Concept Core
From galvanic cells to electrolysis β the electrochemistry framework.
Galvanic Cell β The Electrochemistry Setup
A galvanic cell converts chemical energy to electrical energy. Key components:
Anode (β): Oxidation occurs. Loses electrons: Zn β ZnΒ²βΊ + 2eβ»
Cathode (+): Reduction occurs. Gains electrons: CuΒ²βΊ + 2eβ» β Cu
Salt bridge: maintains electrical neutrality between the two half-cells.
EΒ°cell = EΒ°cathode β EΒ°anode = EΒ°reduction(cathode) β EΒ°reduction(anode)
Standard Electrode Potentials
All EΒ° values are standard reduction potentials (vs SHE at 25Β°C, 1 M, 1 atm).
Higher EΒ°: better oxidising agent (cathode). Lower EΒ°: better reducing agent (anode).
SHE: EΒ°(HβΊ/Hβ) = 0 V (by definition)
EΒ°cell = EΒ°(reduction at cathode) β EΒ°(reduction at anode)
Spontaneous cell: EΒ°cell > 0 β ΞGΒ° < 0
Nernst Equation
Cell potential under non-standard conditions:
E_cell = EΒ°cell β (RT/nF) ln Q
E_cell = EΒ°cell β (0.0592/n) log Q (at 25Β°C)
n = number of electrons transferred. Q = reaction quotient. At equilibrium: E = 0 β EΒ°cell = (0.0592/n) log K
Relationship: ΞGΒ°, EΒ°, and K
ΞGΒ° = βnFEΒ°cell
ΞGΒ° = βRT ln K
nFEΒ°cell = RT ln K
F = Faraday constant = 96485 C/mol β 96500 C/mol
Spontaneous: EΒ°cell > 0 β ΞGΒ° < 0 β K > 1
Electrolysis β Faraday's Laws
First Law: Mass deposited β charge passed: m = ZQ = ZIt
Second Law: Same charge deposits masses in proportion to their equivalent weights.
m = (M Γ I Γ t) / (n Γ F)
where M = molar mass, n = electrons per ion
Conductance & Kohlrausch's Law
Conductance: G = 1/R. Molar conductivity Ξm = ΞΊ/C (ΞΊ = specific conductance).
Kohlrausch's Law: At infinite dilution, ΞΒ°m = Ξ£ λ°(ions). Used to find ΞΒ°m of weak electrolytes from strong ones.
Molar conductivity increases with dilution (more ions dissociate for weak electrolytes).
Formula Vault
All electrochemistry formulas for EAPCET.
Cell EMF
EΒ°cell = EΒ°cat β EΒ°an
Both in standard reduction potential
ΞGΒ° and EΒ°cell
ΞGΒ° = βnFEΒ°cell
F = 96500 C/mol
Nernst Equation
E = EΒ° β (0.0592/n) log Q
At 25Β°C; Q = reaction quotient
EΒ° and K
log K = nEΒ°/0.0592
At equilibrium E = 0
Faraday's Law
m = MIt/(nF)
M = molar mass; n = electrons per ion
Charge Passed
Q = I Γ t
Q in coulombs; I in amperes; t in seconds
Specific Conductance
ΞΊ = G Γ (l/A)
l/A = cell constant
Molar Conductivity
Ξm = ΞΊ Γ 1000/C
C in mol/L; ΞΊ in S/cm
Worked Examples
5 problems β EΒ°cell, Nernst, Faraday, ΞG, and a Daniel cell trap.
EasyCalculate EΒ°cell for Zn-Cu Daniel cellβΎ
Calculate EΒ°cell for the cell Zn|ZnΒ²βΊ||CuΒ²βΊ|Cu. Given EΒ°(ZnΒ²βΊ/Zn) = β0.76 V, EΒ°(CuΒ²βΊ/Cu) = +0.34 V.
1
Zn is oxidised (anode), CuΒ²βΊ is reduced (cathode).
2
EΒ°cell = EΒ°cathode β EΒ°anode = 0.34 β (β0.76) = 0.34 + 0.76 = 1.10 V
β EΒ°cell = 1.10 V
EasyFind ΞGΒ° for a cell with EΒ°cell = 1.10 V, n = 2βΎ
Calculate ΞGΒ° for a galvanic cell with EΒ°cell = 1.10 V and n = 2.
1
ΞGΒ° = βnFEΒ°cell = β2 Γ 96500 Γ 1.10 = β2 Γ 96500 Γ 1.10
2
= β212300 J/mol = β212.3 kJ/mol
β ΞGΒ° = β212.3 kJ/mol (spontaneous)
MediumFind mass of Cu deposited by 2A current for 1 hourβΎ
Calculate the mass of copper deposited when a current of 2 A is passed through CuSOβ solution for 1 hour. (M = 64, n = 2)
1
Charge Q = I Γ t = 2 Γ 3600 = 7200 C
2
m = MQ/(nF) = 64 Γ 7200/(2 Γ 96500) = 460800/193000 = 2.39 g
β Mass of Cu deposited = 2.39 g
EAPCET LevelUse Nernst equation: EΒ°cell = 1.10 V, [ZnΒ²βΊ] = 0.1 M, [CuΒ²βΊ] = 0.01 MβΎ
Calculate E_cell for the Daniel cell when [ZnΒ²βΊ] = 0.1 M, [CuΒ²βΊ] = 0.01 M. EΒ°cell = 1.10 V, n = 2.
1
Nernst equation: E = EΒ° β (0.0592/n) log Q
2
Q = [ZnΒ²βΊ]/[CuΒ²βΊ] = 0.1/0.01 = 10 (products/reactants for cell reaction)
3
E = 1.10 β (0.0592/2) log 10 = 1.10 β 0.0296 Γ 1 = 1.07 V
β E_cell = 1.07 V
Trap QuestionThe species with higher reduction potential is always the cathode β always?βΎ
In a cell with EΒ°(AgβΊ/Ag) = +0.80 V and EΒ°(ZnΒ²βΊ/Zn) = β0.76 V, a student says 'Ag is always the cathode.' Is this correct under all conditions?
1
Under standard conditions: EΒ°cell = 0.80 β (β0.76) = 1.56 V > 0 β Ag is cathode. Correct here.
2
But under non-standard conditions: Nernst equation can change actual E values.
3
If [AgβΊ] is extremely low, E_Ag can drop below E_Zn, reversing the cell direction.
4
Conclusion: At standard conditions, higher EΒ° = cathode. At non-standard conditions, use Nernst equation to find actual E before deciding.
β Under standard conditions yes; under non-standard conditions, use Nernst equation to verify
Mistake DNA
4 electrochemistry errors from EAPCET distractor analysis.
π
Anode-Cathode Sign Confusion
In galvanic cells, anode is negative (β) and cathode is positive (+). In electrolytic cells, it's reversed.
β Wrong
Galvanic cell:
Anode = positive β
Cathode = negative β
β Correct
Galvanic: Anode(β), Cathode(+) β
Electrolytic: Anode(+), Cathode(β) β
Oxidation always at anode
Memory: 'An Ox, Red Cat' β Oxidation at Anode, Reduction at Cathode. This holds for BOTH cell types.
π
EΒ°cell = EΒ°anode β EΒ°cathode (Signs Swapped)
The formula is EΒ°cell = EΒ°cathode β EΒ°anode, both as reduction potentials.
β Wrong
EΒ°cell = EΒ°anode β EΒ°cathode
= (β0.76) β 0.34 = β1.10 V β
(sign error)
β Correct
EΒ°cell = EΒ°cathode β EΒ°anode
= 0.34 β (β0.76) = +1.10 V β
Spontaneous when positive
Both electrode potentials are quoted as STANDARD REDUCTION POTENTIALS. Subtract the anode from the cathode. Positive result = spontaneous cell.
β±οΈ
Forgetting to Convert Hours to Seconds in Faraday's Law
Q = It requires I in amperes and t in seconds. Students forget to convert hours/minutes to seconds.
β Wrong
2A for 1 hour:
m = MΓ2Γ1/(nF) β
(t=1, should be 3600)
β Correct
t = 1h = 3600 s
Q = 2 Γ 3600 = 7200 C β
m = MΓ7200/(nΓ96500) β
Faraday's Law: Q = It where t is in SECONDS. Always convert: 1 min = 60 s; 1 hour = 3600 s.
π’
Nernst Q: Writing Concentration of Solids/Pure Liquids
Pure solids and liquids have activity = 1 and are NOT included in Q for Nernst equation.
β Wrong
Zn + CuΒ²βΊ β ZnΒ²βΊ + Cu:
Q = [ZnΒ²βΊ][Cu]/([Zn][CuΒ²βΊ]) β
(Zn and Cu are solids)
β Correct
Q = [ZnΒ²βΊ]/[CuΒ²βΊ] β
Solids and pure liquids
are excluded from Q
In Q (and K) expressions, only aqueous ions and gases appear. Solid Zn and Cu metal have activity = 1 β they don't contribute to Q.
Chapter Intelligence
Electrochemistry is heavily numerical β practise Faraday calculations and Nernst equation.
EAPCET Weightage (2019β2024)
Faraday's law (electrolysis)~7 ΞGΒ° and EΒ°cell relation~5 Conductance & Kohlrausch~3 High-Yield PYQ Patterns
EΒ°cell from standard potentialsMass deposited by Faraday's lawNernst equation E calculationΞGΒ° = βnFEΒ° numericallog K from EΒ°cellIdentify anode and cathode
Exam Strategy
- For EΒ°cell: always identify which electrode is anode (lower EΒ°/more negative) and which is cathode (higher EΒ°/more positive). Then EΒ°cell = EΒ°cat β EΒ°an.
- For Faraday's Law: Q = It (seconds!), then m = MQ/(nF). The two most common errors: wrong time units, wrong n value.
- Nernst equation: at 25Β°C, the coefficient is 0.0592/n. Calculate Q from the cell reaction (products/reactants of the cell reaction, not just the solution concentrations).
- ΞGΒ° = βnFEΒ°cell. If EΒ°cell > 0, ΞGΒ° < 0 β spontaneous. This is a very fast MCQ connection question.
- Links to Equilibrium: nFEΒ° = RT ln K β log K = nEΒ°/0.0592. This bridges electrochemistry and equilibrium.
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