Reduction-oxidation equilibrium

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L e c t u r e 4
Reduction-oxidation equilibrium in electrolytes
solution
Associate prof . L.V. Vronska Associate prof .
M.M. Mykhalkiv
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Outline

• 1. Reduction-oxidation reactions, main concepts.
• 2. Equilibrium constant of Reduction-oxidation
  reactions.
• 3. Influence of different factors on value of
  redox potential.
• 4. Usage of reduction-oxidation reactions in
  analysis.

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1. Reduction-oxidation reactions, main concepts.

• 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.

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• Redox 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 ? Red
• ?xidizing reducing
• form form
• Those species experiencing an increase in
  their oxidation state are oxidized, while those
  experiencing a decrease in their oxidation state
  are reduced.

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

• Equation Ox n ? Red describes the
  reduction-oxidation half-reaction.
• redox pair is the system of oxidizing and
  reducing forms of substance, in which oxidizing
  form (oxidizer) is an electron acceptor and is
  itself reduced when it accepts electrons,
  reducing form (reducer) is electron donor and is
  itself oxidized when it gives up electrons.

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The most important oxidizing agents

• (NH4)2S2O8, KMnO4, K2Cr2O7, K2CrO4, KBrO3, KClO3,
  KJO3
• Cl2, Br2, J2, JCl, JBr, NaClO, NaBrO, CaOCl2
• H2O2, HNO3, H2SO4(concentrated), MgO2, Na2O2, HCl
  HNO3, H2O2 HCl (Komarovskys mixture)
• Cu2, Fe3, Hg2

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The most important reduction agents

• Zn, Fe, Mg, Al, alkali and alkali-earth metals
• Sn2, Mn2, Fe2
• S2-, SO32-, S2O32-, J-, Br-, C2O42-

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Redox-amphoteric substances

• Mn2 ? MnO2 ? MnO4-
• H2O ? H2O2 ? O2
• NH3, N2O, NO ? NO2- ? NO3-
• S2- ? SO32- ? SO42-

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• Not less two redox pairs take part in redox
  reactions. Reaction products are new oxidizer and
  reducer (weaker, than initial)
• Ox1 Red2 ? Red1 Ox2
• 2Fe3 Sn2 ? 2Fe2 Sn4.
• The analogy to the acid-base reactions it is
  observed
• Acid1 Base2 ? Base1 Acid2

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Electronic theory of Reduction-oxidation reactions
ROR is the process of transport of electrons Protolysis is the process of transport of protons
Red - n Acid nH
Ox n Base nH
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• The standard (normal) oxidation-reduction
  potential of pairs which are soluble forms, is a
  difference of potentials, which arises between
  the standard hydrogen and inactive (platinum)
  electrode dipped into the solution, which
  contains the ?xidizing and reducing forms of one
  redox-pairs (25 ?C, activity of components of
  pair equal 1 mol/L)

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• The standard hydrogen electrode (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 (?N or N) H2SO4 or 1,25 mol/L
  ??l, 1 atm H2, 25C. The corresponding
  half-reaction is assigned an arbitrary potential
  of exactly 0 V
• 2? 2 ? ?2?

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• Standard (normal) OR potential ?0 of pairs which
  contain insoluble metal, is a difference of
  potentials, which arise between the metal
  electrode dipped into the solution of the salt
  (with metal ions activity equal 1 mol/L) and
  standard hydrogen electrode at 25 ?C.
• Standard potential depends for temperature,
  pressure, solvent.

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Electrons flow from the S.H.E. (anode) to the
copper cathode.
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Determination of standard potentials (galvanic
cell)

• (-) Zn ZnSO4 H2SO4 (?2) Pt ()
• ?(-) Zn0 ? Zn2 2
• K() 2? 2 ? ?20

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Determination of standard potentials
Electrons flow from the zinc anode to the S.H.E.
(cathode).
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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
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galvanic cell
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Standard redox potentials are determinated at
activity of oxidizing and reducing forms are
equal 1 mol/L and temperature 25??. This state is
called standard state of substance (but not
standard conditions).

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

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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
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• In the standard conditions ?(??) ?(Red) 1
  mol/L and ??0.
• In the nonstandard conditions

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If ? or ??- ions take part in reactions of
oxidation or reduction

• For example, for redox pair Cr2O72-2Cr3
• Cr2O72- 14H 6 2Cr3 7H2O

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• 2) for redox pair MnO4- Mn2
• MnO4- 8H 5 Mn2 4H2O
• 3) for redox pair SnO32- SnO22-
• SnO32- H2O 2 SnO22- 2OH-

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

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

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Formal potential depends on

• The ionic strength of solution
• Running of competitive reactions
• The concentration of reaction components, which
  isnt oxidizing or reducing forms, but their take
  part in the half-reactions
• The nature and concentration of stranger
  electrolytes.

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• As more oxidation-reduction potential of
  redox-pair as stronger oxidizer is ?xidizing form
  this redox-pair.
• As less oxidation-reduction potential of
  redox-pair as stronger reducer is reducing form
  this redox-pair.

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The direction of passage of reaction depends from
value of electromotive force (EMF), which call
potential of reaction E

• ?MF ? ?0(??) - ?0(Red).
• ?MF (?) ? 0, than passes direct reaction
• ?MF (?) ? 0, than passes return reaction
• ?MF (?) 0 condition of equilibrium

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2. Equilibrium constant of Reduction-oxidation
reactions.
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• Reactions which pass completely, should have a
  equilibrium constant more than 108 (when 99,99
  starting compounds should pass), so
• ?0 ? 0,4 V (n1)
• ?0 ? 0,2 V (n2)

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

• influence of temperature
• influence of catalyst
• influence of solution ionic strengh
• influence of concentration of redox-pair
  components
• influence of solution ??
• influence of precipitation reaction
• influence of complexing
• influence of medium nature

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4. Usage of reduction-oxidation reactions in
analysis.

• For transfer of ions and compounds with the less
  oxidation state on the higher and on the
  contrary
• ?) from Fe2 to Fe3
• ?) from ?sO43- to AsIII

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• 2. For determination of ions which give
  characteristic reactions with an oxidizer or a
  reducer

AsIII
As-3H3
As3
AsV
H
?n2 Mn?4- H2O Mn?2

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• 3. For separation of ions which are oxidised or
  reduced with formation or dissolution of
  precipitate.
• H2O2
• ?n2 Mn?2?.
• ??-
• MnO2H2C2O4H2SO4?MnSO42CO2 2H2O

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• 4. In qualitative analysis.
• 5. For identification of drugs
• Aldehydic groups (formalin, chloraminum, chlorali
  hydras)
• Primary amino group (Anaesthesinum,
  Paracetamolum)
• alkaloids (action of concentrated HNO3 typical
  colour)
• 6. In quantitative analysis
• gravimetric analysis (sulphatic ashes, method of
  precipitation)
• titrimetric analysis (oxidimetry, reductimetry)
• physical-chemical methods (potentiometry,
  coulometry, electric gravimetric analysis,
  polarography).

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Usage of reduction-oxidation reactions in
potentiometry.
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Thanks for your attention!