Oxidation Number
This is an indication of the number of electrons over which the atom
in question has lost or gained control
It is the key to Redox. The rules are
1. The oxidation number of an atom in a pure element = 0
2. The oxidation number of a simple ion = the charge on the ion
3. The sum of all the oxidation numbers in a compound particle = 0
4. The sum of all the oxidation numbers in a molecular ion = the
charge on the ion
5. In a compound, the more electronegative atom has the negative
oxidation number
The following elements very often have a certain oxidation number
Element
Li, Na, K
Mg, Ca, Sr, Ba
Fluorine
*!* Oxygen *!*
Hydrogen
Oxidation number in
compounds
Species
Oxidation number
Cu
Cl2
Cu in CuO
Fe3+
S in SO2
S in SO32Cl in ClOBr in BrO3Cr in Cr2O72Mn in MnO4S in H2SO4
N in KNO3
P in PCl3
O in H2O2
S in S4O62Look at the half-equations you were doing last week. Put in oxidation
number below each chemical and see if you work out a connection between
change in oxidation number and redox
!!! Balancing numbers have nothing to do with oxidation number !!!
Oxidation is
Reduction is
1. Write down the oxidation number of the following
(a)
I in IO3-
(b)
N in N2O5
(c)
Mn in MnO2
(d)
C in the carbonate ion
2. Use oxidation numbers to explain whether the following halfequations are reduction or oxidation
a. Mn2+  MnO4-
b. 2I-  I2 + 2e-
c. Cr2O72-  2Cr3+
3. Which is a better reducing agent: sodium or potassium? Explain
your answer
4. During one process for the manufacture of iodine the following
reaction occurs:
2IO3- + 5SO2 + 4H2O  I2 + 8H+ + 5SO42(a) Deduce the oxidation number of sulphur in
SO2
SO42-
(b) Use your answer to (a) to explain whether SO2 has been oxidised or
reduced in the above reaction
(c) Explain, with reference to oxidation numbers, whether I has been
reduced or oxidised in the reaction
(d) How could you prove iodine was produced in this reaction?
5. The sulphite ion, SO32-, is a good reducing agent as it can easily
change to sulphate. It can convert the Cr2O72- ion to Cr3+.
(a) By considering oxidation numbers, explain what is being reduced in
the reaction and what is being oxidised
(b) What is the reducing agent in this reaction?
6. In the following reaction
5KBr + KBrO3 + 3H2SO4  3Br2 + 3K2SO4 + 3H2O
(a)
Give the oxidation numbers of bromine in
(i)
KBr
(ii)
KBrO3
(iii)
Br2
(b)
Which substance is the oxidising agent in the reaction? Give a
reason for your choice
7. Which is a better oxidising agent: chlorine or iodine? Explain your
answer. Give an example of a reaction that show this
8. Chlorine reacts with sodium hydroxide to give sodium chloride,
sodium chlorate (I) (NaOCl) and water
(a)
Write an equation for this
(b)
Write the oxidation number for the chlorine underneath
(c)
Why is NaOCl called sodium chlorate (I)?
(d)
Write the formula for sodium chlorate (V)
DISPROPORTIONATION is