Oxidation StatesDr. Sobers’ Lecture Slides

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.

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Assigning Oxidation States

See the handout for the list of rules.

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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Recognizing Oxidations and Reductions

Use the Oxidation Numbers

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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Oxidation and ReductionCuCl2(aq) + Mg(s) → MgCl2(aq) + Cu(s)

Copper is being reduced

0 -1+2 +1 0

Magnesium is being oxidized

Decrease in oxidation state

Increase in oxidation state

-1+2

Copper is the oxidizing agent

Magnesium is the reducing agent

It is oxidizing magnesium

It is reducing copper27