Electrochemistry: Multiple Choice Questions

Multiple Choice Questions (MCQs) and Explanations

Here are 40 MCQs based on the “Electrochemistry” notes, with detailed explanations.

Question 1: In a galvanic cell, which electrode is the anode? (A) The electrode where reduction occurs. (B) The positive terminal. (C) The electrode where oxidation occurs. (D) The electrode connected to the positive terminal of the voltmeter.

Explanation 1:

Correct Answer: (C)
Reasoning: By definition, oxidation always occurs at the anode, regardless of the cell type. In a galvanic cell, the anode is the negative terminal.

Question 2: The standard electrode potential of SHE (Standard Hydrogen Electrode) is: (A) 1.00 V (B) 0.50 V (C) 0.00 V (D) −1.00 V

Explanation 2:

Correct Answer: (C)
Reasoning: The standard hydrogen electrode (SHE) is defined as having a standard electrode potential of 0.00 V and serves as a reference for measuring other electrode potentials.

Question 3: What is the relationship between ΔG∘ and Ecell∘ for a spontaneous reaction? (A) ΔG∘=nFEcell∘ (B) ΔG∘=−nFEcell∘ (C) ΔG∘=RTlnEcell∘ (D) ΔG∘=−RTlnEcell∘

Explanation 3:

Correct Answer: (B)
Reasoning: The standard Gibbs energy change (ΔG∘) for a redox reaction in an electrochemical cell is related to the standard cell potential (Ecell∘) by the equation ΔG∘=−nFEcell∘. A spontaneous reaction has a negative ΔG∘ and a positive Ecell∘.

Question 4: According to Faraday’s First Law of Electrolysis, the mass of substance deposited or liberated at an electrode is directly proportional to the: (A) Resistance of the electrolyte. (B) Time of electrolysis. (C) Quantity of electricity passed. (D) Concentration of the electrolyte.

Explanation 4:

Correct Answer: (C)
Reasoning: Faraday’s First Law states W∝Q, where W is the mass deposited and Q is the quantity of electricity (charge) passed.

Question 5: The unit of molar conductivity (Λm) is: (A) S cm−1 (B) Ω cm (C) S cm2 mol−1 (D) Ω−1 m−1

Explanation 5:

Correct Answer: (C)
Reasoning: Molar conductivity is conductivity (S cm−1) divided by molarity (mol L−1 or mol cm−3). When molarity is in mol cm−3 (which is Molarity / 1000), the units become S cm2 mol−1.

Question 6: Which of the following is a primary battery? (A) Lead-acid battery (B) Nickel-cadmium cell (C) Dry cell (D) Fuel cell

Explanation 6:

Correct Answer: (C)
Reasoning: Primary batteries are non-rechargeable. The dry cell (Leclanché cell) is a common example of a primary battery. Lead-acid and nickel-cadmium cells are secondary (rechargeable) batteries, and fuel cells continuously produce electricity as long as reactants are supplied.

Question 7: Corrosion of iron is commonly known as: (A) Galvanization (B) Rusting (C) Electrolysis (D) Reduction

Explanation 7:

Correct Answer: (B)
Reasoning: Rusting is the specific term for the corrosion of iron, which forms hydrated ferric oxide (Fe2O3⋅xH2O).

Question 8: The Nernst equation at 298 K for the reaction Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s) is: (A) Ecell=Ecell∘−20.0592log[Cu2+][Zn2+] (B) Ecell=Ecell∘−20.0592log[Zn2+][Cu2+] (C) Ecell=Ecell∘+20.0592log[Cu2+][Zn2+] (D) Ecell=Ecell∘−20.0592log[Zn2+][Cu2+]

Explanation 8:

Correct Answer: (A)
Reasoning: The Nernst equation is Ecell=Ecell∘−n0.0592logQ. For this reaction, n=2 (two electrons exchanged), and Q=[Reactants][Products]=[Cu2+][Zn2+] (solids are not included in Q).

Question 9: For a strong electrolyte, how does molar conductivity (Λm) change with dilution? (A) Decreases (B) Increases (C) Remains constant (D) First decreases then increases

Explanation 9:

Correct Answer: (B)
Reasoning: For strong electrolytes, molar conductivity increases slightly with dilution. This is because interionic attractions decrease with increased distance between ions upon dilution, allowing ions to move more freely.

Question 10: According to Kohlrausch’s Law, the molar conductivity of a weak electrolyte at infinite dilution can be calculated from: (A) The conductivity of its concentrated solution. (B) The sum of the limiting molar conductivities of its constituent ions. (C) The product of conductivity and molarity. (D) The resistivity of the solution.

Explanation 10:

Correct Answer: (B)
Reasoning: Kohlrausch’s Law states that at infinite dilution, the molar conductivity of an electrolyte is the sum of the individual limiting molar conductivities of its cation and anion (Λm∘=λc∘+λa∘). This is particularly useful for weak electrolytes, whose Λm∘ cannot be found by extrapolation.

Question 11: In an electrolytic cell, oxidation occurs at the: (A) Cathode (B) Anode (C) Salt bridge (D) Electrolyte

Explanation 11:

Correct Answer: (B)
Reasoning: Oxidation always occurs at the anode, regardless of whether it’s a galvanic or electrolytic cell. In an electrolytic cell, the anode is the positive terminal.

Question 12: If the Ecell∘ for a reaction is positive, then ΔG∘ will be: (A) Positive (B) Negative (C) Zero (D) Cannot be determined

Explanation 12:

Correct Answer: (B)
Reasoning: From ΔG∘=−nFEcell∘, if Ecell∘ is positive, then ΔG∘ will be negative, indicating a spontaneous reaction under standard conditions.

Question 13: The main purpose of a salt bridge in an electrochemical cell is to: (A) Generate voltage. (B) Provide a path for electron flow. (C) Maintain electrical neutrality. (D) Store charge.

Explanation 13:

Correct Answer: (C)
Reasoning: The salt bridge allows the migration of ions between the two half-cells, preventing charge build-up and maintaining electrical neutrality, which is essential for the continuous flow of current.

Question 14: Which of the following metals is most commonly used for sacrificial protection of iron? (A) Copper (B) Silver (C) Zinc (D) Lead

Explanation 14:

Correct Answer: (C)
Reasoning: Zinc is more electropositive than iron, so it preferentially corrodes (oxidizes) when coated on iron, providing cathodic protection. This process is called galvanization.

Question 15: What happens to the conductivity of an electrolytic solution with dilution? (A) Decreases (B) Increases (C) Remains constant (D) First decreases then increases

Explanation 15:

Correct Answer: (A)
Reasoning: Conductivity (κ) is the conductance of a unit volume of solution. Upon dilution, the number of ions per unit volume decreases, leading to a decrease in conductivity. Molar conductivity, however, increases with dilution.

Question 16: A fuel cell converts the chemical energy of a fuel directly into: (A) Heat energy (B) Mechanical energy (C) Electrical energy (D) Light energy

Explanation 16:

Correct Answer: (C)
Reasoning: Fuel cells are electrochemical devices that convert the chemical energy of a fuel (like H2) and an oxidant (like O2) directly into electrical energy without combustion.

Question 17: Which constant is used to relate ΔTf (depression in freezing point) to molality? (A) Kb (B) Kf (C) R (D) F

Explanation 17:

Correct Answer: (B)
Reasoning: Kf is the molal depression constant (or cryoscopic constant) specific to the solvent, used in the equation ΔTf=Kf×m.

Question 18: The phenomenon of electrochemical corrosion requires the presence of: (A) Metal and acid (B) Metal and base (C) Metal, oxygen, and water (D) Metal and salt

Explanation 18:

Correct Answer: (C)
Reasoning: Electrochemical corrosion, like the rusting of iron, requires the metal, an electrolyte (water or moisture), and an oxidizing agent (oxygen from air). The presence of salts can accelerate the process.

Question 19: Which of the following is a unit of resistivity? (A) Siemens (S) (B) Ohm (Ω) (C) Ohm-meter (Ω m) (D) Siemens per meter (S m−1)

Explanation 19:

Correct Answer: (C)
Reasoning: Resistivity (ρ) is the resistance of a conductor of unit length and unit cross-sectional area. Its unit is Ohm-meter (Ω m). Ohm is for resistance, Siemens for conductance, and Siemens per meter for conductivity.

Question 20: If the degree of dissociation (α) of a weak electrolyte is 0.5 and it dissociates into 3 ions, what is its van’t Hoff factor (i)? (A) 1.5 (B) 2.0 (C) 1.0 (D) 2.5

Explanation 20:

Correct Answer: (B)
Reasoning: For dissociation, i=1+(n−1)α. Given α=0.5 and n=3. i=1+(3−1)×0.5=1+2×0.5=1+1=2.

Question 21: In a lead-acid battery, what is the role of PbO2 during discharging? (A) It acts as the anode and gets oxidized. (B) It acts as the cathode and gets reduced. (C) It acts as the electrolyte. (D) It is an inert electrode.

Explanation 21:

Correct Answer: (B)
Reasoning: During discharging, PbO2 is the cathode where reduction occurs: PbO2(s)+SO42−(aq)+4H+(aq)+2e−→PbSO4(s)+2H2O(l).

Question 22: Which equation directly relates Ecell∘ to the equilibrium constant Kc? (A) Ecell∘=nFRTlnKc (B) Ecell∘=−nFEcell∘ (C) Ecell∘=Kc×RT (D) Ecell∘=RTnFlnKc

Explanation 22:

Correct Answer: (A)
Reasoning: The relationship is derived from ΔG∘=−nFEcell∘ and ΔG∘=−RTlnKc, leading to Ecell∘=nFRTlnKc.

Question 23: When a salt bridge is removed from an electrochemical cell, the voltage drops to zero because: (A) The electrodes lose their charge. (B) The circuit becomes open. (C) Electrical neutrality is not maintained. (D) The electrolyte evaporates.

Explanation 23:

Correct Answer: (C)
Reasoning: The salt bridge’s primary function is to maintain electrical neutrality in the half-cells. Without it, charge builds up, preventing the further flow of electrons and thus stopping the reaction and dropping the voltage to zero.

Question 24: If the molar conductivity of CH3COOH at a given concentration is Λm and at infinite dilution is Λm∘, the degree of dissociation (α) is given by: (A) α=Λm×Λm∘ (B) α=Λm/Λm∘ (C) α=Λm∘/Λm (D) α=Λm+Λm∘

Explanation 24:

Correct Answer: (B)
Reasoning: The degree of dissociation (α) for a weak electrolyte is the ratio of its molar conductivity at a given concentration to its molar conductivity at infinite dilution (α=Λm∘Λm).

Question 25: Which of the following materials is typically used as the cathode in a dry cell? (A) Zinc container (B) Carbon (graphite) rod (C) Manganese dioxide (D) Both (B) and (C) as a mixture around the rod.

Explanation 25:

Correct Answer: (D)
Reasoning: In a dry cell, the central carbon (graphite) rod acts as the cathode, but it is surrounded by a paste of powdered MnO2 and carbon powder, which participates in the cathodic reduction reaction.

Question 26: Hydrogen-oxygen fuel cells generate water as a byproduct. This makes them: (A) Highly polluting. (B) Non-polluting. (C) Less efficient than other cells. (D) Suitable only for small-scale applications.

Explanation 26:

Correct Answer: (B)
Reasoning: The overall reaction in a hydrogen-oxygen fuel cell is the formation of water from hydrogen and oxygen. This means there are no harmful emissions, making them non-polluting and environmentally friendly.

Question 27: The unit of electrochemical equivalent (Z) is: (A) g s−1 (B) C g−1 (C) g C−1 (D) mol C−1

Explanation 27:

Correct Answer: (C)
Reasoning: From Faraday’s First Law, W=ZIt. So, Z=W/(It)=W/Q. Since W is mass (g) and Q is charge (C), the unit of Z is g C−1.

Question 28: Why does the molar conductivity of a weak electrolyte increase significantly with dilution? (A) Increase in mobility of ions. (B) Increase in the number of ions due to increased dissociation. (C) Decrease in interionic attraction. (D) Decrease in resistivity.

Explanation 28:

Correct Answer: (B)
Reasoning: For weak electrolytes, dilution causes a significant increase in the degree of dissociation (α), leading to a large increase in the number of free ions in the solution, which is the primary reason for the increase in molar conductivity.

Question 29: A concentration cell is formed when: (A) Two different metals are dipped in the same electrolyte. (B) Same metal electrodes are dipped in solutions of different concentrations of its own ions. (C) Different electrolytes are used with the same electrodes. (D) A fuel cell is used.

Explanation 29:

Correct Answer: (B)
Reasoning: A concentration cell is an electrochemical cell in which the two half-cells are composed of the same electrode material but differ in the concentration of the electrolyte solution. The potential difference arises from the tendency to equalize concentrations.

Question 30: Which of the following defines a non-spontaneous chemical reaction driven by electrical energy? (A) Galvanic cell (B) Electrolysis (C) Corrosion (D) Fuel cell

Explanation 30:

Correct Answer: (B)
Reasoning: Electrolysis is the process of using external electrical energy to force a non-spontaneous redox reaction to occur in an electrolytic cell.

Question 31: If the temperature of an electrochemical cell is increased, the cell potential (Ecell) will: (A) Always increase. (B) Always decrease. (C) May increase or decrease depending on the reaction entropy (ΔS). (D) Remain constant.

Explanation 31:

Correct Answer: (C)
Reasoning: The temperature dependence of cell potential is given by (∂T∂Ecell)P=nFΔS. Therefore, the change in cell potential with temperature depends on the entropy change (ΔS) of the cell reaction. If ΔS is positive, Ecell increases with T; if ΔS is negative, Ecell decreases with T.

Question 32: The mercury cell is preferred for hearing aids and watches because: (A) It is cheap. (B) It gives a constant voltage throughout its life. (C) It is rechargeable. (D) It has high capacity.

Explanation 32:

Correct Answer: (B)
Reasoning: The mercury cell provides a constant voltage (approximately 1.35 V) throughout its operating life because the overall cell reaction does not involve the change in concentration of any ions in the solution. This is crucial for devices requiring a steady power supply.

Question 33: The relationship Ecell∘=n0.0592log10Kc is applicable at what temperature? (A) 0 K (B) 273 K (C) 298 K (D) Any temperature

Explanation 33:

Correct Answer: (C)
Reasoning: The factor 0.0592 comes from the conversion of FRTln to F2.303RTlog10 at a specific temperature of 298 K (25∘C).

Question 34: Which term is known as the reciprocal of specific resistance? (A) Conductance (B) Resistance (C) Conductivity (D) Molar conductivity

Explanation 34:

Correct Answer: (C)
Reasoning: Specific resistance is resistivity (ρ), and its reciprocal is conductivity (κ). Conductance is the reciprocal of resistance.

Question 35: In the process of rusting of iron, what is the fate of Fe2+ ions initially formed at the anode? (A) They are reduced to Fe. (B) They remain as Fe2+ in solution. (C) They are further oxidized to Fe3+ and form hydrated ferric oxide. (D) They migrate to the cathode.

Explanation 35:

Correct Answer: (C)
Reasoning: The Fe2+ ions produced at the anode are highly unstable and are further oxidized by atmospheric oxygen to Fe3+ ions, which then combine with water to form hydrated ferric oxide (Fe2O3⋅xH2O), which is rust.

Question 36: How does the conductivity of a solution of a strong electrolyte change with increasing concentration? (A) Increases (B) Decreases (C) Remains constant (D) First increases then decreases

Explanation 36:

Correct Answer: (A)
Reasoning: As the concentration of a strong electrolyte increases, the number of ions per unit volume increases, leading to higher conductivity. However, molar conductivity decreases slightly due to increased interionic interactions.

Question 37: Which law states that the masses of different substances liberated by the same quantity of electricity are proportional to their equivalent masses? (A) Henry’s Law (B) Raoult’s Law (C) Faraday’s First Law (D) Faraday’s Second Law

Explanation 37:

Correct Answer: (D)
Reasoning: This is the statement of Faraday’s Second Law of Electrolysis.

Question 38: What is the condition for a solution to be hypotonic relative to a cell? (A) Solution has higher osmotic pressure than the cell. (B) Solution has lower osmotic pressure than the cell. (C) Solution has same osmotic pressure as the cell. (D) Solution is saturated.

Explanation 38:

Correct Answer: (B)
Reasoning: A hypotonic solution has a lower solute concentration and thus a lower osmotic pressure than the cell. This causes water to move from the hypotonic solution into the cell, leading to swelling or even bursting of the cell.

Question 39: In the context of corrosion, what is “galvanization”? (A) Coating iron with copper. (B) Painting iron to prevent rust. (C) Coating iron with zinc. (D) Alloying iron with carbon.

Explanation 39:

Correct Answer: (C)
Reasoning: Galvanization is a method of preventing corrosion by coating iron with a layer of zinc. Zinc acts as a sacrificial anode, protecting the iron even if the coating is scratched.

Question 40: Which of the following is an advantage of fuel cells? (A) They are easy to recharge. (B) They produce a lot of waste products. (C) They have high efficiency and are pollution-free. (D) They are very small and portable.

Explanation 40:

Correct Answer: (C)
Reasoning: Fuel cells are known for their high energy conversion efficiency and environmentally friendly nature, as their byproducts are often harmless (like water in the case of H2-O2 fuel cell). They are not typically recharged in the traditional sense but rather continuously supplied with fuel.