Oxidation States
Dr. Sobers’ Lecture Slides
The Oxidation State
Also known as the oxidation number
The oxidation state is used to determine whether an
element has been oxidized or reduced. The oxidation
state is not always a real, quantitative, physical constant.
The oxidation state can be the charge on an atom:
MgCl2
Oxidation State:
Mg2+
Cl-
+2
-1
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The Oxidation State
For covalently bonded substances, it is not as simple
as an ionic charge.
A covalent bond is a sharing of electrons. The electrons
are associated with more than one atomic nuclei. This
holds the nuclei together.
The electrons may not be equally shared. This creates
a polar bond.
The electronegativity of a covalently bonded atom is
its ability to attract electrons towards itself.
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Example: Chlorine
Sodium chloride is an ionic compound. In sodium
chloride, the chloride ion has a charge and an oxidation
state of -1. The oxidation state of sodium is +1.
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Example: Chlorine
In a chlorine molecule, the chlorine atoms are
covalently bonded. The two atoms share electrons
equally and the oxidation state is 0.
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Example: Chlorine
The two atoms of a hydrogen chloride molecule are covalently
bonded. The electrons are not shared equally because chlorine is
more electronegative than hydrogen.
There are no ions but the oxidation state of chlorine in HCl is -1 and
the oxidation state of hydrogen is +1.
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Assigning Oxidation
States
See the handout for the list of rules.
Rule 1: Free Elements
Free elements have an oxidation state of zero
Example
Oxidation State
O2(g)
0
Fe(s)
0
O3(g)
0
C(graphite)
0
C(diamond)
0
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Rule 2: Monatomic Ions
The oxidation state of monatomic ions is the charge of the ion
Example
Oxidation State
O2-
-2
Fe3+
+3
Na+
+1
I-
-1
V4+
+4
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Rule 3: Fluorine in Compounds
Fluorine in a compound always has an oxidation state of -1
Example
NaF
Comments and Oxidation States
These are monatomic ions. Rule 2 applies
F is -1
CF4
Na is +1
This is a molecular compound.
F is -1 See rule 7 for C
Fluorine is the most electronegative element
so it is -1 when bonded to other atoms.
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Rule 4: Hydrogen in Compounds
Hydrogen in a compound usually has an oxidation state of +1
Example
CH4
Comments and Oxidation States
A molecular compound. Carbon is more
electronegative.
H is +1
NaH
See rule 7 for C
This is an ionic compound - see rule 2
Na is +1
The hydride ion is -1
An exception to rule 4 is the hydride ion.
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Rule 5: Oxygen in Compounds
Oxygen in a compound usually has an oxidation state of -2
Example
MgO
Comments and Oxidation States
These are monatomic ions. See rule 2
Mg is +2
H2O
O is -2
A molecular compound - oxygen is more
electronegative
O is -2
H is +1 (rule 4)
But why is covalently bonded oxygen -2 and not -1?
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Rule 5: Oxygen in Compounds
Oxygen in a compound usually has an oxidation state of -2
But why is covalently bonded oxygen -2 and not -1?
After all, fluorine is more electronegative but only -1 in
compounds.
Oxygen usually makes two bonds and fluorine makes one.
The electronegative oxygen pulls electron density from two
atoms.
Compound:
H-F
H-O-H
Oxidation State: +1 -1
+1 -2 +1
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Rule 5: Oxygen in Compounds
Oxygen in a compound usually has an oxidation state of -2
Example
F2O
Comments and Oxidation States
Fluorine is more electronegative than
oxygen so electron density is pulled away
from oxygen.
F-O-F
-1 +2 -1
Exception to rule 5: oxygen bonded to fluorine.
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Rule 5: Oxygen in Compounds
Oxygen in a compound usually has an oxidation state of -2
Example
H2O2
Comments and Oxidation States
In hydrogen peroxide, each oxygen atom
is bonded to another oxygen and to one
less electronegative element.
H-O-O-H
+1 -1 -1 +1
Each oxygen has only one bond to a less electronegative atom
Exception to rule 5: oxygen in a peroxide is -1.
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Rule 6: Halogens other than
fluorine in Compounds
Chlorine, bromine and iodine in a compound usually
have an oxidation state of -1
Example
NaCl
Comments and Oxidation States
These are monatomic ions. See rule 2
Na is +1
CBr4
Cl is -1
This is a molecular compound and
bromine is more electronegative.
Br is -1
See rule 7 for C
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Rule 6: Halogens other than
fluorine in Compounds
Chlorine, bromine and iodine in a compound usually
have an oxidation state of -1
Example
FCl
Comments and Oxidation States
Because fluorine is more electronegative, it is -1
F is -1
ClBr
Cl is +1
Because chlorine is more electronegative, it is -1
Cl is -1
Br is +1
Exception to rule 6: when bonded to a more electronegative
element.
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Rule 7: The Sum of the Oxidation
States in a formula
The sum of the oxidation states in a formula is equal to
the charge on the formula.
Example
Comments and Oxidation States
CH4
By rule 4, hydrogen is +1
Assign the variable x to carbon
Create an algebraic equation
x + 4(+1) = 0
4 hydrogens, each +1
x = -4
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Rule 7: The Sum of the Oxidation
States in a formula
The sum of the oxidation states in a formula is equal to
the charge on the formula.
Example
Comments and Oxidation States
C3H8
By rule 4, hydrogen is +1
Average oxidation state of C = x
3x + 8(+1) = 0
3x = -8
x = -3/8
The average for carbon is -3/8
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Rule 7: The Sum of the Oxidation
States in a formula
The sum of the oxidation states in a formula is equal to
the charge on the formula.
Example
Comments and Oxidation States
MnO4-
By rule 5, oxygen is -2
Oxidation state of Mn = x
x + 4(-2) = -1
charge on formula = -1
x = +7
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Rule 7: The Sum of the Oxidation
States in a formula
The sum of the oxidation states in a formula is equal to
the charge on the formula.
Example
KMnO4
+1 +7 -2
Comments and Oxidation States
Try last one again but with a cation
K+
MnO4-
Monatomic ion, potassium, is +1
By rule 5, oxygen is -2
Oxidation state of Mn = x
+1 + x + 4(-2) = 0
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x = +7
Rule 7: The Sum of the Oxidation
States in a formula
The sum of the oxidation states in a formula is equal to
the charge on the formula.
Example
ClO2-
Comments and Oxidation States
By rule 5, oxygen is +2
Oxidation state of Cl = x
x + 2(-2) = -1
charge on formula = -1
x = +3
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Lower Number Rules Take
Precedence
The rules are presented so that lower numbered rules have
priority.
If an exception is forgotten or not realized, then the order
of the rules can help.
In FCl the rule about fluorine being -1 in a compound
takes precedence over the rule about other halogens
usually being -1 in a compound.
In H2O2 the sum must be zero. The rule for hydrogen
takes precedence over the rule for oxygen so the
oxidation state of -1 is calculated.
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Recognizing Oxidations
and Reductions
Use the Oxidation Numbers
Oxidation and Reduction
C3H8(g) + O2(g)
-3/8 +1
0
→ CO2(g)
+4 -2
+ H2O(g)
+1 -2
Carbon is being oxidized
Increase in oxidation state
Oxygen is being reduced
Decrease in oxidation state
Both carbon and oxygen are more electronegative than
hydrogen. Oxygen is more electronegative than carbon
but hydrogen is +1 when bonded to either. The
oxidation state is not quantitatively meaningful but
allows for recognition of oxidation and reduction.
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Oxidation and Reduction
CuCl2(aq) + Mg(s)
0
+2 -1
→
MgCl2(aq)
+2 -1
+ Cu(s)
+1 0
Copper is being reduced
Decrease in oxidation state
Magnesium is being oxidized
Increase in oxidation state
Copper is the oxidizing agent
It is oxidizing magnesium
Magnesium is the reducing agent
It is reducing copper
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