oxyanions:
• These are negative ions containing an
element and oxygen: sulphate, nitrate,
carbonate.
• Oxidation numbers are used again to
disambiguate similar oxyanions:
E.g.
• NO2Nitrate(III)
N: O.N. = +3
• NO3Nitrate(V)
N: O.N. = +5
Now complete the questions
Answer on white boards
Page 3 redox starters for 10
Oxidation numbers
• Know what oxidation numbers are
• Understand oxidation and reduction in terms of
electron transfer and changes in oxidation number
• Calculate oxidation numbers of elements in
compounds and ions
• Use Roman numerals to indicate the oxidation
number of an elements in a compound or ion
• To be able to write ionic half equations
• Combine ionic half equations to construct full ionic
equations
Definitions
OIL RIG
Oxidation is loss of electrons
An increase in oxidation number
Reduction is gain of electrons
A decrease in oxidation number
Definitions
Redox
A reaction in which oxidation and reduction
both occur is known as a redox reaction.
Electron transfer
• This is a redox equation:
Mg + Cl2  MgCl2
• But this fact is concealed in the equation.
• Instead we can write two half equations:
Mg
 Mg2+ + 2eCl2 + 2e-  2Cl• But which is oxidation and which
reduction?
Electron transfer
• Using oxidation numbers:
Mg + Cl2  MgCl2
Mg: 0
 +2
0 
-1
Cl:
Cl:
0 -1
Using oxidation numbers
Work out what has been reduced and what
has been oxidised in the following reaction:
MnO2(s) + 4HCl(aq)  MnCl2(aq) + 2H2O(l) + Cl2(g)
Now complete the questions on white
boards
Page 4 starter for 10
Redox eq
Combining half equations
Mg
 Mg2+ + 2eCl2 + 2e-  2ClHow would you combine these to write an
overall ionic equation?
Displacement of halogens
If a halogen is added to a solution of a compound containing a
less reactive halogen, it will react with the compound and form
a new one.
This is called displacement.
fluorine +
F2
sodium
chloride
+ 2NaCl

sodium
fluoride
+ chlorine

2NaF
+
Cl2
A more reactive halogen will always displace a less reactive
halide from its compounds in solution.
Halogen displacement reactions
Halogen displacement reactions are redox reactions.
Cl2 + 2KBr  2KCl + Br2
Write two half equations two show the electron transfer
involved
Cl2 + 2e-  2Cl-
2Br-  Br2 + 2e-
What has been oxidized and what has been reduced?

Chlorine has gained electrons, so it is reduced to Cl- ions.

Bromide ions have lost electrons, so they have been
oxidized to bromine.
Halogen displacement reactions
Halogen displacement reactions are redox reactions.
Cl2 + 2KBr  2KCl + Br2
Write two half equations two show the electron transfer
involved
Cl2 + 2e-  2Cl-
2Br-  Br2 + 2e-
Can you combine these to write an overall ionic equation?
Challenge work
Redox equations exam questions
Complete worksheet
Combining half equations to write full
equations
Page 5 starter for 10
Summary Exercise
Now, on your own, without notes:
1. Work out the O.N. of:
a. Sulphur in SO32b. Nitrogen in NH4+
c. Carbon in CO32-
2. Are the underlined atoms oxidised or reduced:
a. Mg(s) + Cl2(g)  MgCl2(s)
b. 2S2O32- + I2(g)  2I- + S4O62c. 2Fe3+ + 2I-  2Fe2+ + I2
Exercise -answers
Now, on your own, without notes:
1. Work out the O.N. of:
a. Sulphur in SO32b. Nitrogen in NH4+
c. Carbon in CO32-
+4
-3
+4
2. Are the underlined atoms oxidised or reduced:
a. Mg(s) + Cl2(g)  MgCl2(s)
b. 2S2O32- + I2(g)  2I- + S4O62c. 2Fe3+ + 2I-  2Fe2+ + I2
ox (0+2)
ox (+2+2.5)
red (+3+2)
Oxidation numbers
• Know what oxidation numbers are
• Understand oxidation and reduction in terms of
electron transfer and changes in oxidation number
• Calculate oxidation numbers of elements in
compounds and ions
• Know redox reactions are
• Use Roman numerals to indicate the oxidation
number of an elements in a compound or ion
• To be able to write ionic half equations
• Combine ionic half equations to construct full ionic
equations
Oxidation numbers
• Know what oxidation numbers are
• Understand oxidation and reduction in terms of
electron transfer and changes in oxidation number
• Calculate oxidation numbers of elements in
compounds and ions
• Know redox reactions are
• Use Roman numerals to indicate the oxidation
number of an elements in a compound or ion
• To be able to write ionic half equations
• Combine ionic half equations to construct full ionic
equations