Reduction-oxidation equilibrium Reduction-oxidation equilibrium in in electrolyte’s solutionelectrolyte’s solution

LL ee cc tt uu rr ee 4 4

Associate profAssociate prof . L.V. Vronska. L.V. VronskaAssociate profAssociate prof . M.M. Mykhalkiv

OutlineOutline

1. Reduction-oxidation reactions, main concepts.1. Reduction-oxidation reactions, main concepts.

2. Equilibrium constant of Reduction-oxidation 2. Equilibrium constant of Reduction-oxidation reactions.reactions.

3. Influence of different factors on value of 3. Influence of different factors on value of redox potential.redox potential.

4. Usage of reduction-oxidation reactions in 4. Usage of reduction-oxidation reactions in analysis. analysis.

1. Reduction-oxidation reactions1. Reduction-oxidation reactions, , mainmain conceptsconcepts..

Oxidation state (oxidation number)– the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.

Oxidation - a loss of electrons. Reduction - a gain of electrons. Reducing agent (reductant or reducer) - a species that

donates electrons to another species. Oxidizing agent (oxidant or oxidizer) - a species that

accepts electrons from another species.

Redox reactionRedox reaction - an electron-transfer reaction.

As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state.

Ox + ne Ox + ne Red Red

ооxidizingxidizing reducingreducing

formform formform

Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced.

The pair of an oxidizing and reducing agent that The pair of an oxidizing and reducing agent that are involved in a particular reaction is called aare involved in a particular reaction is called a redox pairredox pair..

Equation Equation Ox + n Ox + n Red Red describes the describes the reduction-oxidation half-reaction.reduction-oxidation half-reaction.

redox pairredox pair – is the system of oxidizing and – is the system of oxidizing and reducing forms of substance, in which oxidizing reducing forms of substance, in which oxidizing form (form (oxidizeroxidizer) is an electron acceptor and is ) is an electron acceptor and is itself reduced when it accepts electrons, itself reduced when it accepts electrons, reducing form (reducing form (reducerreducer) is electron donor and is ) is electron donor and is itself oxidized when it gives up electrons.itself oxidized when it gives up electrons.

e

The most important oxidizing agentsThe most important oxidizing agents:: (NH(NH44))22SS22OO88, KMnO, KMnO44, K, K22CrCr22OO77, K, K22CrOCrO44, ,

KBrOKBrO33, KClO, KClO33, KJO, KJO33

ClCl22, Br, Br22, J, J22, JCl, JBr, NaClO, NaBrO, , JCl, JBr, NaClO, NaBrO, CaOClCaOCl22

HH22OO22, HNO, HNO33, H, H22SOSO44(concentrated), MgO(concentrated), MgO22, , NaNa22OO22, HCl + HNO, HCl + HNO33, H, H22OO22 + HCl + HCl (Komarovsky’s(Komarovsky’s mixturemixture))

CuCu2+2+, Fe, Fe3+3+, Hg, Hg2+2+

The most important reduction agentsThe most important reduction agents::

Zn, Fe, Mg, Al, alkali and alkali-earth Zn, Fe, Mg, Al, alkali and alkali-earth metalsmetals

SnSn2+2+, Mn, Mn2+2+, Fe, Fe2+2+

SS2-2-, SO, SO332-2-, S, S22OO33

2-2-, J, J--, Br, Br--, C, C22OO442-2-

RedoxRedox-amphoteric -amphoteric substancessubstances::

MnMn2+2+ MnOMnO22 MnO MnO44--

HH22O O HH22OO22 O O22

NHNH33, N, N22O, NO O, NO NONO22-- NO NO33

--

SS2-2- SOSO332-2- SO SO44

2-2-

Not less two redox pairs take part in redox Not less two redox pairs take part in redox reactions. Reaction products are new oxidizer reactions. Reaction products are new oxidizer and reducer (weaker, than initial):and reducer (weaker, than initial):

OxOx11 + Red + Red22 Red Red11 + Ox + Ox22

2Fe2Fe3+3+ + Sn + Sn2+2+ 2Fe 2Fe2+2+ + Sn + Sn4+4+..

The analogy to the acid-base reactions it is The analogy to the acid-base reactions it is observed:observed:

AcidAcid11 + Base + Base22 Base Base11 + Acid + Acid22

ElectronicElectronic theory of Reduction-oxidation theory of Reduction-oxidation reactionsreactions

ROR – is the process ROR – is the process of electrons of electrons transporttransport

Protolysis – is the Protolysis – is the process of transport process of transport of protons of protons

Red - nRed - n Acid – nHAcid – nH++

Ox + nOx + n Base + nHBase + nH++ e

e

The The standard (normal) oxidation-standard (normal) oxidation-reduction potential of pairsreduction potential of pairs which are which are soluble forms, is a difference of soluble forms, is a difference of potentials, which arises between the potentials, which arises between the standard hydrogen and inactive standard hydrogen and inactive (platinum) electrode dipped into the (platinum) electrode dipped into the solution, which contains the оxidizing solution, which contains the оxidizing and reducing forms of one redox-pairsand reducing forms of one redox-pairs (25 (25 C, activity of components of pair equal C, activity of components of pair equal 1 mol/L) 1 mol/L)

The standard hydrogen electrode standard hydrogen electrode ((S.H.E.)S.H.E.) It consists of a platinum electrode in contact with H2 gas and aqueous H+ ions

at standard-state conditions [1 mol/L (СN or

N) H2SO4 or 1,25 mol/L НСl, 1 atm H2,

25°C]. The corresponding half-reaction is assigned an arbitrary potential of exactly 0 V:

2Н2Н+ + + 2 + 2 Н Н22e

Standard (normal) OR potential ЕStandard (normal) OR potential Е00 of of pairs which contain insoluble metalpairs which contain insoluble metal, is a , is a difference of potentials, which arise difference of potentials, which arise between the metal electrode dipped into the between the metal electrode dipped into the solution of the salt (with metal ion’s solution of the salt (with metal ion’s activity equal 1 mol/L) and standard activity equal 1 mol/L) and standard hydrogen electrode at 25 hydrogen electrode at 25 C. C.

Standard potential depends for Standard potential depends for temperature, pressure, solvent.temperature, pressure, solvent.

Electrons flow from the S.H.E. (anode) to the copper cathode.

(-) Zn (-) Zn || ZnSO ZnSO44 |||| H H22SOSO44 || (Н (Н22) Pt (+)) Pt (+)

А(-): ZnА(-): Zn00 ZnZn2+2+ + 2 + 2

KK(+): 2Н(+): 2Н++ + 2 + 2 НН2200

e

e

Determination of standard potentials Determination of standard potentials (galvanic cell):(galvanic cell):

Determination of standard potentials

Electrons flow from the zinc anode to the S.H.E. (cathode).

If electrons flow from the metal anode to the S.H.E. (cathode), than standard potentials with “-”. If Electrons flow from the S.H.E. (anode) to the metal cathode, than standard potentials with “+”.

galvanic cellgalvanic cell

Standard redox potentials are determinated at activity of oxidizing and reducing forms are equal 1 mol/L and temperature 2525СС.. This state is called This state is called standard state of substancestandard state of substance (but not standard (but not standard conditions). conditions).

Nernst equationNernst equation - an equation relating electrochemical potential to the concentrations of products and reactants::

.a

aln

nF

RTEE

dRe

Ox0

Substituting appropriate values for R and F, assuming a temperature of 25 °C (298 K), and switching from ln to log gives the potential in volts as

.a

alg

n

059,0EE

dRe

Ox0

In the standard conditions:In the standard conditions: аа((ОхОх)) = = аа((RedRed) ) == 1 mol 1 mol//LL andand Е=ЕЕ=Е00..

In the nonstandard conditions:In the nonstandard conditions:

.]d[Re

]Ox[lg

n

059,0EE 0

IfIf Н Н++ oror ОН ОН-- ionsions take part in reactions of take part in reactions of oxidation or reductionoxidation or reduction::

For exampleFor example, , for redox pair Crfor redox pair Cr22OO772-2-|2Cr|2Cr3+3+::

CrCr22OO772-2- + 14H + 14H++ + 6 + 6 = 2Cr= 2Cr3+3+ + 7H + 7H22OO

e

.]Cr[

]H][OCr[lg

6

059,0EE

23

142720

2) 2) for redox pair for redox pair MnOMnO44--|| Mn Mn2+2+::

MnOMnO44-- + 8H + 8H++ + 5 = Mn + 5 = Mn2+2+ + 4H + 4H22OO

3) 3) for redox pair for redox pair SnOSnO332-2- | | SnOSnO22

2-2-

SnOSnO332-2- + H + H22O + 2 = SnOO + 2 = SnO22

2-2- + 2OH + 2OH--

e

e

.a

aalg

5059,0

EE2

4

Mn

8HMnO0

.aa

alg

2059,0

EE2OHSnO

SnO0

22

23

Real redox potential – – it is potential of redox it is potential of redox pair than components of reaction are in real pair than components of reaction are in real condition, not standard.condition, not standard.

Formal redox potential – – it is potential of it is potential of redox pair when concentration of reaction redox pair when concentration of reaction components is formal (concentration of components is formal (concentration of reagents is equal 1 mol/L, but concentrations reagents is equal 1 mol/L, but concentrations of other compounds in solution are certain).of other compounds in solution are certain).

Formal potential depends onFormal potential depends on::

The ionic strength of solutionThe ionic strength of solution Running ofRunning of competitive reactionscompetitive reactions The concentration of reaction components, The concentration of reaction components,

which isn’t oxidizing or reducing forms, which isn’t oxidizing or reducing forms, but their take part in the half-reactionsbut their take part in the half-reactions

The nature andThe nature and concentration of stranger concentration of stranger electrolyteselectrolytes..

As As more oxidation-reduction potentialmore oxidation-reduction potential of of redox-pair as redox-pair as stronger oxidizer is stronger oxidizer is ооxidizingxidizing formform this redox-pair. this redox-pair.

As As less oxidation-reduction potentialless oxidation-reduction potential of of

redox-pair as redox-pair as stronger reducer is stronger reducer is reducing formreducing form this redox-pair. this redox-pair.

The direction of passage of reactionThe direction of passage of reaction

depends from value of depends from value of electromotive forceelectromotive force

(EMF), which call (EMF), which call potential of reactionpotential of reaction E E

ЕMF = Е = ЕЕMF = Е = Е00(Ох) - Е(Ох) - Е00(Red).(Red).

ЕMF (Е) ЕMF (Е) 0, than passes 0, than passes direct reactiondirect reaction ЕMF (Е) ЕMF (Е) 0, than passes 0, than passes returnreturn reactionreaction ЕMF (Е) = 0 ЕMF (Е) = 0 condition of equilibriumcondition of equilibrium

2. 2. Equilibrium constant of Reduction-Equilibrium constant of Reduction-oxidation reactions.oxidation reactions.

eq

02

01 Klg

059,0

n)EE(

059,0

n)ЕЕ(

eq

02

01

10K

Reactions which Reactions which pass completelypass completely, should , should have a have a equilibrium constantequilibrium constant more than 10more than 1088 (when 99,99 % starting compounds should (when 99,99 % starting compounds should pass), so: pass), so:

ЕЕ00 + 0,4 V (n=1) + 0,4 V (n=1) ЕЕ00 + 0,2 V (n=2) + 0,2 V (n=2)

8059,0

n)EE( 02

01

3. 3. Influence of different factors on Influence of different factors on value of redox potential.value of redox potential.

influence of temperatureinfluence of temperature influence of catalystinfluence of catalyst influence of solution ionic strenghinfluence of solution ionic strengh influence of concentration of redox-pair componentsinfluence of concentration of redox-pair components influence of solution рНinfluence of solution рН influence of precipitation reactioninfluence of precipitation reaction influence of complexinginfluence of complexing influence of medium natureinfluence of medium nature

4. 4. Usage of reduction-oxidation Usage of reduction-oxidation reactions in analysis.reactions in analysis.

1. For transfer of ions and compounds with the less oxidation state on the higher and on the contrary:

а) from Fe2+ to Fe3+

б) from АsO43- to AsIII

2.2. For determination of ions which give For determination of ions which give characteristic reactions with an characteristic reactions with an oxidizer or a reducer:oxidizer or a reducer:

H+

AsIII

Мn2+ MnО4-

H2O MnО2

AsVAs-3H3 As+3

3.3. For separation of ions which are For separation of ions which are oxidized or reduced with formation or oxidized or reduced with formation or dissolution of precipitate.dissolution of precipitate.

HH22OO22

МnМn2+2+ MnО MnО22..

ОН-ОН-

MnOMnO22+H+H22CC22OO44+H+H22SOSO44MnSOMnSO44+2CO+2CO22+ 2H+ 2H22OO

4.4. In qualitative analysis.In qualitative analysis.

5.5. For identification of drugs:For identification of drugs: Aldehydic groups Aldehydic groups ((formalinformalin, , chloraminumchloraminum, , chlorali chlorali

hydrashydras)) Primary amino groupPrimary amino group ( (AnaesthesinumAnaesthesinum, , ParacetamolumParacetamolum)) alkaloids alkaloids (action of (action of concentratedconcentrated HNO HNO33 – – typical colourtypical colour))

6.6. In quantitative analysis:In quantitative analysis: gravimetric analysisgravimetric analysis ( (sulphaticsulphatic ashesashes, , method of method of

precipitationprecipitation);); titrimetric analysistitrimetric analysis ( (oxidimetryoxidimetry, , reductimetryreductimetry);); physical-chemical methodsphysical-chemical methods (potentiometry, coulometry,

electric gravimetric analysis, polarography).

Usage of Usage of reduction-reduction-oxidation oxidation reactions in reactions in potentiometry..

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