Electrochemistry Flashcards
Electrolytic/galvanic/voltaic cell is for
Converts chemical energy of spontaneous redox rn into electrical energy.
Salt bridge
Connects two solutions without mixing them and maintaining electrical neutrality
EMF
P.d between 2 electrodes of a cell when no current is drawn from it
Electrode potential
Pd between electrode & electrolyte
Or
The potential of electrode when it shows a tendency to undergo oxidation or reduction in its salt sol
Standard electrode potential
Electrode potential when conc. of all species is unity
Cell potential
Pd between 2 electrodes of a galvanic cell
E(cell) =
E(cath) - E(ano)
Same for standard E(cell)
When is reduced form of standard E.P more stable than hydrogen gas?
When it is > 0
Which has max and min tendency to get reduced?
Flourine & Lithium
Fluorine is a strong ___ and weak ___
Oxidizing agent & weak reducing agent
E(Metal(n+)/M) =
E°(M(n+)/M) - 2.303RT/nF log[M]/[Mn+]
1 Faraday is
96500C
If E° is -ve
It is a stronger reducing agent
Oxidation potential
Potential of a electrode when it loses e’
Which element will get reduced easily
Element with greater oxidation potential
Which will act as cathode or anode?
One with higher reduction potential will be cathode
Movement of e’s is from
Anode to cathode
E°(cell) in terms of K(c) is
E°(cell) = 0.0591/n logK(c)
Work done by a cell
W = EMF × nF
Electrical work done by the system is
Spontaneous
ΔG =
-nFE(cell)
When is a rn spontaneous?
ΔG < 0 and E°(cell) > 0
ΔG° =
-2.303RT logK(c)
EMF of Cu
0.34V
Zn EmF
-0.76V
Will H+ or Cu²+ ions get easily reduced?
Cu²+
Inert electrodes examples
Platinum or gold
What do inert electrodes do
Provide surface for oxidation or reduction
dG is ___ and depends on
Extensive prop; n
Purification is also known as
Electrolytic refining
Impurities present below anode is
Anode mud
Na and Mg are produced by
Electrolysis of their fused chlorides
Aluminium is produced by
Electrolysis of Aluminium Oxide in the presence of cryolite
Products of electrolysis depends on
Conc. of electrolyte, nature of electrolyte and electrode
Conductivity depends on
Nature of material, temp, pressure
Semiconductors eg
Silicon, doped silicon, gallium arsenide
Substances that are electronically conducting
Carbon-black, graphite
Substances having low conductivity
Glass, ceramics
Superconductors are
Having 0 resistivity or infinite conductivity
Metals and alloys at low temperatures behave as
Superconductors
Electronic conductance depends on
Nature of material
No of valence e’s
Temperature(decreases with rise in temp)
Electrolytic solutions depends on
Nature of electrolyte added
Size of ions
Nature of solvent & its viscosity
Conc of electrolyte
Temperature(increases)
Faraday’s 1st law
Amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to quantity of electricity passed through the electrolyte
Faraday’s 2nd law
The amounts of different substances liberated by the same quantity of electricity passing through the electrolyte solution are proportional to their chemical equivalent weights
Equivalent weight/mass w =
At. mass/no of e’s lost or gained
To carry out electrochemical processes such as liberating O2 from aq NaCl, the extra potential applied is
Overpotential
Products of electrolysis if we use molten NaCl
Na+ and Cl2 gas
Which product is liberated at anode and cathode for NaCl
Na at cathode and Cl at anode
During electrolysis of aq NaCl products are liberated at
NaOH, Cl & H are the ions
H gets liberated at Cathode and Cl2 at anode
If conc H2SO4 is used
SO4 2- undergoes oxidation
S2O8 is called
Persulphate
W =
ZQ
Electrochemical equivalent is
Amount of substance deposited when electricity passed is 1C
Z =
Equivalent mass/96500
Weak electrolytes conduct
Less
When size of ion increase and salvation is more, conductance?
Increases
Viscosity is more conductance is
Less due to less ion movement
1/rho =
K kappa which is conductivity
Unit of K
1/(ohm×m) or S/m(siemen)
1/R =
G conductance
K =
GL/A
Conductivity
The conductance of ions present in unit volume of electrolytic solution
Cell constant
L/A denoted by G*
G* = R×K
Upon dilution conductivity
Decreases since no of ions present in unit volume decreases
Molar conductivity
Conductance of ions in 1 mole of electrolyte dissolved in V volume of the solution kept between electrodes which which are unit distance apart and have area large enough to accommodate 1 mole of electrolyte
Λm =
K/C = KV where C is conc of solution
When volume increases conc
Decreases
On dilution, molar conductivity
Increases even though conductivity decreases
Unit of Λm
m²/(ohm mol)
Limiting molar conductivity is
Molar conductivity at max dilution
For strong and weak electrolytes, molar conductivity
Increases since no of ions increases and degree of dissociation increases
Debye Huckel Onsager equation for strong electrolytes
Λm = Λ°m - AC^½
When conc increases, Λm
Decreases
Λ°m can be found out for strong electrolyte using
graph of debye equation as y=mx+c
Kohlrausch’s law of independent migration
Limiting molar conductivity of an electrolyte is the sum of limiting molar conductivity of anion & cation in the sol
Kohlrausch’s law applications
To find Λ°m of weak electrolytes
To find degree of dissociation
To find equilibrium constant
Degree of dissociation in terms of Λm
Alpha = Λm/Λ°m
Primary battery eg
Dry/leclanche cell & mercury cell
Secondary battery eg
Lead storage & Ni-Cd
Describe leclanche cell
Zn as anode, graphite rod as cathode, graphite rod surrounded by MnO2 & powdered C which undergoes reduction, paste of NH4Cl is electrolyte
1.5V supply
Leclanche cell rns
Cathode: 2MnO2 + (2NH4+) + 2e –> 2MnO(OH) + 2NH3
Anode: Zn –> (Zn²+) + 2e
Net: you can write
Rn does not stop
Describe Mercury cell
Zn as anode, HgO as cathode, KOH as electrolyte, Carbon rod is surrounded by paste of HgO & KOH
1.3V supply
Mercury cell rns
Anode: Zn –> ZnO + 2e
Balanced: Zn + 2OH –> ZnO + H2O + 2e
Cathode: HgO + 2e –> Hg
Balanced: H2O + HgO + 2e –> Hg + 2OH
Net: you can write
Describe lead storage
Pb grids filled with Pb as anode, Pb grids with PbO2 as cathode, 38% H2SO4 as electrolyte
2v supply
Lead storage rns
Anode: Pb + SO4 –> PbSO4 + 2e
Cathode: PbO2 + SO4 + H + 2e –> PbSO4 + H2O
Balanced: PbO2 + SO4 + 4H +2e –> PbSO4 + 2H2O
Net: Pb + PbO2 + 2H2SO4 –> 2PbSO4 + 2H2O
Describe Ni-Cd cell
Cd as anode, NiO2 as cathode, KOH as electrolyte
1.2V supply
Cd-Ni rns
Anode: 2OH + cd –> Cd(OH)2 + 2e +2H2O
Cathode: NiO2 + 2H2O + 2e –> Ni(OH)2 + 2OH
Net: you can write
Hydrogen oxygen fuel cell rns
Anode: h2 -> h2O + 2e
Balanced: h2 + 2OH -> 2h2O + 2e
Cathode: O2 -> 2H2O
Balanced: O2 + 4e + 2H2O -> 4OH
Net: you can write
Which cell is used most? Why?
Hydrogen-oxy fuel cell as it is eco-friendly with no harmful emissions
Rusting of iron
Anode: 2Fe –> 2Fe + 4e
H2O + CO2 -> H2CO3
Cathode: O2 + 4H + 4e -> 2H2O
Fe²+ + O2 + H2O -> Fe2O3 + H+
What is rust
Hydrated ferric oxide
List two advantages of cadmium cell over lead storage battery
Cadmium cell has a longer life cycle and performes good at low temperatures as compared to lead storage cells.
It can operate practically at its full rated capacity at high discharge rates.
Application Nickel-Cadmium storage cell
Portable power tools, photographic equipment, flashlights
Which is +ve and -ve in electrolytic cell?
Anode is +ve & cathode is -ve
When E(ext) > 1.1V
Rn in backwards
By 2nd law W1/W2 =
E1/E2
Uses of electrochemical cell
finding pH of solns, find solubility product, find eq. const, get thermodynamic properties
A: E(cell) is an intensive parameter but ΔG is an extensive prop
R: Its value depends on n
A
A substance is a good reducing agent if
it’s oxidation potential is more
Atm to pascal conversion
multiply by 1.01×10⁵PA
