Electrochemistry
In the electrorefining of copper, impure copper acts as:
A
Cathode
B
Anode
C
Salt bridge
D
Electrolyte
In electrorefining, impure copper acts as anode and undergoes oxidation. Pure copper deposits at cathode. More reactive impurities go into solution.
The Gibbs free energy change for an electrochemical cell reaction is related to cell potential by:
A
ΔG = nFE
B
ΔG° = -nFE°
C
ΔG = E/nF
D
ΔG° = nFE°
Correct Answer:
B. ΔG° = -nFE°
The standard free energy change ΔG° = -nFE°cell, where n is moles of electrons, F is Faraday constant, and E° is standard cell potential.
During the electrolysis of aqueous CuSO₄ solution with copper electrodes, which reaction occurs at the cathode?
A
SO₄²⁻ → products
B
Cu²⁺ + 2e⁻ → Cu
C
H₂O → H⁺ + OH⁻
D
2H⁺ + 2e⁻ → H₂
Correct Answer:
B. Cu²⁺ + 2e⁻ → Cu
At cathode with copper electrodes in CuSO₄: Cu²⁺ ions are preferentially reduced as their reduction potential (+0.34 V) is higher than H⁺ (-0.83 V).
The standard reduction potential of H⁺/H₂ electrode is taken as 0.00 V because:
A
Hydrogen is the most abundant element
B
It serves as a reference electrode for all other half-cells
C
Hydrogen has zero atomic number
D
It's the easiest element to oxidize
Correct Answer:
B. It serves as a reference electrode for all other half-cells
SHE (Standard Hydrogen Electrode) is the arbitrary reference against which all other reduction potentials are measured.
The relationship between equivalent conductance (Λ) and molar conductance (Λm) is:
A
Λ = Λm × n
B
Λm = Λ × n
C
Λ = Λm/n
D
Λm = Λ/M
Correct Answer:
A. Λ = Λm × n
Λm = κ × 1000/c (molar conductance), Λ = κ × 1000/(c/n) (equivalent conductance). Therefore, Λm = Λ/n or Λ = Λm × n.
In a Daniel cell, the mass of zinc electrode decreases and copper electrode increases. This indicates:
A
Zn acts as anode and Cu as cathode
B
Both electrodes are anodes
C
Both electrodes are cathodes
D
Cu acts as anode and Zn as cathode
Correct Answer:
A. Zn acts as anode and Cu as cathode
Anode undergoes oxidation (Zn → Zn²⁺ + 2e⁻), losing mass. Cathode undergoes reduction (Cu²⁺ + 2e⁻ → Cu), gaining mass.
A current of 5 A is passed through an electrolytic cell for 1930 seconds. Calculate the number of moles of electrons transferred. (Faraday constant = 96500 C/mol)
A
0.1 mol
B
0.5 mol
C
1.0 mol
D
2.0 mol
Correct Answer:
A. 0.1 mol
Charge = I × t = 5 × 1930 = 9650 C. Moles of e⁻ = 9650/96500 = 0.1 mol.
The molar conductivity of strong electrolytes follows which relationship with concentration?
A
Increases linearly with concentration
B
Decreases with √concentration according to Debye-Hückel law
C
Remains independent of concentration
D
Increases exponentially with concentration
Correct Answer:
B. Decreases with √concentration according to Debye-Hückel law
For strong electrolytes: Λm = Λ°m - A√c (Debye-Hückel-Onsager equation), showing decrease with √c.
In the electrolysis of molten NaCl using inert electrodes, what is produced at the cathode?
A
Cl₂ gas
B
Na metal
C
O₂ gas
D
NaOH solution
Correct Answer:
B. Na metal
At cathode: Na⁺ + e⁻ → Na (reduction). At anode: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation).
The conductivity of a solution decreased upon dilution. This is because:
A
Molar conductivity remains constant
B
Number of ions per unit volume decreases
C
Degree of ionization increases
D
Ionic mobility increases significantly
Correct Answer:
B. Number of ions per unit volume decreases
Conductivity (κ) = concentration × molar conductivity. Upon dilution, concentration decreases faster than molar conductivity increases.
