Sulphur, Nitrogen, Halogen, Inert Gases

11. Which among the following is the popular method for manufacture of amm

Which among the following is the popular method for manufacture of ammonia ?

Ostwald's process
Haber-Bosch process
Electric furnace process
Electrolysis process
This question was previously asked in
UPSC NDA-1 – 2023
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The industrial manufacture of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂) is achieved through the Haber-Bosch process. This process involves reacting nitrogen from the air with hydrogen, usually derived from natural gas, at high temperature (400-450°C) and high pressure (150-250 atm) over an iron catalyst.
– Ammonia (NH₃) is synthesized from N₂ and H₂.
– The industrial method is called the Haber-Bosch process.
– It uses high temperature, high pressure, and an iron catalyst.
Ostwald’s process is used for the industrial production of nitric acid (HNO₃) from ammonia. Electrolysis is a process used to decompose substances using electricity. Electric furnaces are used for melting metals and other high-temperature processes, but not specifically for the Haber-Bosch synthesis.

12. Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but

Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but they do not react with each other to form oxides of nitrogen because

the reaction requires initiation by a catalyst
oxides of nitrogen are unstable
the reaction is endothermic and requires very high temperature
the stoichiometry of N₂ and O₂ in air is not ideal for the reaction to take place
This question was previously asked in
UPSC NDA-1 – 2019
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Dinitrogen (N₂) and dioxygen (O₂) molecules are very stable due to strong bonds (triple bond in N₂, double bond in O₂). The reaction between them to form oxides of nitrogen, such as nitric oxide (NO), is highly endothermic (requires energy input) and has a high activation energy barrier. Therefore, this reaction requires very high temperatures (e.g., found in lightning strikes or internal combustion engines) to proceed significantly. At normal atmospheric temperatures, there isn’t enough energy to break the strong N≡N bond and initiate the reaction.
– The reaction N₂ + O₂ → 2NO is endothermic (ΔH > 0).
– The strong triple bond in N₂ requires a large amount of energy to break.
– Standard atmospheric temperatures are insufficient to overcome the activation energy barrier for this reaction.
While a catalyst could potentially lower the activation energy, the primary reason the reaction doesn’t occur spontaneously in the air is the high energy requirement. Oxides of nitrogen, once formed, vary in stability, but their stability is not the reason for their lack of formation under normal conditions. The stoichiometry in air is about 4:1 N₂:O₂, which is not ideal for NO formation (1:1), but this ratio doesn’t prevent the reaction from occurring at all; it just affects equilibrium yield.

13. Radon is

Radon is

an inert gas
an artificial fibre
an explosive
a metal
This question was previously asked in
UPSC NDA-1 – 2017
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The question asks for the classification of Radon.
Radon (Rn) is a chemical element with atomic number 86. It is a radioactive gas. Based on its position in the periodic table (Group 18), Radon belongs to the group of noble gases (also known as inert gases). Noble gases are characterized by their filled valence electron shells, making them very unreactive chemically.
Let’s evaluate the options:
A) an inert gas: Correct. Radon is a noble gas, which is synonymous with being an inert gas due to its very low chemical reactivity.
B) an artificial fibre: Incorrect. Artificial fibres are synthetic textiles (e.g., nylon, polyester).
C) an explosive: Incorrect. Explosives are substances that undergo rapid expansion in volume with the release of large amounts of energy (e.g., TNT, gunpowder).
D) a metal: Incorrect. Radon is a non-metal (specifically, a gas at standard conditions).
Radon is a decay product of radium, which is itself a decay product of uranium. It is the heaviest noble gas. All isotopes of radon are radioactive. Radon is a significant environmental hazard, as it can accumulate in buildings, particularly basements, posing a health risk (lung cancer) due to inhalation of its radioactive decay products.

14. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

List I
(Noble gas)		 List II
(Use)
A. Argon 1. In lights for advertising display
B. Neon 2. Airport landing lights and in light houses
C. Krypton 3. Light in photographer’s flash gun
D. Xenon 4. In tungsten filament to last longer

Code :

3 1 2 4
3 2 1 4
4 2 1 3
4 1 2 3
This question was previously asked in
UPSC NDA-1 – 2017
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The correct match is A-4, B-1, C-2, D-3. This option correctly pairs each noble gas with one of its common or significant uses listed.
– Argon is widely used as an inert filling gas in incandescent light bulbs (tungsten filament lamps) to prevent the oxidation of the filament and reduce its evaporation, thereby increasing the bulb’s lifespan (List II – 4).
– Neon is famously used in “neon signs” for advertising displays, producing a characteristic red-orange glow when an electric current is passed through it (List II – 1).
– Krypton is used in various lighting applications, including some high-efficiency fluorescent lamps, halogen lamps, and is also used in some airport lighting systems and blended into other high-intensity discharge lamps (List II – 2 is a plausible use, although Xenon is more primary).
– Xenon is used in high-intensity discharge lamps such as those found in cinema projectors, high-end car headlights, airport landing lights, lighthouses (List II – 2), and in photographic flash lamps/guns (List II – 3).
Based on typical primary uses, A-4, B-1, C-3, D-2 would be the most accurate. However, examining the options provided, Option D matches A-4, B-1, C-2, and D-3. Matches A-4, B-1, and D-3 are correct and common uses. While Krypton (C) is not the primary gas for airport landing lights (which is usually Xenon), it is sometimes used in specific systems or blends. Xenon (D) is also a primary gas for photographer’s flash guns. Given the options, D provides the best set of matches, correctly pairing Argon with tungsten filaments, Neon with advertising displays, and Xenon with flash guns, leaving Krypton with a plausible secondary use in airport lighting.

15. Which one of the following oxides of nitrogen is known as ‘anhydride’

Which one of the following oxides of nitrogen is known as ‘anhydride’ of nitric acid ?

N<sub>2</sub>O
N<sub>2</sub>O<sub>3</sub>
NO<sub>2</sub>
N<sub>2</sub>O<sub>5</sub>
This question was previously asked in
UPSC NDA-1 – 2016
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An acid anhydride is a compound that reacts with water to form an acid. The anhydride of nitric acid (HNO₃) is formed by removing water from one or more molecules of the acid.
The formula for nitric acid is HNO₃. Taking two molecules and removing one water molecule:
2HNO₃ → H₂N₂O₆
H₂N₂O₆ – H₂O → N₂O₅
Thus, N₂O₅ (dinitrogen pentoxide) is the anhydride of nitric acid. It reacts with water to form nitric acid: N₂O₅ + H₂O → 2HNO₃.
– Anhydride: A compound that yields an acid when combined with water.
– To find the anhydride of an acid (like HNO₃), remove water molecule(s) from the acid formula while maintaining the correct ratio of other elements.
– N₂O₅ is dinitrogen pentoxide.
Similarly, the anhydride of sulfuric acid (H₂SO₄) is SO₃ (SO₃ + H₂O → H₂SO₄). The anhydride of carbonic acid (H₂CO₃) is CO₂ (CO₂ + H₂O ⇌ H₂CO₃). Oxides of non-metals are often acidic anhydrides.

16. Statement-I: Colour of nitrogen dioxide changes to colourless at low t

Statement-I: Colour of nitrogen dioxide changes to colourless at low temperature.
Statement-II: At low temperature Nitrogen tetroxide (N₂O₄) is formed which is colourless.

Both the statements are individually true and Statement II is the correct explanation of Statement I
Both the statements are individually true but Statement II is not the correct explanation of Statement I
Statement I is true but Statement II is false
Statement I is false but Statement II is true
This question was previously asked in
UPSC NDA-1 – 2015
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Both Statement I and Statement II are individually true, and Statement II is the correct explanation of Statement I.
Statement I is true; the reddish-brown colour of nitrogen dioxide gas disappears at low temperatures. Statement II is also true; at low temperatures, nitrogen dioxide dimerizes to form colourless nitrogen tetroxide (N₂O₄).
Nitrogen dioxide (NO₂) is an equilibrium mixture with its dimer, dinitrogen tetroxide (N₂O₄):
2NO₂(g) (reddish-brown) ⇌ N₂O₄(g) (colourless)
This dimerization reaction is exothermic (ΔH < 0). According to Le Chatelier's principle, decreasing the temperature shifts the equilibrium towards the exothermic reaction, favoring the formation of N₂O₄. As the temperature decreases, more NO₂ dimerizes into colourless N₂O₄, causing the colour of the gas mixture to fade to colourless. Statement II correctly identifies the formation of colourless N₂O₄ as the reason for the colour change.

17. Which one of the following elements is not monatomic?

Which one of the following elements is not monatomic?

Hydrogen
Helium
Neon
Argon
This question was previously asked in
UPSC Geoscientist – 2022
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The correct answer is Hydrogen.
Monatomic elements exist as single atoms under normal conditions. Noble gases (Group 18) are typically monatomic.
Helium (He), Neon (Ne), and Argon (Ar) are noble gases and exist as stable, single atoms. Hydrogen (H), along with nitrogen (N), oxygen (O), fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), exists as diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) in its elemental form under standard conditions. Therefore, hydrogen is not monatomic.

18. Which one of the following has the similar physical state with water a

Which one of the following has the similar physical state with water at room temperature?

Iodine
Bromine
Chlorine
Fluorine
This question was previously asked in
UPSC Geoscientist – 2020
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Room temperature is typically considered to be around 20-25°C. Water (H₂O) is a liquid at this temperature range and standard atmospheric pressure. Let’s examine the physical states of the given halogens (Group 17 elements) at room temperature:
A) Iodine (I₂) is a dark purple-black solid at room temperature.
B) Bromine (Br₂) is a reddish-brown liquid at room temperature. It is one of the few elements that are liquid under standard conditions.
C) Chlorine (Cl₂) is a yellowish-green gas at room temperature.
D) Fluorine (F₂) is a pale yellow gas at room temperature.
Comparing these states, Bromine (liquid) has a similar physical state to water (liquid) at room temperature.
At room temperature (20-25°C) and standard pressure, water is a liquid. Among the given halogens, only Bromine is a liquid.
The physical states of the halogens at room temperature follow a trend based on increasing molecular size and thus increasing strength of London dispersion forces: Fluorine (gas), Chlorine (gas), Bromine (liquid), Iodine (solid), Astatine (solid, highly radioactive). This trend relates to their boiling points and melting points.

19. Which one among the following is used in bleaching of paper ?

Which one among the following is used in bleaching of paper ?

Chloride
Chlorobenzene
Ammonium chloride
Hydrochloric acid
This question was previously asked in
UPSC CDS-2 – 2024
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Chloride (referring to chlorine-based compounds) is used in the bleaching of paper.
Bleaching of paper pulp is primarily done using oxidizing agents to remove lignin and whiten the pulp. Historically, elemental chlorine (Cl₂) was used, but it has largely been replaced by more environmentally friendly chlorine-based compounds like chlorine dioxide (ClO₂) or processes using oxygen, hydrogen peroxide, or ozone. Among the given options, “Chloride” is the most general term that points towards the use of chlorine-based bleaching agents.
Options B (Chlorobenzene) is an organic solvent and chemical intermediate, not a paper bleach. Option C (Ammonium chloride) is a salt used in various applications but not paper bleaching. Option D (Hydrochloric acid) is a strong acid, used in some industrial processes but not as the primary bleaching agent for paper pulp.

20. Which one of the following is monatomic?

Which one of the following is monatomic?

Hydrogen
Sulphur
Phosphorus
Helium
This question was previously asked in
UPSC CDS-2 – 2019
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A monatomic substance consists of individual atoms that are not bonded together to form molecules under standard conditions. Hydrogen exists as diatomic molecules (H2). Sulphur exists as polyatomic molecules, commonly S8 rings. Phosphorus commonly exists as P4 tetrahedra. Helium is a noble gas and exists as single atoms (He) under standard conditions, hence it is monatomic.
Monatomic substances are composed of single atoms; noble gases like Helium are monatomic.
All noble gases (Helium, Neon, Argon, Krypton, Xenon, Radon) are monatomic under typical conditions because they have stable electron configurations.
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