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The Oxidation StateAlso 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:
Mg2+MgCl2 Cl-
Oxidation State: +2 -1
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The Oxidation StateFor 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.
Rule 1: Free ElementsFree elements have an oxidation state of zero
Example Oxidation State
O2(g) 0
Fe(s) 0
O3(g) 0
C(graphite)
C(diamond)
0
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Rule 2: Monatomic IonsThe oxidation state of monatomic ions is the charge of the ion
Example Oxidation State
O2- -2
Fe3+ +3
Na+ +1
I-
V4+
-1
+410
Rule 3: Fluorine in CompoundsFluorine in a compound always has an oxidation state of -1
Example Comments and Oxidation States
NaF
Na is +1F is -1
These are monatomic ions. Rule 2 applies
CF4
F is -1 See rule 7 for CThis is a molecular compound.
Fluorine is the most electronegative element so it is -1 when bonded to other atoms.
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Rule 4: Hydrogen in CompoundsHydrogen in a compound usually has an oxidation state of +1
Example Comments and Oxidation States
CH4
H is +1 See rule 7 for C
A molecular compound. Carbon is more electronegative.
NaHNa is +1 The hydride ion is -1This is an ionic compound - see rule 2
An exception to rule 4 is the hydride ion.12
Rule 5: Oxygen in CompoundsOxygen in a compound usually has an oxidation state of -2
Example Comments and Oxidation States
MgO
Mg is +2 O is -2
These are monatomic ions. See rule 2
H2O
O is -2 H is +1 (rule 4)
A molecular compound - oxygen is more electronegative
But why is covalently bonded oxygen -2 and not -1?13
Rule 5: Oxygen in CompoundsOxygen in a compound usually has an oxidation state of -2
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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.
H-O-HH-F+1 +1+1 -1 -2
Compound:Oxidation State:
But why is covalently bonded oxygen -2 and not -1?
Rule 5: Oxygen in CompoundsOxygen in a compound usually has an oxidation state of -2
Example Comments and Oxidation States
F2O Fluorine is more electronegative than oxygen so electron density is pulled away from oxygen.
Exception to rule 5: oxygen bonded to fluorine.
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F-O-F-1 -1+2
Rule 5: Oxygen in CompoundsOxygen in a compound usually has an oxidation state of -2
Example Comments and Oxidation States
H2O2 In hydrogen peroxide, each oxygen atom is bonded to another oxygen and to one less electronegative element.
Exception to rule 5: oxygen in a peroxide is -1.16
H - O - O - H+1 -1 +1-1
Each oxygen has only one bond to a less electronegative atom
Rule 6: Halogens other than fluorine in Compounds
Chlorine, bromine and iodine in a compound usually have an oxidation state of -1
Example Comments and Oxidation States
NaCl
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Na is +1 Cl is -1
These are monatomic ions. See rule 2
CBr4
Br is -1 See rule 7 for C
This is a molecular compound and bromine is more electronegative.
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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 Comments and Oxidation States
FCl
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F is -1 Cl is +1
Because fluorine is more electronegative, it is -1
ClBr
Cl is -1
Because chlorine is more electronegative, it 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 +1Assign the variable x to carbonCreate an algebraic equation
x + 4(+1) = 0
x = -4
4 hydrogens, each +1
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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 +1Average oxidation state of C = x
3x + 8(+1) = 03x = -8x = -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 -2Oxidation state of Mn = x
x + 4(-2) = -1x = +7
charge on formula = -1
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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
KMnO4
By rule 5, oxygen is -2Oxidation state of Mn = x+1 + x + 4(-2) = 0 x = +7
Try last one again but with a cation
Monatomic ion, potassium, is +1
K+ MnO4-+1 -2+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 Comments and Oxidation States
ClO2- By rule 5, oxygen is +2
Oxidation state of Cl = x
x + 2(-2) = -1
x = +3
charge on formula = -1
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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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Oxidation and ReductionC3H8(g) + O2(g) → CO2(g) + H2O(g)
Carbon is being oxidized
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.
+1-3/8 0 -2+4 +1 -2
Oxygen is being reduced
Increase in oxidation state
Decrease in oxidation state
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