Oxidation-Reduction Reactions Oxidation Numbers Oxidation Numbers and Nomenclature Identifying Oxidation-Reduction Reactions Writing Equations for Oxidation-Reduction

Oxidation-Reduction ReactionsOxidation-Reduction Reactions

Oxidation NumbersOxidation Numbers

Oxidation Numbers and NomenclatureOxidation Numbers and Nomenclature

Identifying Oxidation-Reduction ReactionsIdentifying Oxidation-Reduction Reactions

Writing Equations for Oxidation-Reduction Writing Equations for Oxidation-Reduction ReactionsReactions

Oxidation-Reduction TitrationsOxidation-Reduction Titrations

Oxidation by OxygenOxidation by Oxygen

Oxidation NumbersOxidation Numbers

Definition:Definition:

An oxidation number is a number that An oxidation number is a number that reflects the electrons gained, lost, or reflects the electrons gained, lost, or shared when an element reacts.shared when an element reacts.

Remember that when you lose electrons Remember that when you lose electrons you become more positive and when you you become more positive and when you gain electrons you become more negative.gain electrons you become more negative.

Rules for Assigning Oxidation Rules for Assigning Oxidation NumbersNumbers

1.1. The oxidation # of any uncombined element is zero.The oxidation # of any uncombined element is zero.2.2. The oxidation # of a monoatomic ion equals its’ charge.The oxidation # of a monoatomic ion equals its’ charge.3.3. The more electrongative element in a binary compound is The more electrongative element in a binary compound is

assigned the number equal to the charge it would have if it assigned the number equal to the charge it would have if it were an ion.were an ion.

4.4. The oxidation number of fluorine is always -1.The oxidation number of fluorine is always -1.5.5. Oxygen has an oxidation number of -2.Oxygen has an oxidation number of -2.6.6. Hydrogen has an oxidation number of +1.Hydrogen has an oxidation number of +1.7.7. In compounds, Group 1 and 2 elements and aluminum In compounds, Group 1 and 2 elements and aluminum

have oxidation numbers of +1, +2, and +3 respectively.have oxidation numbers of +1, +2, and +3 respectively.8.8. The sum of the oxidation numbers of atoms in a The sum of the oxidation numbers of atoms in a

compound is zero.compound is zero.9.9. The sum of the oxidation # in a polyattomic ion is equal to The sum of the oxidation # in a polyattomic ion is equal to

the charge of the ion.the charge of the ion.

Oxidation number or stateOxidation number or state

When dealing with simple ions, this is easy When dealing with simple ions, this is easy to determine. It is simply the charge on to determine. It is simply the charge on the ion.the ion.

ExamplesExamples Group IA (1)Group IA (1) +1+1 Group IIA (2)Group IIA (2) +2+2 Group VIIA (17)Group VIIA (17) -1 -1

OxygenOxygen -2 usually -2 usually HydrogenHydrogen +1 if bonded to +1 if bonded to

nonmetalnonmetal HydrogenHydrogen -1 if bonded to metal-1 if bonded to metal

Oxidation numberOxidation number

For elements in their elemental state, the For elements in their elemental state, the oxidation number is also pretty straightforward.oxidation number is also pretty straightforward.

Since all of the atoms are the same, the Since all of the atoms are the same, the electrons are shared equally so the oxidation electrons are shared equally so the oxidation number is zero.number is zero.

ExamplesExamples

The atoms in NThe atoms in N22, Na, P, Na, P44, H, H22 and O and O22 all all have oxidation numbers of zero.have oxidation numbers of zero.

Oxidation numbersOxidation numbers

With polar covalent bonds electrons are With polar covalent bonds electrons are shared but not equally.shared but not equally.

For electrons that are shared in these For electrons that are shared in these compounds, we assign the shared electrons compounds, we assign the shared electrons to the most electronegative element.to the most electronegative element.

We are just acting as though the We are just acting as though the electronegativity difference was large electronegativity difference was large enough for the transfer of electrons to occur.enough for the transfer of electrons to occur.

ExampleExampleAssign the oxidation states for all elements Assign the oxidation states for all elements in water. in water.

The electronegativities are: The electronegativities are:

H = 2.2, O = 3.5H = 2.2, O = 3.5

The electrons from both hydrogen are The electrons from both hydrogen are assigned to the oxygen.assigned to the oxygen.

Oxidation numbers:Oxidation numbers: O = -2O = -2

H = +1H = +1

Oxidation numbersOxidation numbersMany elements have more than one Many elements have more than one possible oxidation number.possible oxidation number.

Often, it is possible to determine the Often, it is possible to determine the oxidation number of those elements in a oxidation number of those elements in a compound simply by looking at what you do compound simply by looking at what you do know.know.

Follow the previous rules and then assign Follow the previous rules and then assign an oxidation number that insures that the an oxidation number that insures that the overall compound has no net charge.overall compound has no net charge.

ExampleExample

Find the oxidation state for all elements in:Find the oxidation state for all elements in:

HNO3HNO3

Hydrogen we know - it must be +11

Oxygen should be -2 2 in this case.

What about nitrogen?

ExampleExample

See what you do know and find the See what you do know and find the difference.difference.

HNOHNO33

Oxygen is -2and we have 3of them = -6

Oxygen is -2and we have 3of them = -6

We know that His assigned avalue of +1

We know that His assigned avalue of +1

OK, so what's left over??

(+1) + (-6) + charge on nitrogen = 0

Nitrogen must have a value of +5

Determining Oxidation NumbersDetermining Oxidation Numbers

SOSO22 NONO33--

PClPCl55 SOSO442-2-

HH22OO NHNH44++

HH22SOSO33 MnOMnO44--

Tl+3+1

Hg+2+1

Au+3+1

Hf+4

Lu+3

Li+1

Na+1

Cs+1

Rb+1

K+1

Fr+1

Pt+4+2

Ir+4+3

Os+8+6

Re+7+6+4

W+6+4

Ta+5

H+1

He

RnAt-1

Po+2

Bi+5+3

Pb+4+2

Cd+2

Ag+1

Zr+4

Y+3

Pd+4+2

Rh+4+3+2

Ru+8 +6

+4+3

Tc+7+6+4

Mo+6+4+3

Nb+5+4

Lr+3

Ba+2

Be+2

Mg+2

Sr+2

Ca+2

Ra+2

Zn+2

Cu+2+1

Ti+4+3+2

Sc3+

Ni+2

Co+3+2

Fe+3+2

Mn+7 +6+4 +3

+2

Cr+6+3+2

V+5 +4

+3+2

In+3

Xe+6+4+2

I+7 +5

+1-1

Te+6+4-2

Sb+5+3-3

Sn+4+2

Ga+3

Kr+4+2

Br+5+1-1

Se6+4+2-

As5+3+3-

Ge+4-4

Al+3

ArCl

+7 +5+3 +1

-1

S+6 +4

+2-2

P+5+3-3

Si+4-4

B+3

NeF-1

O-1-2

N+5 +4+3 +2+1 -3

C+4-2-4

Common oxidation numbersCommon oxidation numbers

Oxidation numbers and Oxidation numbers and the periodic tablethe periodic table

Some observed trends in compounds.Some observed trends in compounds.

Metals have positive oxidation numbers.Metals have positive oxidation numbers.

Transition metals typically have more than one Transition metals typically have more than one oxidation number.oxidation number.

Nonmetals and semimetals have both positive and Nonmetals and semimetals have both positive and negative oxidation numbers.negative oxidation numbers.

No element exists in a compound with an oxidation No element exists in a compound with an oxidation number greater than +8.number greater than +8.

The most negative oxidation numbers equals 8 - the The most negative oxidation numbers equals 8 - the group numbergroup number

Oxidation number and nomenclatureOxidation number and nomenclature

Stock systemStock system

For metals with several possible oxidation For metals with several possible oxidation numbers, use Roman numeral in the name.numbers, use Roman numeral in the name.

FeSOFeSO44 iron(II) sulfateiron(II) sulfate

FeFe22(SO(SO44))3 3 iron (III) sulfateiron (III) sulfate

CuCu22OO copper(I) oxidecopper(I) oxide

CuOCuO copper(II) oxidecopper(II) oxide

PbClPbCl22 lead(II) chloridelead(II) chloride

PbClPbCl44 lead(IV) chloridelead(IV) chloride

Identifying oxidation-reduction Identifying oxidation-reduction reactions.reactions.

Oxidation-Reduction - REDOXOxidation-Reduction - REDOX

A chemical reaction where there is a net change in the A chemical reaction where there is a net change in the oxidation number of one or more species.oxidation number of one or more species.

Both an oxidation and a reduction must occur during Both an oxidation and a reduction must occur during the reaction.the reaction.

Mg (s) + Cl2 (g) MgCl2 (s)

Here the oxidation number of Mg has changed fromzero to +2. Cl has changed from zero to -1.

Here the oxidation number of Mg has changed fromzero to +2. Cl has changed from zero to -1.

REDOX reactionsREDOX reactionsOxidationOxidation

An increase in oxidation number.An increase in oxidation number.

ReductionReduction

A decrease in oxidation number.A decrease in oxidation number.

If the oxidation number of any element If the oxidation number of any element changes in the course of a reaction, the changes in the course of a reaction, the reaction is oxidation-reduction.reaction is oxidation-reduction.

Example.Example.2 Fe(NO2 Fe(NO33))3 3 (aq)(aq) + Zn + Zn(s) (s) 2 Fe(NO 2 Fe(NO33))2 2 (aq)(aq) + +

Zn(NOZn(NO33))2 2 (aq)(aq)

Redox ReactionRedox Reaction

Fe(s) + OFe(s) + O22(g) (g) Fe Fe22OO33(s)(s)

Iron is oxidized (0 to +3)Iron is oxidized (0 to +3)

and Oand O2 2 is reduced (0 to -2)is reduced (0 to -2)

Half ReactionsHalf Reactions

Fe Fe Fe Fe3+3+ + 3e + 3e- - (oxidation)(oxidation)

OO22 + 4e + 4e-- 2 O 2 O2-2- (reduction) (reduction)

Half reactionsHalf reactionsExample.Example.

Half-reactions can be of the ‘net ionic’ form. Balance Half-reactions can be of the ‘net ionic’ form. Balance the followthe follow

FeFe3+3+ + Zn + Zn (s) (s) Fe Fe2+2+ + Zn + Zn2+2+

2 ( Fe2 ( Fe3+3+ + e + e-- Fe Fe2+2+)) (reduction)(reduction)

ZnZn(s)(s) Zn Zn2+2+ + 2e + 2e-- (oxidation)(oxidation)

2Fe2Fe3+3+ + Zn + Zn (s)(s) 2Fe 2Fe2+2+ + Zn + Zn2+2+

ExampleExample

2Fe(NO2Fe(NO33))3 3 (aq)(aq) + Zn + Zn(s)(s) 2Fe(NO 2Fe(NO33))22 (aq) (aq) Zn(NO Zn(NO33))2 2 (aq)(aq)

+3 0 +2 +2

Fe3+ is reduced to Fe2+

Zn is oxidized to Zn2+

NO3- is a spectator ion.

Fe3+ is reduced to Fe2+

Zn is oxidized to Zn2+

NO3- is a spectator ion.

Zinc - Copper Voltaic CellZinc - Copper Voltaic Cell

(s)2(aq)

2(aq)(s) CuCuZnZn

A Zinc-Copper Voltaic Cell

Zn electrode

Anode (-)

Oxidation

Half Reaction:

Zn Zn2+ + 2 e-

Cu electrode

Cathode (+)

Reduction

Half Reaction:

Cu2+ + 2 e- Cu

Voltaic Cell cont…Voltaic Cell cont…

In a voltaic cell the redox reaction is In a voltaic cell the redox reaction is spontaneous and produces an electric spontaneous and produces an electric current. This current can be measured current. This current can be measured using a voltmeter.using a voltmeter.

Standard Electrode Potentials

• A standard electrode potential, Eo, is based on the tendency for reduction to occur at the electrode.

• The cell voltage, called the standard cell potential (Eo

cell), is the difference between the standard potential of the cathode and that of the anode.

Eocell = Eo (cathode) – Eo (anode)

Standard Electrode PotentialsStandard Electrode Potentials

Balancing REDOX equationsBalancing REDOX equations

Many REDOX equations can be balanced by Many REDOX equations can be balanced by inspection.inspection.

HH22SS (g) (g) + H + H22OO22 (aq)(aq) S S (s) (s) + 2 H + 2 H22OO (l) (l)

However, others are more difficult.However, others are more difficult.

2KMnO2KMnO4 4 (aq)(aq) + H + H22OO2 2 (l)(l) + 3H + 3H22SOSO44 (aq) (aq)

2MnSO2MnSO44 (aq) (aq) + K + K22SOSO44 (aq) (aq) + 3O+ 3O22 (g) (g) + 4H + 4H22OO (l)(l)


Half-Reaction method.Half-Reaction method.

With this approach, the reaction is broken into two parts.With this approach, the reaction is broken into two parts.

Oxidation half-reaction.Oxidation half-reaction. The portion of the reaction The portion of the reaction where electrons are lost.where electrons are lost.

A AA An+n+ + ne + ne--

Reduction half-reaction.Reduction half-reaction. The portion of the reaction The portion of the reaction where electrons are gained.where electrons are gained.

meme-- + B B + B Bm-m-


The goal is then to make sure that the same number of The goal is then to make sure that the same number of electrons are being produced and consumed.electrons are being produced and consumed.

(m) ( A A(m) ( A An+n+ + ne + ne-- ) )

(n) (me(n) (me-- + B B + B Bm- m- ) )

nB + mA mAnB + mA mAn+n+ + nB + nBm+m+

When properly balanced, the electrons will cancel When properly balanced, the electrons will cancel out. out.

Half reactionsHalf reactions

Another ExampleAnother Example

Determine the balanced equation for the reaction Determine the balanced equation for the reaction of Feof Fe2+2+ with Cr with Cr22OO77

2-2- in an acidic solution. in an acidic solution.

FeFe2+2+ + Cr + Cr22OO772-2- Fe Fe3+3+ + Cr + Cr3+3+

The two half-reactions would be:The two half-reactions would be:

FeFe2+2+ FeFe3+3+

CrCr22OO772-2- CrCr3+3+

H+


First, balance each half-reaction for all elements except hydrogen First, balance each half-reaction for all elements except hydrogen and oxygen.and oxygen.

FeFe2+2+ FeFe3+3+

CrCr22OO772-2- 2Cr2Cr3+3+

Next, balance each half-reaction with respect to oxygen by adding Next, balance each half-reaction with respect to oxygen by adding an appropriate number of Han appropriate number of H22O. O.

FeFe2+2+ FeFe3+3+

CrCr22OO772-2- 2Cr2Cr3+3+ + 7H + 7H22OO


Remember that this reaction occurs in an acid solution Remember that this reaction occurs in an acid solution so we can add Hso we can add H++ as needed. as needed.

FeFe2+2+ FeFe3+3+

14H14H++ + Cr + Cr22OO772-2- 2Cr2Cr3+3+ + 7H + 7H22OO

Now we need to know how many electrons are produced Now we need to know how many electrons are produced or consumed and place them in our half-reactions.or consumed and place them in our half-reactions.

For iron, one eFor iron, one e-- is produced. is produced.

For dichromate, six eFor dichromate, six e-- are consumed. are consumed.

Half-reactionsHalf-reactions FeFe2+2+ Fe Fe3+3+ + e + e--

6e6e-- + 14 H + 14 H++ + Cr + Cr22OO772-2- 2Cr 2Cr3+3+ + 7H + 7H22OO

We need the same number of electrons produced and We need the same number of electrons produced and consumed so:consumed so:

6Fe6Fe2+2+ 6Fe 6Fe3+3+ + 6e + 6e--

6e6e-- + 14H + 14H++ + Cr + Cr22OO772-2- 2Cr 2Cr3+3+ + 7H + 7H22OO

As our final step, we need to combine the half-reactions and As our final step, we need to combine the half-reactions and cancel out the electrons.cancel out the electrons.

Half-reactionsHalf-reactions

6Fe6Fe2+2+ + 14H + 14H+ + + Cr+ Cr22OO772-2-

6Fe6Fe3+3+ + 2Cr + 2Cr3+3+ + 7H + 7H22OO

In this reaction, FeIn this reaction, Fe2+2+ is oxidized and the is oxidized and the dichromate ion is reduced.dichromate ion is reduced.

This reaction is used for the determination of iron This reaction is used for the determination of iron by titration.by titration.

Disproportionation reactionsDisproportionation reactions

In some reactions, the same species is both oxidized and In some reactions, the same species is both oxidized and reduced.reduced.

ExamplesExamples

2H2H22OO22 (l) (l) 2H 2H22OO (l) (l) + O + O22 (g) (g)

3Br3Br22 (aq) (aq) +6OH+6OH- - (aq)(aq) BrO BrO33--(aq) (aq) +5Br+5Br--(aq)(aq) +3H +3H22OO(l)(l)

For this to occur, the species must be in an For this to occur, the species must be in an intermediate oxidation state. Both a higher and lower intermediate oxidation state. Both a higher and lower oxidation state must exit. oxidation state must exit.

Oxidation-reduction titrationsOxidation-reduction titrations

REDOX reactions can also serve as the basis for REDOX reactions can also serve as the basis for titrations.titrations.

For example, we can determine the amount of iron For example, we can determine the amount of iron in an ore by titration.in an ore by titration.

Initially, we must dissolve the sample. This results Initially, we must dissolve the sample. This results in both iron(II) and iron(III) being produced in solution.in both iron(II) and iron(III) being produced in solution.

The first step is to get all of the iron into one The first step is to get all of the iron into one oxidation state.oxidation state.

Sample preparationSample preparation

One option is to use a reductor. You slowly wash your One option is to use a reductor. You slowly wash your sample through the column with water.sample through the column with water.

Jones ReductorJones Reductor

Zn(Hg)Zn(Hg) ZnZn2+2+ + Hg + Hg(l)(l) + 2e + 2e

An amalgam is used to prevent An amalgam is used to prevent

Zn + 2HZn + 2H++ ZnZn2+2+ + H + H22(g)(g)

For iron, we getFor iron, we get

FeFe2+2+ + Fe + Fe3+3+ Fe Fe2+2+

reductor

TitrantsTitrants

Now that all of our iron is in a single oxidation state, Now that all of our iron is in a single oxidation state, we’re ready to do a titration.we’re ready to do a titration.

We need an oxidizing agent to convert all the iron We need an oxidizing agent to convert all the iron from Fefrom Fe2+2+ to Fe to Fe3+3+..

Primary standardPrimary standard

A material that is available in pure form.A material that is available in pure form.

Ideally, it should be something that we can directly Ideally, it should be something that we can directly weigh out, dissolve and then titrate with.weigh out, dissolve and then titrate with.

Common titrantsCommon titrants

Oxidizing titrantsOxidizing titrants

Dichromate -Dichromate - CrCr22OO772-2-

The potassium salt is a primary standard The potassium salt is a primary standard material.material.

Very stable solutions. If air is kept out, it can last Very stable solutions. If air is kept out, it can last for years.for years.

It is a very strong oxidizing agent. It is a very strong oxidizing agent.

Need an indicator such as diphenylamine Need an indicator such as diphenylamine sulfonic acid.sulfonic acid.


Dichromate -Dichromate - CrCr22OO772-2-

The potassium salt is a primary standard The potassium salt is a primary standard material.material.

Very stable solutions. If air is kept out, it can last Very stable solutions. If air is kept out, it can last for years.for years.

It is a very strong oxidizing agent. It is a very strong oxidizing agent.

Need an indicator such as diphenylamine Need an indicator such as diphenylamine sulfonic acid.sulfonic acid.



Permanganate - MnOPermanganate - MnO44--

The potassium salt is the most commonly used. It is The potassium salt is the most commonly used. It is not a primary standard.not a primary standard.

Solutions must be standardized - typically use Solutions must be standardized - typically use NaNa22CC22OO4 4 ( a primary standard material.) ( a primary standard material.)

Reagent slowly degrades and MnOReagent slowly degrades and MnO22 must be must be

removedremoved No indicator is needed - excess reagent produces a No indicator is needed - excess reagent produces a

pink solution.pink solution.


Permanganate - MnOPermanganate - MnO44--

The potassium salt is the most commonly used. It is The potassium salt is the most commonly used. It is not a primary standard.not a primary standard.

Solutions must be standardized - typically use Solutions must be standardized - typically use NaNa22CC22OO4 4 ( a primary standard material.) ( a primary standard material.)

Reagent slowly degrades and MnOReagent slowly degrades and MnO22 must be must be

removedremoved No indicator is needed - excess reagent produces a No indicator is needed - excess reagent produces a

pink solution.pink solution.


The standardization of MnOThe standardization of MnO442-2- with oxalate with oxalate

involves the following reaction.involves the following reaction.

2MnO2MnO442-2-(aq)(aq) + 5C + 5C22OO44

2-2-(aq)(aq) + 8H + 8H++(aq)(aq)

2Mn2Mn2+2+(aq)(aq) + 10CO + 10CO22 (g) (g) + 8H + 8H22OO (l) (l)

Since MnOSince MnO442-2- in an intense magenta color in an intense magenta color

and Mnand Mn2+2+ is a faint green, detecting the is a faint green, detecting the endpoint for the titration is easy.endpoint for the titration is easy.

Oxidation by oxygenOxidation by oxygen

Oxygen is not the strongest of oxidizing agents but it is Oxygen is not the strongest of oxidizing agents but it is about 19% of our atmosphere.about 19% of our atmosphere.

It is able to react with all other elements except:It is able to react with all other elements except:noble gasesnoble gases

halogenshalogens

noble metals like goldnoble metals like gold

One very common reaction for oxygen is:One very common reaction for oxygen is:

CombustionCombustion - rapid oxidation accompanied by heat and - rapid oxidation accompanied by heat and usually light.usually light.

CombustionCombustion

ExamplesExamples

CHCH44(g)(g) + 2O+ 2O22(g)(g) COCO22(g)(g) + 2H + 2H22OO(g)(g)

SS(s)(s) + O+ O22(g)(g) SOSO22(g)(g)

NN22(g)(g) + O+ O22(g)(g) 2 NO2 NO(g)(g)

2NO2NO(g)(g) + O+ O2 2 (g)(g) 2 NO2 NO22(g)(g)

Note.Note. All of these oxides contribute to air All of these oxides contribute to air pollution. pollution. COCO22 contributes to the contributes to the “greenhouse effect.”“greenhouse effect.”

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Oxidation-Reduction Reactions Oxidation Numbers Oxidation Numbers and Nomenclature Identifying Oxidation-Reduction Reactions Writing Equations for Oxidation-Reduction