Electrochemistry and Electrolysis – NDA Chemistry PYQs

Q1. NDA 2012-II
Which one among the following is an electrochemical cell that cannot be charged?
(a) Electrolytic cell (b) Storage cell (c) Primary cell (d) Fuel cell
Answer: (c) Primary cell
A primary cell (like a dry cell) converts chemical energy to electricity only once. When the chemicals are used up, it cannot be recharged. A storage cell (secondary cell) can be recharged. A fuel cell is not rechargeable in the traditional sense but works differently. An electrolytic cell uses electricity, it does not produce it.

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Q2. CDS 2009-I
Which of the following statements about the commonly used automobile battery are true?
I. It is usually a lead-acid battery.
II. It has six cells with a potential of 2V each.
III. Its cells work as galvanic cells while discharging power.
IV. Its cells work as electrolytic cells while recharging.
(a) I, II, III and IV (b) I, II and III (c) III and IV (d) I, II and IV (Wait — verify options)
Answer: (a) I, II, III and IV
All four statements are correct. The standard automobile battery is a lead-acid battery. It has 6 cells each producing 2V (total 12V). During use (discharge), the cells act as galvanic cells converting chemical energy to electrical energy. During recharging, the cells act as electrolytic cells converting electrical energy back to chemical energy.

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Q3. CDS 2011-II
Which one among the following will you put into pure water in order to pass electric current through it?
(a) Kerosene (b) Mustard oil (c) Lemon juice (d) Sugar
Answer: (c) Lemon juice
Lemon juice contains citric acid. Citric acid is a weak electrolyte. When dissolved in water, it ionises to some extent, producing H⁺ and citrate ions. These ions carry the electric current. Kerosene, mustard oil, and sugar are non-electrolytes. They do not produce ions.

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Q4. CDS 2011-II
Sacrificial anode protects iron from rusting. Which one of the following metals CANNOT be used as a sacrificial anode?
(a) Tin (b) Zinc (c) Magnesium (d) Aluminium
Answer: (a) Tin
The sacrificial anode must be MORE reactive than iron so that it corrodes preferentially and protects the iron. In the reactivity series: Mg > Al > Zn > Fe > Sn. Tin is LESS reactive than iron. If tin were attached to iron, the iron would corrode, not the tin. Zinc, magnesium, and aluminium are all more reactive than iron and can serve as sacrificial anodes.

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Q5. CDS 2015-I
Electricity is produced through a dry cell from:
(a) Chemical energy (b) Thermal energy (c) Mechanical energy (d) Nuclear energy
Answer: (a) Chemical energy
A dry cell is a primary galvanic cell. It converts the chemical energy stored in its reactants (zinc and manganese dioxide) directly into electrical energy. No thermal, mechanical, or nuclear energy is involved.

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Q6. CDS 2011-I
When items or jewellery made of metals such as copper or nickel are placed in a solution having a salt of gold, a thin film of gold is deposited by:
(a) thermal evaporation (b) electroplating (c) sublimation (d) chemical vapour deposition
Answer: (b) Electroplating
When copper or nickel jewellery is placed in a gold salt solution and current is passed, gold ions from the solution are reduced and deposit as a thin layer on the jewellery surface. The jewellery acts as the cathode. This process is electroplating.

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Q7. CDS 2016-II
In case of a standard hydrogen electrode:
(a) absolute electrode potential is not zero
(b) absolute electrode potential is zero
(c) both absolute and standard electrode potential values are zero
(d) electrode potential is zero only at 25°C
Answer: (d) electrode potential is zero only at 25°C
The standard electrode potential of the hydrogen electrode is defined as zero. This zero value is measured under standard conditions at 25°C. The absolute electrode potential (the actual potential relative to a vacuum reference) is NOT zero. So neither (a) which says absolute is not zero (true but incomplete) nor (c) which says both are zero (the absolute is not zero) is fully correct. The standard potential is zero only at the defined conditions (25°C, 1 atm H₂, 1 mol/L H⁺).

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Q8. CDS 2008-I
Misch metal is widely used in the manufacture of which of the following?
(a) Material of car brake (b) Smoke detectors (c) Cigarette lighters (d) Emergency lights
Answer: (c) Cigarette lighters
Misch metal is an alloy of rare earth metals (mainly cerium with lanthanum and other lanthanides). When struck, it produces bright sparks. This property makes it ideal for use in the flint mechanism of cigarette lighters. When the flint wheel is turned, it strikes the misch metal and sparks ignite the lighter fuel.

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Q9. NDA 2012-I
Which of the following statements regarding oxidation and reduction are correct?

1. In oxidation, loss of electron takes place whereas in reduction, gain of electron takes place.
2. In oxidation, gain of electron takes place whereas in reduction, loss of electron takes place.
3. Oxidising agent decreases the oxidation number but reducing agent increases the oxidation number.
4. Oxidising agent increases the oxidation number but reducing agent reduces the oxidation number.
   Code: (a) 1 and 3 (b) 2 and 4 (c) 1 and 4 (d) 2 and 3
   Answer: (c) 1 and 4
   Statement 1 is correct: oxidation = loss of electrons, reduction = gain of electrons. Statement 4 is correct: an oxidising agent oxidises others by taking their electrons, so the substance being oxidised increases in oxidation number; a reducing agent reduces others by giving electrons, so the substance being reduced decreases in oxidation number.

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Q10. NDA 2012-I
The following equation is an example of a redox reaction: 2FeBr₃(aq) + 3Cl₂(g) = 2FeCl₃(aq) + 3Br₂(l). Which one among the following statements is INCORRECT for this redox reaction?
(a) Oxidising agents are themselves reduced
(b) Reducing agents gain or appear to gain electrons
(c) Reducing agents are themselves oxidised
(d) Oxidising agents oxidise other substances
Answer: (b) Reducing agents gain or appear to gain electrons
This statement is incorrect. Reducing agents LOSE electrons (they are oxidised). It is oxidising agents that gain electrons (they are reduced). Statements (a), (c), and (d) are all correct.