Oxidation Numbers
An oxidation number is the charge an atom in a compound would have if the
electron pairs in the bonds belonged entirely to the more electronegative
atoms.
rules …
1)
2)
3)
4)
5)
6)
7)
the
the
the
the
the
the
the
oxidation number of hydrogen will be +1 (can be -1 if a hydride)
oxidation number of oxygen will be -2 (can be -1 if a peroxide)
oxidation numbers of group 1, 2 and 3 metals will be +1, +2 and +3
oxidation number of elements is 0
oxidation number of monoatomic ion is equal to the charge on the ion
oxidation number of neutral compounds must add to equal 0
oxidation number of polyatomic ions must add to equal the charge
oxidation number (+)
1
2
1
1
2
2
2
2
2
2
2
2
2
3
4
5
6
names of elements
copper – cuprum
mercury
iron – ferrum
cobalt
nickel
lead – plumbum
tin – stannum
manganese
chromium
antimony
positive ions
NH4+1
3
3
3
4
4
4
3
3
negative ions
OH-1
NO3-1
SO4-2
PO4-3
CO3-2
ClO3-1
6
5
Using Oxidation to Balance Redox Equations
Redox reactions involve the loss and gain of electrons.
Reduction is the gain of electrons.
Oxidation is the loss of electrons.
OIL
(oxidation is loss) RIG
(reduction is gain)
In redox reactions, one element must be reduced while, at the same time,
another is oxidized.
The element being reduced is allowing the other element to be oxidized and so
it is called the oxidizing agent. The element being oxidized is allowing the other
element to be reduced and so is called the reducing agent.
2 Na + Cl2  2 NaCl
oxidation number
0
0
+1 and -1
reduced
ox agent
no
no
yes
yes
oxidized
red agent
yes
yes
no
no
rules
1)
2)
3)
4)
5)
determine the two elements whose ox #’s have changed
temporarily balance only those elements that have changed
calculate the total increase and decrease in ox #’s
make the magnitude of the increase (number not sign) equal the
magnitude of the decrease
balance the rest of the equation as you would any other equation
Balancing Equations Using Oxidation Numbers
1.
Mg + HCl  MgCl2 + H2
2.
Fe + V2O3  Fe2O3 + VO
3.
KMnO4 + KNO2 + H2SO4  MnSO4 + H2O + KNO3 + K2SO4
4.
K2Cr2O7 + SnCl2 + HCl  CrCl3 + SnCl4 + KCl + H2O
5.
KMnO4 + NaCl + H2SO4  Cl2 + K2SO4 + MnSO4 + H2O + Na2SO4
6.
K2Cr2O7 + H2O + S  SO2 + KOH + Cr2O3
Electrochemical Oxidation Numbers Worksheet
1. Assign oxidation numbers to each atom in NiCl2, Mg2TiO4, K2Cr2O7, SO32-.
2. Assign oxidation numbers to the underlined atoms. (a) ClO4-, (b) CrCl3, (c)
SnS2, (d) Au(NO3)3.
3. Assign oxidation numbers to the elements in the following: (a) Ca(VO3)2,
(b) SnCl4, (c) MnO4-, (d) MnO2.
4. Supply the oxidation number of the underlined element in the following
formulas;
a) Zn3(PO4)2, b) NaNO2, c) SnBr2, d) HSbO2,
e) Mg(MnO4)2, f) NH4NO3