Redox

1
Redox Electrochemistry

• Electrochemical Process
• Chemical E -gt Electrical E
• OR
• Electrical E -gt Chemical E
• Both involve redox rxns

2
Oxidation-Reduction Rxns

• Chemical changes th occur when e-s are
  transferred between elements
• Oxidation is the principle source of E on the
  earth
• Combustion of fuels
• Burning wood
• Burning food in our bodies
• Oxidation- originally meant the combination of an
  element w/ oxygen to produce oxides

3
Re-dox Rxns

• Reduction - has meant loss of oxygen fr a comp
• Refers to the fact th when a metal oxide is
  reduced to a metal, there is a considerable
  decrease in volume.
• Redox rxns Always go together if an element
  is oxidized, another is reduced

4
Re-dox Rxns

• LEO goes GER (todays meaning)
• Lose es oxidized
• Gain e-s reduced
• Half rxns show either oxidation or reduction
  alone
• e-s lost gained must be equal
• 1/2 rxns can be added to give the overall
  balanced equation

5
Oxidation Numbers

• Allows us to keep track of e-s
• Oxidation the actual charge of a monatomic
  ion otherwise, arbitrary charge using a set of
  rules
• Oxidation Ox increases
• Reduction Ox decreases

6
Rules for assigning Ox s

• 1. Free element 0.
• 2. Monoatomic ion charge of ion.
• 3. Nonmetals are usu negative
• Oxygen -2
• Hydrogen is 1 when bonded to nonmetals
• HCl
• Hydrogen is -1 when bonded to metals
• LiH

7

Rules for assigning Ox s -cont

• Fluorine -1
• Other halogens are -1 unless bonded w/ O, then
  they are .
• 4. Sum of Ox s of all atoms in a neutral comp
  0. NaCl-
• 5. The sum of the Ox s in a PA ion the charge
  of the ion. H3O

8
Oxidizing Reducing Agents

• Oxidizing agent- sub th is reduced
• Common oxidizing agents halogens, O2, and
  oxyanions such as MnO4-, Cr2O72-, and NO3-
• Reducing agent - the sub th is oxidized
• Common reducing agents H2, active metals such
  as the alkali alkali earth metals

9
Balancing Equations by the Half Rxn Method

• If the rxn is in acidic conditions, H and H2O
  can be added to either the R or P to balance H
  and O
• MnO4-(aq) C2O42-(aq) -gt Mn2(aq) CO2 (aq)

10
MnO4-(aq) C2O42-(aq) -gt Mn2(aq) CO2 (aq)

• Divide equation into 2 incomplete half rxns.
• Balance ea half rxn
• Balance the elements other than H O
• Balance O by adding H2O
• Balance H by adding H
• Balance the charge by adding e-s to the side w/
  the greater positive charge
• Multiply ea half rxn by an integer so e-s gained
  lost balance.
• Add 2 half rxns, cancel like species.
• Check to see it is balanced.

11
Voltaic Cells

• The first battery
• Also called a Galvanic Cell
• Energy (e-s) produced in a spontaneous redox rxn
  are transferred through an external pathway so
  can be used to perform electrical work

12
Voltaic Cells-cont

• Consists of 2 half cells
• Composed of a metal in a soln of its own ions
  (electrolyte)
• A diff metal in a soln of its own ions makes up
  the other half cell
• An external circuit connects the metals
• Spontaneous redox rxn

13
Voltaic Cells-cont

• Anode electrode where oxidation occurs
• Cathode electrode where reduction occurs
• An Ox A Red Cat Studied Redox
• Anode Oxidation (LEO)
• Cathode Reduction (GER)
• E-s flow fr A -gt C
• Anode -gt Cathode
• Salt bridge- porous U-shaped tube of electrolyte
  th separates the half cells
• Needed to maintain neutrality by allowing
  migration of ions

14
The Activity Series

• An arrangement of elements from most active
  (easily oxidized) to least active (most easily
  reduced)

15
The Activity Series Cells

• Electrochemical cell consists of
• 2 different metal electrodes
• Electrolyte soln
• When the electrodes are connected, an electric
  current flows
• More reactive metal loses e-s, wh flow fr it to
  the other metal
• Further apart the metals are on the activity
  series greater voltage produced

16
Cell EMF

• Std Reduction Potential Table
• -provides reduction voltages of half cells under
  std conditions
• - the more the voltage value of 2 half rxns is
  the one th is reduced (cathode)
• - the less () half rxn will be oxidized (anode)

17
Cell EMF- cont

• Cell potential
• Ecell Ered (cathode) - Ered (anode)
• Intensive property, so changing coefficients WILL
  NOT change the Ecell
• The more positive the Ecell , the greater the
  driving force for reduction

18
Dry Cell Battery

• Voltaic cell in wh the electrolyte is a paste
• Flashlight battery 1.5V drops not
  re-chargeable
• Zn container filled w/ a thick moist paste of
• MnO2, ZnCl2, NH4Cl, H2O
• Graphite rod is embedded in paste

19
Dry Cell Battery-cont

• Anode - Zn container
• Cathode- graphite rod
• Half rxns ox Zn -gt Zn2 2e- (anode)
• red 2MnO4 2NH4 2e-s -gt Mn2O3
  2NH3 H2O (cathode)
• Graphite is not reduced only a conductor

20
Lead Storage Battery

• Rechargeable 12V car battery
• Battery - a group of cells connected together
• 12V 6 2V cells joined together
• Ea cell contains 2 Pb electrodes
• anode spongy Pb
• cathode PbO2

21
Lead Storage Battery-cont

• Electrodes are immersed in H2SO4(aq)
• Ox Pb SO42- -gt PbSO4 2e-
• Red PbO2 4H SO42- 2e- -gt
• PbSO4 2H2O
• Overall rxn
• Pb PbO2 H2SO4 -gt 2PbSO4 2H2O
• As electrical E is discharged
• PbSO4 increases H2SO4 decreases
• Non-spontaneous recharge can occur if a D.C.
  current passes through cell in reverse.

22
Fuel Cells

• Voltaic cells in wh fuel subs undergo oxidation
  fr wh electrical E is obtainted continuously
• Hydrogen-Oxygen FC
• 3 compartments separated fr 1 another by 2
  pourous electrodes of C
• Oxygen (oxidizer) fed into cathode
• Hydrogen (fuel) fed into anode

23
Hydrogen-Oxygen FC-cont

• Gases diffuse through electrodes into hot
  concentrated KOH(aq)- electrolyte
• e-s fr oxidation (anode) pass through an external
  circuit to reduce the cathode
• Ox 2H2 4OH- -gt 4H2O 4e-
• Red O2 2H2O 4e -gt 4OH-
• Overall rxn 2H2 O2 -gt 2H2O
• Apollo, space shuttle water source

24
Corrosion

• Returning metals to their natural states-
  usu.compounded w/ O2 (or S)
• Oxidation of the metal decreases its strength
  attractiveness
• Costs the US billions each year!

25
Corrosion-cont

• Al forms a thin protective coating of Al2O3 that
  prevents further corrosion
• Cr, Ni other metals do the same
• Fe forms iron oxide (Fe2O3), rust
• Scales off easily, so th a new surface is exposed
  to rusting
• Patina- green sub formed when Cu reacts w/ S in
  the air forming copper sulfate
• Silver tarnish- Ag2S

26
Preventing Corrosion

• Coating metal
• Painting
• Oil
• Plating- putting a thin layer of noncorrosive
  metal on top
• Galvanized steel- Zn on steel
• Alloys
• Stainless steel (contains Cr Ni)

27
Preventing Corrosion-cont

• Cathodic Protection
• A more reactive metal, such as Mg, is put in
  contact w/ Fe or steel
• Mg is oxidized instead
• Sacrificial protection
• Mg must be replaced as it dissolves away

28
Electrolysis

• Passing an electric current through a sub,
  causing it to decompose into new subs.
• All ionic subs when molten or in soln can be
  electrolyzed
• Causes a nonspontaneous re-dox rxn to occur

29
Electrolysis-cont

• Electrolytic Cell electrolysis apparatus
• Electrodes- 2 inert conducting rods
• Made of graphite or platinum
• Carry current into the electrolyte
• Cathode (-) electrode
• Joined to (-) battery terminal
• Attracts cations
• Anode () electrode
• Joined to () battery terminal
• Attracts anions

30
Electrolysis -cont

• An Ox A Red Cat Studied Redox
• Anode Oxidation (LEO)
• Cathode Reduction (GER)
• E-s flow fr A -gt C
• Anode -gt Cathode

31
Electrolysis of Molten Salts

• Can be prohibitive due to high melting pts
• 2NaCl(l) -gt 2Na(l) Cl2(g)
• Cathode 2Na(l) 2e- -gt 2Na (l)
• Anode 2Cl- (l)-gt Cl2(g) 2e-
• 2Na (l) 2Cl -(l) -gt 2Na (l) Cl2(g)
• Important industrial process for the production
  of active metals such as Na and Al

32
Hall Process

• Bauxite (Al2O3 ore) is made molten in a sodium
  soln (cryolite) and electrolyzed to produce Al
  metal
• Uses 4.5 of all electricity in US

33
Electrolysis of Aqueous Solns

• Complicated by the presence of water
• Water maybe either
• Oxidized to produce O2(g) OR
• Reduced to produce H2(g)
• Rather than the ions of the salt

34
Electrolysis of NaCl(aq)

• Produces H2(g), Cl2(g) and NaOH(aq)
• Cathode 2H2O(l) 2e- -gt H2(g)
  2OH-(aq)
• Anode 2Cl-(aq) -gt Cl2(g) 2e-
• 2H2O(l) 2Cl- -gt H2(g) 2OH-(aq)

35
Electroplating

• An electrolytic cell
• Cathode object to be plated
• Anode - plating metal
• Electrolyte- soln of the plating metal