Electrochemical equilibria Flashcards
1
Q
2 types of cells
A
chemical energy
converts chemical to electrical
ex batteries and fuel cells
electrical energy
converts electrical to chemical
ex: cells used in electroplating
2
Q
Electrode potential
A
metal rod is dipped in a solution of its own ions,
the electrical potential developed at the interface of the metal and its solution = E
3
Q
If ionization is faster than deposition
A
metal acquires net (-) charge,
increases the rate of deposition. =>equilibrium
- the metal electrode gets
negatively charged and attracts the layer
of positive ions at the interface - an electrical double layer is formed at the interface of metal and solution => Helmholtz electrical double layer
4
Q
deposition is faster than ionization
A
the metal acquires net (+) charge,
increases rate of ionization.
=> equilibrium.
* The metal electrode gets (+) charged and attracts the layer
of (-) ions at the interface,
- an electrical double layer is formed at the interface of metal =>
solution known as Helmholtz electrical double layer
5
Q
Standard electrode potential
A
The potential developed at the interface of metal and solution, when the metal is in contact with a solution of its own ions having unit
concentration at 298 K
in gas electrodes the partial pressure of gas is maintained at 1 ATM
6
Q
Cell potential
A
The difference in electrode potentials of the electrodes constituting the cell => Ecell
7
Q
Standard cell potential
A
- Ecell depends on concentration of the ions in the cell, temp and the partial pressures of any gases involved in the cell reaction.
- When all the concentrations are 1M, all partial pressures of gases are 1atm and temp is 298K, the emf => Standard cell potential, Eo cell
8
Q
Electrochemical Series
A
predict the electrochemical behavior of an electrode –
electrolyte system,
elements are arranged in the order of their standard
reduction potentials.
=> electrochemical series.
* A negative value =
oxidation tendency
while a positive value
= reduction tendency
9
Q
Metal-metal ion electrode
A
Metal in contact with a solution of its own ions
e.g., Zn/Zn2+, Cu/Cu2+, Ag/Ag+
10
Q
Metal-Metal insoluble salt-
A
metal in contact with a sparingly soluble salt of the same metal dipped in a solution of soluble
salt of the same anion
ex: Hg/Hg2Cl2/KCl, Ag/AgCl(s)/HCl
11
Q
Gas Electrode
A
gas bubbling about an inert metal foil, immersed in solution
containing ions to which the gas is reversible.
metal provides electrical contact => equilibrium btw the gas and its ions
e.g.,Hydrogen electrode Pt/H2/H+,Chlorine electrode Pt/Cl2/Cl-
12
Q
Amalgam Electrode
A
similar to metal- metal ion
metal amalgam is in contact with a solution containing its own ions
e.g., Lead amalgam electrode Pb-Hg/Pb2+
13
Q
Oxidation - reduction electrode
A
inert metal such as platinum immersed in a solution
= redox system.
- metal => electrical contact.
- potential arises due to the tendency of one form to change in to other form.
Pt/Fe2+,Fe3+,Pt/Ce3+,Ce4+ , Pt/Sn2+,Sn4+
14
Q
Quinhydrone electrode
A
inert metal such as platinum immersed in a
solution => quinone, hydroquinone
metal => electrical contact
potential => tendency of quinone to change
to hydroquinone
* Pt/Q,QH2
15
Q
Ion selective electrode:(membrane electrode)
A
consists of a membrane in contact with a solution, with which it can exchange ions.
e.g., glass electrode: selective to H+, Na+, K+ etc
16
Q
Reference electrodes
A
Electrodes whose potentials are accurately known, stable and
with reference to which the electrode potential of any electrode can be measured
combined with indicator electrode and emf of the cell is measured
17
Q
Primary reference electrode: Standard hydrogen electrode
A
value of 0.0 V
Gas electrode
* Pt/H2/H+
* 2H+ + 2e- ⇌ H2
18
Q
Disadvantages of SHE:
A
Maintaining concentration of H+ ions at 1M and pressure of H2
gas at 1 atm is difficult.
- Platinum is highly susceptible to poisoning by different impurities in gas
- It cannot be used with oxidizing and reducing environment
19
Q
Secondary reference electrodes:
A
whose electrode potentials are accurately known and
remain stable over a long period of time and can be easily assembled.
With respect to these electrodes , electrode potentials of other
electrodes can be assigned
e.g.,Calomel electrode, silver silver chloride electrode
20
Q
Calomel electrode
A
Most widely used reference electrode
- Metal-insoluble salt –ion electrode
Construction:
* A glass tube with layer of mercury
over a paste of insoluble salt Hg2Cl2
(calomel) + Hg and the next layer is a solution
of KCl
- A Pt wire dipped in Hg = electrical contact
- Tube is fitted with a side tube to fill KCl
solution of known concentration and another
side tube which connects to the salt bridge
21
Q
Decinormal electrode conc of KCL
A
0.1M
22
Q
Concentration cells
A
An electrochemical cell with identical electrodes in
contact with a solution of identical species but of different concentration
electrolyte has spontaneous tendency to diffuse from a solution of higher concentration to a solution of lower concentration which is the driving force for potential
23
Q
Expression for cell potential for Cu electrodes
Electrolyte concentration
A
c2/c1 => Cu/Cu2+(c1)//Cu2+(c2)/Cu
The emf of the cell is positive only if c2 > c1
When c2 = c1, the emf of the cell becomes 0
concentration of ions
increases at anode decreases at cathode
current is drawn from the cell c1 increases and
c2 decreases
24
Q
Electrode concentration cell
A
2 identical electrodes of
different activity which are dipped in the same solution of electrolyte
c1/c2
Na-Hg(c1)/Na+/Na-Hg(c2)
25
Gas electrode concentration cell in terms of partial pressure
Hydrogen cell
p1/p2
other cells p2/p1
26
Ion selective electrodes (ISE)
Selectively respond to a specific ion in a mixture
Potential developed is a due to concentration of that ion
a membrane which is capable of exchanging the
specific ion with solution with which it is in contact
27
Glass electrode
Ion-selective electrode
* Responds to H+ ion
* pH sensitive;
* Consists of a glass membrane => exchanging H+ ions
28
Construction of glass electrode
end => a bulb of very thin glass membrane => special glass, CORNING
015
glass bulb is filled with solution of known pH = reference solution
silver - silver chloride electrode dipped in
reference solution => internal reference electrode
also provides external electrical contact
electrode is immersed in a solution containing H+
= analyte
Ag/AgCl/HCl/glass
29
working of glass bulb
inner and outer surfaces of the glass membrane can exchange H+ ions
with the solution they are in contact with
hydrated glass membrane brings about ion exchange reaction
btw singly charged cations in the interstices of glass lattice and protons from the solution
* potential is developed, => function of H+ of the solution
30
glass electrode potential has 3 components and the formula
The boundary potential The potential of internal reference electrode Asymmetric potential
EG = Eb +Eref + Easymmetric
31
how does Asymmetric potential arise
due to difference in responses of inner and
outer surfaces of the glass bulb, due to differing conditions of stress on two
glass surfaces
32
how to determine ph of glass electrode
the glass electrode is dipped in a solution of known pH(buffer
solution) and combined with calomel electrode, the emf of the cell is
measured from which E0
G can be evaluated
33
Crystalline / solid state membrane electrodes:
Single crystal LaF3 selective to F-
Polycrystalline such as Ag2S selective to S2-
34
Non-crystalline membrane electrodes:
Glass membrane selective to H+, Na+
35
Liquid membrane electrodes:
An ion-exchanger is dissolved in a viscous organic liquid membrane; used for Ca+, K+
36
Immobilised liquid in a rigid polymer:
e.g., immobilized ion exchanger in PVC matrix ; used for Ca+
,NO3-
37
Advantages of glass electrode and applications
in chemical, industrial, agricultural and
biological labs
can be used in oxidizing and reducing environments and
metal ions
* not get poisoned
* used for very small volumes
* Accurate results can be obtained btw pH 1 - 9 = ordinary electrodes.
glass electrodes =
pH 1 to 14
* Simple to operate and used with portable instruments
38
Disadvantages of glass electrode:
Because of high resistance of glass, a simple potentiometer cannot be
used. = sensitive potentiometer for emf measurements
* Glass membrane is very delicate,
* At very high pH levels over a pH of 9 , Alkaline error is observed H+
solution + Na+Gl- ⇌ Na+
solution + H+Gl
When the Sodium ion l=
high, some of the H+ ions in the gel layer
around the sensitive electrode membrane
=> replaced by Na+ ions
The electrode may eventually respond to
Na+ instead of H+ ions, giving a false lower
pH value than the actual value
39
Corrosion
destruction /deterioration & consequent loss of metal
through direct chemical or electrochemical attack by the environment
40
Rusting of iron
- a reddish brown scale
* Due hydrated ferric oxide (Fe2O3.xH2O)
41
Scaling of Copper
Green scales formed on copper articles
* Due to cupric carbonate
(CuCO3 + Cu(OH)2)
42
Why do metals undergo corrosion?
Most metals in nature = compounds
* Metals extracted from ores by reduction
(endothermic process),
* Pure metals => at higher energy state = tendency
to revert back to their combined state
43
Dry corrosion
Direct chemical attack due to affinity of the metal to certain gases
* In the absence of the electrolytic medium
44
Wet corrosion :
Electrochemical attack on the metal
* in the presence of moisture or conducting medium
45
Electrochemical theory of corrosion
large number of tiny galvanic cells => formation of separate anodic, cathodic regions
anode => oxidation => converted into its ions,=>e- metal => corrosion
cathode => reduction, metal cannot be reduced further, => unaffected by corrosion
46
small corrosion current
electrons liberated at the anodic region migrate to the cathodic region
47
corrosion product
metal ions formed at the anode, some anions(-) formed at the cathode diffuse towards each other through the conducting
medium and =>
corrosion product
btw anode and cathode
continues as long as both anodic and cathodic
reactions take place simultaneously
48
Cathode: types of reactions
Liberation of hydrogen
Absorption of oxygen
49
formation of Fe(OH)2
Fe2+ and OH- ions move towards one another and Fe(OH)2 is formed
somewhere btw anode and cathode regions
oxidizing environment, Fe(OH)2 is oxidized to hydrated ferric oxide = yellow rust
2[Fe2O3.3H2O]
presence of limited oxygen
Fe(OH)2 is converted into
magnetic oxide of iron (Fe3O4) = black rust
→Fe3O4.3H2O
50
Differential metal corrosion/galvanic corrosion
two dissimilar metals are in contact with each other and corrosive environment
differ in their electrode potentials.
metal with lower electrode potential = anode and the other= cathode
anodic metal = oxidation and gets corroded.
cathodic metal = unaffected
* The driving force= diff in electrode potentials of the two metals
51
Differential aeration corrosion
metal is exposed to different concentrations of air
lower concentration= anodic and undergoes corrosion
* Other part of the metal = cathodic and remains unaffected
difference in concentration = p.d => corrosion current to flow
52
Waterline corrosion
case of differential aeration corrosion
metal is half immersed in water
part immersed exposed to less O2,
hence = anode and gets corroded
cathode remains unaffected by corrosion
distinct brown line is formed just below the water line due to the
deposition of rust
53
Pitting Corrosion
small anodic area and large cathodic area =
accelerated corrosion at the anodic area
A localized and accelerated corrosion
formation of pits or pin holes, around which the metal is relatively unattacked
small anodic area and =
in accelerated corrosion
54
case 1 pitting corrosion
dust particles or oil drops get deposited over the metal surface
covered by dust which is less aerated becomes
anodic
demand for electrons is high from the cathode
= accelerated corrosion forming a deep and narrow pit
55
Case II pitting corrosion
break down of the protective film on a metal surface
* Peeling off of a small tin coating on iron gives rise to a small anodic area
(Fe) and large cathodic area (Sn)
56
Stress Corrosion AKA Stress Corrosion Cracking(SCC)
part of the material
is under stress and exposed to specific corrosive environment
stress can be external stress acting on the metal during service conditions or residual stress or both
Due to stress = crack
*The metal atoms under stress = higher energy levels
= more reactive, anode
= corrosion cell
57
* Conditions for stress corrosion
Tensile stress
* Specific corrosive environment
under specific corrosive environment the stressed part undergoes
corrosion
crack deepens = breakdown of structure
58
Specific corrosive environment for brass and steel
Brass - ammoniacal solution or ammonia vapours
* Steel – NaOH and chloride ions
59
Factors affecting
rate of corrosion
Nature of the metal
Metals with lower electrode potential values are more reactive =
more susceptible to corrosion
* Galvanic series => arranges metals and alloys in the order of their tendency to undergo corrosion in a particular environment
60
Factors affecting
rate of corrosion
Nature of the corrosion product of Cr, Al
corrosion product formed on the surface = protective film
=>insoluble, stable, uniform, and
non porous
A thin continuous film on surface = barrier btw fresh metal surface and the corrosion
environment
61
Factors affecting
rate of corrosion
Nature of the corrosion product of Fe, Zn
no protective flim
corrosion product is soluble, unstable, non uniform, and porous,
fresh metal surface is exposed to the corrosion environment and
62
Hydrogen overvoltage
extra voltage over the theoretical voltage
required for an electrode reaction to take place
for evolution of gases
if overvoltage of a gas is high on a surface,
liberation of the gas on that surface will be less
A metal with low hydrogen overvoltage on its surface = more corrosion
*If cathodic reaction is hydrogen evolution type, hydrogen gas is
liberated easily = more corrosion
63
Temperature corrsion relation
rate of corrosion increases as the temp increases
*Increase in conductance of the medium- ions
migrate fast
*Decrease in the polarization effects at the anodic and cathodic sites
*Increase in solubility of corrosion product in the medium
*Breakdown in the protective film
if corrosion due to O2, CO2, rate of corrosion decreases as the solubility of these gases decreases
64
pH Corrosion Chemistry
lower the pH = higher corrosion rate
*At low pH ,concentration of H+ ions is high; H+ ions for cathodic reaction
*Corrosion product becomes more soluble in acidic medium
some metals like Al undergo fast corrosion in
highly alkaline solution as their corrosion product is
soluble in alkaline medium
65
Polarization at anodic and cathodic region
drift in electrode potential of an electrode Due to change in concentration of species
66
Anodic polarisation
liberating metal ions which move towards cathodic area
Metal ions cannot move freely due to
opposing factors:
lower conductivity of the medium
Metal ions get concentrated at anodic area and this decreases tendency of metal to undergo oxidation ; corrosion rate falls
Anodic polarisation is due to accumulation of ions in the anodic region
67
Cathodic polarisation
*Due to retarded movement of cathodic reactant (O2, H+) towards the cathodic surface
or retarded removal
region
rate decreases slows down
corrosion
due to depletion of ions in the cathodic region
68
Protective coatings
Metallic coating
Corrosion of base metal prevented by coating a layer of another metal
over it
69
Anodic metal coating
Coating metal is more active with respect to the base metal. Even if the base metal is not completely covered, it will not
undergo pitting corrosion
e.g., Coating of Zn or Mg on iron
70
Galvanizing
*Iron sheet is passed through organic solvent or caustic solution to remove oil or grease present on it
* It is washed with dil. H2SO4 to remove any rust, scale or dust present on the surface (pickling)
treated with a mixture of aqueous solution of ZnCl2 and NH4Cl which acts as flux which prevents oxidation and then dried
* Finally it is dipped in molten zinc at 425-430o C
* Excess zinc present on iron sheet is removed by passing through hot rollers
71
Application of galvanisation
Galvanization of iron is carried out to produce roofing sheets, fencing wire, buckets, bolts, nuts, pipes etc
Advantage:
Even if the Zn coating peels
the base metal (Fe) does not get corroded because the base metal = cathode
DIS
Galvanized articles are not used for preparing and storing food because zinc dissolves in dilute acids producing toxic zinc
compounds
72
Cathodic metal coating
Coating metal is less active with respect to the base metal
Base metal should be completely covered and
If some part of the base metal is left uncovered, then it undergoes pitting corrosion due to formation of small anodic and large cathodic area
e.g.,Coating of Sn or Ni metal over iron
73
Inorganic coating AKA chemical conversion coating
* Surface of the base metal = protective coating through appropriate chemical modification
*The protective film (corrosion product) is an inherent part of the metal
74
Anodizing
Induced oxidation in an oxidising environment by making the article
anode and passing current
Oxidation of outer layer of metal to its metal oxide covers the metal
= protective layer
Done for non-ferrous metals like Al, Cr, Ti
74
Anodizing of Aluminum
* Article is cleaned, degreased, and polished and taken as anode
* It is immersed in an electrolyte consisting of 5-10% chromic acid, sulfuric
acid, phosphoric acid, oxalic acid or their mixtures maintained at around 40 C
* Lead or steel is taken as cathode
* Voltage above 40V is applied
* Outer layer of Al is oxidised to Al2O3
Al2O3 formed is slightly porous in nature
made compact by sealing by immersing the anodised article in boiling water or steam
Al2O3 is converted into Al2O3.H2O which occupies higher volume; the pores
are sealed
Anodized articles can produced in good colour finishes by
*Adsorbing an organic dye after anodising and then sealing
*Depositing a metal like Ni, Co etc., into the pores
75
Anodizing of Aluminum applications
* Anodized Al is used as an attractive, highly durable, corrosion
resistant material in exteriors for roofs, walls, buildings and also in
window frames, office partitions, tiffin carriers, etc.
76
Corrosion Inhibitors
Chemical substances which are added in small quantities to the
corrosive environment to decrease the rate of corrosion
slow down or inhibit the anodic or cathodic reaction
used only in confined environment
action of an inhibitor depends on the nature of the metal to be protected as well as corrosive environment
77
Anodic inhibitors
If the formation of Mn+ is prevented, the corrosion process is retarded
achieved by the addition of large anions such as
chromate(CrO42), tungstate (WO42), etc.
* These ions combine with Mn+ and form precipitate which covers the
surface of the anode
effective only when sufficient amount is added
* If insufficient quantity, then a part of the anodic region is exposed
Formation of small anodic area
= intense corrosion
* Salts like Na2CrO4 , Na2WO4
78
Cathodic inhibitors
liberation of hydrogen
* absorption of oxygen
* Inhibiting these reactions will slow down the
cathodic reaction
which in turn slows down the anodic reaction; corrosion is inhibited
79
Inhibition of hydrogen liberation:
Preventing the diffusion of H+ ions to the cathode-
* By the addition of certain organic compounds:
nitrogen or sulfur
* Aliphatic amines, urea, thiourea, mercaptans, heterocyclic
widely used
adsorbed on the cathodic region forming a protective film, preventing the H+ ions from coming in contact with cathodic metal surface
80
Inhibition of hydrogen liberation:
By increasing the hydrogen overvoltage on the metal surface
When Hydrogen overvoltage = high , liberation of H2 will be
difficult
addition of oxides of arsenic, antimony(As2O3, Sb2O3) or salts like sodium meta-arsenite(NaAsO2)
* They deposit on the cathode region
* makes hydrogen overvoltage high;
liberation of H2 is reduced
81
Inhibition of oxygen absorption
By adding Oxygen scavengers like hydrazine
N2H4 + O2 → N2 + 2H2O
* By adding reducing agents sodium sulfite etc.
2Na2SO3 + O2 →2Na2SO4
82
Inhibition of oxygen absorption
By decreasing the diffusion rate of oxygen to cathode
By adding salts with large cations such as ZnSO4, MgSO4, NiSO4 etc.
* The cations of the salts (Zn2+, Mg2+) migrate towards the cathode surface and react with the hydroxyl ions formed at
the cathode
The hydroxides get deposited on the cathodic sites
* The protective film impermeable to oxygen prevents its diffusion to the cathodic region
83
Limitations of inhibiters
contaminate the environment
* Many of the inhibitors are toxic, cannot be used in
* Can be used only in closed systems in which corrosive environment is either contained or re-circulated
84
Cathodic protection
converting it completely into cathode and no part of it is allowed to act as anode
* Cathode does not undergo corrosion
85
Sacrificial anode method
metal structure converted = cathode by connecting it to a more active metal
* This active metal (example: zinc, magnesium) acts as
an auxiliary anode
* These metals being more active, acts as anode and undergo preferential corrosion
anodic metals are sacrificed to protect the metal
86
Sacrificial anode, ADVS, DISV
ADV;
The method is simple
* Low installation cost
* Does not require power supply
DIS
Involves recurring expenditure for replacement of consumed
anodes
87
Impressed current method
applying a direct current
*protected metal is made cathodic by connecting it to the cathode of the external source of current
* Anode of the external source is usually connected to an inert electrode
like graphite; Platinum, silicon, iron
* A backfill of coke ,bentonite is used to improve efficiency of the anode
* The metal structure being cathode does not undergo corrosion
* Anode being inert remains unaffected
88
Impressed current method ADV and DIS
ADV
*One installation can protect large area of metal
DIS
*Rather expensive, since it needs high current for safe protection of structure
*If the impressed current is not uniform on the entire surface of the protected structure, localized corrosion takes place on the protected metal
