Questions & explanations
1. The correct cell notation for a Daniell cell is:
- A) Zn(s) | ZnSO₄(aq) || CuSO₄(aq) | Cu(s)
- B) Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
- C) Zn(s) | Zn²⁺(aq) | Cu²⁺(aq) | Cu(s)
- D) Cu(s) | CuSO₄(aq) || ZnSO₄(aq) | Zn(s)
Answer: A) Zn(s) | ZnSO₄(aq) || CuSO₄(aq) | Cu(s)
In the standard cell notation, the anode (where oxidation occurs) is written on the left, followed by its electrolyte; then a double vertical line (‖) represents the salt bridge; then the cathode electrolyte and electrode on the right. For the Daniell cell, the anode is zinc metal in zinc sulfate solution, and the cathode is copper metal in copper sulfate solution. Option B uses ion symbols instead of the salt, which is also sometimes used but A is the conventional representation. Option C uses a single vertical line instead of a double line for the salt bridge, and option D reverses the order (copper on left, zinc on right).
2. Which of the following statements correctly distinguishes a galvanic cell from an electrolytic cell?
- A) In a galvanic cell, a non-spontaneous chemical reaction is driven by electrical energy; in an electrolytic cell, a spontaneous reaction generates electricity.
- B) In a galvanic cell, the anode is positive and the cathode is negative; in an electrolytic cell, the anode is negative and the cathode is positive.
- C) In a galvanic cell, chemical energy is spontaneously converted into electrical energy; in an electrolytic cell, electrical energy is used to drive a non-spontaneous chemical reaction.
- D) In a galvanic cell, reduction occurs at the anode; in an electrolytic cell, oxidation occurs at the cathode.
Answer: C) In a galvanic cell, chemical energy is spontaneously converted into electrical energy; in an electrolytic cell, electrical energy is used to drive a non-spontaneous chemical reaction.
A galvanic (voltaic) cell produces electricity from a spontaneous redox reaction, while an electrolytic cell uses an external electrical source to force a non-spontaneous reaction to occur. Option A reverses the definitions, option B incorrectly describes electrode signs (in a galvanic cell, anode is negative, cathode positive; in an electrolytic cell, anode is positive, cathode negative), and option D incorrectly assigns oxidation/reduction sites (oxidation occurs at the anode in both cells).
3. Which of the following is an advantage of a hydrogen-oxygen fuel cell over conventional batteries?
- It can be recharged multiple times.
- It produces electricity continuously as long as fuel is supplied.
- It operates at very high temperatures only.
- It uses solid electrodes that do not corrode.
Answer: It produces electricity continuously as long as fuel is supplied.
A hydrogen-oxygen fuel cell is a type of galvanic cell (distinct from conventional primary and secondary cells) that generates electricity directly from the chemical reaction between hydrogen and oxygen. Unlike conventional batteries that store a fixed amount of energy and require recharging or replacement, a fuel cell can produce electrical energy continuously as long as hydrogen and oxygen are supplied to it. This continuous operation is a key advantage.
4. In the measurement of conductance of an electrolyte using a conductivity cell, the cell constant (k) is determined experimentally. Which of the following relations correctly gives the cell constant if κ is the specific conductivity of a standard solution and G is the measured conductance?
- k = κ / R
- k = G × κ
- k = κ / G
- k = R × κ
Answer: k = κ / G
The cell constant (k) is defined as l/A for a conductivity cell. The measured conductance G = κ A/l = κ / k, therefore k = κ / G. Option C correctly represents this relationship. Option A uses resistance (R) instead of conductance (G), which is incorrect because conductance is the reciprocal of resistance. Option B multiplies G and κ, which would give units of κ², not l/A. Option D multiplies R and κ, which is also dimensionally inconsistent.
5. What is the primary function of the salt bridge in a Daniell cell?
- A) To allow the flow of electrons between the two half-cells.
- B) To maintain electrical neutrality by allowing ions to migrate between the half-cells.
- C) To prevent the two solutions from mixing completely.
- D) To provide a path for the electric current to flow in the external circuit.
Answer: B) To maintain electrical neutrality by allowing ions to migrate between the half-cells.
The salt bridge completes the internal circuit by permitting the movement of ions from one half-cell to the other, thereby balancing the charge buildup that results from the redox reactions. Electron flow occurs through the external wire, not through the salt bridge (so A and D are incorrect). While the salt bridge also helps prevent gross mixing, the primary purpose is to maintain electrical neutrality (C is only a secondary effect).
6. Based on the electrochemical series, which of the following reactions is feasible under standard conditions?
- A) Cu(s) + ZnSO₄(aq) → CuSO₄(aq) + Zn(s)
- B) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
- C) Fe(s) + Al₂(SO₄)₃(aq) → FeSO₄(aq) + Al(s)
- D) Ag(s) + FeSO₄(aq) → Ag₂SO₄(aq) + Fe(s)
Answer: B) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
The electrochemical series arranges metals in order of decreasing reducing power. Zinc is more reactive (has a more negative reduction potential) than copper, so it can displace copper from its salt solution. In contrast, copper cannot displace zinc (A), iron cannot displace aluminium (C), and silver cannot displace iron (D) because the reacting metal is less reactive than the metal in the salt.
7. Rusting of iron is an electrochemical process. Which reaction occurs at the anode during the rusting of iron?
- Fe → Fe²⁺ + 2e⁻
- O₂ + 2H₂O + 4e⁻ → 4OH⁻
- Fe²⁺ + 2OH⁻ → Fe(OH)₂
- 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃
Answer: Fe → Fe²⁺ + 2e⁻
During rusting, the surface of iron acts as the anode where iron atoms lose electrons and go into solution as Fe²⁺ ions. This oxidation half-reaction is the initial step in the electrochemical corrosion of iron. The electrons released flow to cathodic areas where oxygen reduction occurs (option B is the cathodic reaction). Options C and D are subsequent chemical steps leading to rust formation.
8. A concentration cell is constructed with two copper electrodes immersed in CuSO₄ solutions of 0.1 M and 1.0 M concentrations. Which of the following statements is correct?
- The cell potential is zero because both electrodes are copper.
- The cell will produce electricity until the concentrations become equal.
- Electrons flow from the higher concentration to the lower concentration.
- The standard cell potential is 0.059 V.
Answer: The cell will produce electricity until the concentrations become equal.
In a concentration cell, the potential arises from the concentration difference; the cell operates until the concentrations equalize (Q=1, E=0). The other statements are false: potential is non‑zero when concentrations differ, electrons flow from lower to higher concentration, and the standard cell potential (both half‑cells same) is zero.
9. The electrode potential of the standard hydrogen electrode (SHE) is defined as:
- A) +0.34 V
- B) 0.00 V
- C) -0.76 V
- D) +1.00 V
Answer: B) 0.00 V
By international convention, the standard hydrogen electrode (SHE) is assigned an electrode potential of exactly 0.00 V at all temperatures. It serves as the reference for measuring all other standard electrode potentials. The other values are standard reduction potentials of other electrodes (e.g., Cu²⁺/Cu is +0.34 V, Zn²⁺/Zn is -0.76 V).
10. For a galvanic cell reaction where n = 2 and the standard cell potential E°cell = 1.10 V, what is the standard Gibbs free energy change ΔG°? (Faraday constant F = 96500 C mol⁻¹)
- A) –212.3 kJ
- B) +212.3 kJ
- C) –424.6 kJ
- D) +424.6 kJ
Answer: A) –212.3 kJ
The relationship is ΔG° = – n F E°cell. Substituting n=2, F=96500 C mol⁻¹, and E°cell=1.10 V gives ΔG° = – (2)(96500)(1.10) = –212300 J = –212.3 kJ. The negative sign indicates the reaction is spontaneous. Option C would result if n were taken as 4, and options B and D are positive, which would correspond to non-spontaneous reactions.
11. Electrical conductance (G) of a solution is defined as:
- G = 1/R
- G = R
- G = κl/A
- G = ρl/A
Answer: G = 1/R
Conductance is the reciprocal of resistance, i.e., G = 1/R. The other options are incorrect: G = R defines resistance, G = κl/A is dimensionally wrong (κl/A has units of conductance? actually κl/A would be S·m/m² = S/m, not S), and G = ρl/A is the formula for resistance (R = ρl/A).
12. During the discharge of a lead storage battery, the reaction occurring at the anode is:
- Pb + SO₄²⁻ → PbSO₄ + 2e⁻
- PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O
- Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
- PbSO₄ + 2e⁻ → Pb + SO₄²⁻
Answer: Pb + SO₄²⁻ → PbSO₄ + 2e⁻
In a lead storage battery during discharge, the anode (lead plate) undergoes oxidation. Lead metal loses two electrons to form Pb²⁺, which immediately combines with sulfate ions from the electrolyte to form insoluble lead sulfate. This reaction is the correct anodic half-reaction.