Carbon and Its Various Forms

11. Which one of the following statements about diamond and graphite is no

Which one of the following statements about diamond and graphite is not correct ?

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Diamond has a tetrahedral structure, whereas graphite has a hexagonal planar structure.
Both physical and chemical properties of diamond and graphite are different.
Graphite is soft but diamond is hard.
Graphite is a good conductor of electricity while diamond is not.
This question was previously asked in
UPSC NDA-2 – 2019
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Statement B, “Both physical and chemical properties of diamond and graphite are different”, is arguably the least accurate or potentially misleading statement compared to the others, which are clear factual distinctions. While their physical properties are vastly different, their chemical properties, stemming from being elemental carbon, share some fundamental similarities (e.g., both combust in oxygen to form CO2, although under different conditions and rates). The difference in chemical reactivity is significant due to their structures, but claiming *all* chemical properties are different might be considered an overstatement in some contexts. Statements A, C, and D are unequivocally correct and describe key differences between diamond and graphite. In competitive exams, if multiple statements are true, the question might hinge on identifying the statement that is not *completely* true or contains a subtle inaccuracy or overgeneralization.
– Diamond and graphite are allotropes of carbon with distinct crystal structures (tetrahedral for diamond, hexagonal planar layers for graphite).
– These structural differences lead to significant differences in physical properties like hardness, density, electrical conductivity, and thermal conductivity.
– While their reactivity differs significantly (graphite is more reactive than diamond), they are both carbon and participate in similar fundamental chemical reactions like combustion.
– Diamond is an electrical insulator and the hardest known natural material.
– Graphite is a good electrical conductor and is soft, used as a lubricant.
– The difference in properties arises from the difference in bonding and structure: sp3 hybridization in diamond forms a rigid 3D network, while sp2 hybridization in graphite forms layers held together by weak van der Waals forces.
– Statement B is considered “not correct” likely because the term “different” might be interpreted as “completely distinct with no commonalities”, which isn’t strictly true for chemical properties as they are both forms of carbon.

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12. Which one of the following statements is not correct?

Which one of the following statements is not correct?

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All carbons in diamond are linked by carbon-carbon single bond.
Graphite is layered structure in which layers are held together by weak van der Waals forces.
Graphite layers are formed by hexagonal rings of carbon atoms.
Graphite layers are held together by carbon-carbon single bond.
This question was previously asked in
UPSC NDA-2 – 2018
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The statement that graphite layers are held together by carbon-carbon single bonds is incorrect. Graphite layers (graphene sheets) are themselves held together by covalent bonds within the plane, but the forces *between* these layers are weak van der Waals forces.
– Diamond has a 3D tetrahedral structure where each carbon atom is covalently bonded to four other carbon atoms by single bonds (sp³ hybridization).
– Graphite has a layered structure. Each layer consists of carbon atoms arranged in hexagonal rings, with strong covalent bonds (sp² hybridization) within the layer.
– The layers in graphite are held together by relatively weak van der Waals forces. This allows the layers to slide easily over each other, giving graphite its properties as a lubricant and its softness.
The delocalized electrons within the layers of graphite allow it to conduct electricity, unlike diamond, which is an electrical insulator. The carbon-carbon bond length within a graphite layer is about 0.142 nm, while the distance between layers is about 0.335 nm.

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13. How much CO₂ is produced on heating of 1 kg of carbon ?

How much CO₂ is produced on heating of 1 kg of carbon ?

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11/3 kg
3/11 kg
4/3 kg
3/4 kg
This question was previously asked in
UPSC NDA-2 – 2017
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The balanced chemical equation for the complete combustion of carbon is:
C (s) + O₂ (g) → CO₂ (g)
The molar mass of Carbon (C) is approximately 12 g/mol.
The molar mass of Carbon Dioxide (CO₂) is approximately 12 g/mol (for C) + 2 * 16 g/mol (for O) = 44 g/mol.
According to the stoichiometry of the balanced equation, 1 mole of carbon reacts to produce 1 mole of carbon dioxide.
Therefore, 12 grams of carbon produce 44 grams of carbon dioxide.
To find the amount of CO₂ produced from 1 kg (1000 g) of carbon, we can use the ratio:
(Mass of CO₂ produced / Mass of C reacted) = (Molar mass of CO₂ / Molar mass of C)
Mass of CO₂ produced = (44 g / 12 g) * 1000 g
Mass of CO₂ produced = (11/3) * 1000 g = 11000/3 g
Converting grams to kilograms: 11000/3 g = (11000/3) / 1000 kg = 11/3 kg.
– Chemical reaction: C + O₂ → CO₂
– Molar mass ratio of CO₂ to C is 44:12, which simplifies to 11:3.
– The mass of CO₂ produced is 11/3 times the mass of carbon reacted.
This calculation assumes complete combustion of pure carbon. In real-world scenarios, combustion might be incomplete (producing CO) or the fuel might contain impurities, affecting the actual CO₂ yield. The calculation uses approximate standard atomic weights.

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14. Graphite is a much better conductor of heat and electricity than diamo

Graphite is a much better conductor of heat and electricity than diamond. This is due to the fact that each carbon atom in graphite:

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undergoes sp² hybridization and forms three sigma bonds with three neighbouring carbon atoms
undergoes sp³ hybridization
is tetrahedrally bonded
is free from van der Waals force
This question was previously asked in
UPSC NDA-2 – 2015
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The correct answer is A) undergoes sp² hybridization and forms three sigma bonds with three neighbouring carbon atoms.
Graphite’s structure consists of layers of carbon atoms arranged in hexagonal lattices. Each carbon atom in graphite is sp² hybridized and forms three strong sigma bonds with three adjacent carbon atoms within the same layer. The remaining unhybridized p-orbital on each carbon atom overlaps sideways with p-orbitals of neighbouring atoms, forming a delocalized pi electron system across the layer. These delocalized electrons are free to move within the layers, making graphite a good conductor of heat and electricity.
In contrast, diamond has a tetrahedral structure where each carbon atom is sp³ hybridized and forms four strong sigma bonds with four neighbouring carbon atoms. All valence electrons are localized in these covalent bonds, so there are no free electrons to conduct electricity, making diamond an excellent electrical insulator. The strong covalent network also makes diamond an excellent thermal conductor, but the question specifically asks about electrical conductivity compared to diamond, and graphite’s delocalized electrons are key. Graphite’s thermal conductivity is also generally higher than diamond in some directions due to strong in-plane bonds and vibration modes. However, the primary reason for its *better* electrical conductivity than diamond is the presence of delocalized electrons, which stems from its sp² hybridization and bonding structure described in option A.

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15. Which among the following statements is not correct with respect to al

Which among the following statements is not correct with respect to allotropes of Carbon ?

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Graphite is a good conductor of electricity.
Diamond is the hardest substance known.
Fullerene is one of the allotropes of carbon.
Next to diamond, graphite is the second hardest known substance.
This question was previously asked in
UPSC NDA-1 – 2023
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Diamond is indeed the hardest naturally occurring substance known. Graphite, another allotrope of carbon, has a layered structure that allows the layers to slide over each other. This makes graphite very soft, greasy to touch, and suitable for use as a lubricant and in pencil leads. It is far from being the second hardest known substance; many materials, both natural and synthetic, are significantly harder than graphite.
– Allotropes are different structural modifications of an element.
– Diamond is the hardest known substance.
– Graphite is soft and used as a lubricant.
– Fullerene is an allotrope of carbon.
– Graphite’s soft nature contradicts the statement that it is the second hardest substance.
Other allotropes of carbon include carbon nanotubes, graphene, and amorphous carbon. The different properties of carbon allotropes arise from the different ways carbon atoms are bonded together in their structures.

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16. Which one of the following allotropes of carbon is isomorphous with cr

Which one of the following allotropes of carbon is isomorphous with crystalline silicon?

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Coke
Diamond
Graphite
Coal
This question was previously asked in
UPSC NDA-1 – 2022
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The correct answer is Diamond. Diamond is a crystalline allotrope of carbon that is isomorphous with crystalline silicon because they share the same crystal structure.
– Isomorphism refers to the phenomenon where two or more substances crystallize in the same crystal structure.
– Crystalline silicon has a diamond cubic crystal structure, where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement.
– Diamond, which is also a crystalline allotrope of carbon, also has a diamond cubic structure, with carbon atoms replacing silicon atoms in the same arrangement.
– Coke, Graphite, and Coal are other forms of carbon, but they do not share the same crystal structure as crystalline silicon. Graphite has a layered hexagonal structure, while Coke and Coal are largely amorphous or contain various carbonaceous materials without a single, defined crystal structure.
The similarity in structure between diamond and silicon is due to both carbon and silicon belonging to Group 14 of the periodic table and forming strong covalent bonds in a tetrahedral geometry. This structural similarity has implications for their properties, although the bond strength (C-C vs Si-Si) leads to significant differences in hardness and melting point.

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17. Which one of the following materials is not an allotrope of carbon ?

Which one of the following materials is not an allotrope of carbon ?

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Diamond
Graphene
Fly ash
Fullerene
This question was previously asked in
UPSC NDA-1 – 2021
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Fly ash is a residue produced from the combustion of pulverized coal in thermal power plants. It is primarily composed of fine particles of various oxides and minerals, such as silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), iron oxide (Fe₂O₃), and calcium oxide (CaO), along with some unburnt carbon. It is not a pure form of carbon existing in a specific structural modification like diamond, graphene, or fullerene.
Allotropes are different forms of the same element in the same physical state. Carbon exists in many allotropic forms due to its ability to bond in various ways. Common allotropes include diamond (tetrahedral sp³ bonding), graphite (layered sp² bonding), graphene (single layer of graphite), fullerenes (spherical or elliptical sp² bonding, like C₆₀), carbon nanotubes, and lonsdaleite.
Fly ash is often used as a pozzolanic material in concrete mixtures, improving its properties. While it contains some unburnt carbon, its composition is complex and includes many non-carbon components, classifying it as an industrial byproduct rather than an allotrope of carbon.

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18. Which one of the following is the purest form of Carbon ?

Which one of the following is the purest form of Carbon ?

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Charcoal
Coke
Fullerene
Carbon black
This question was previously asked in
UPSC NDA-1 – 2018
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Among the given options, Fullerene is considered one of the purest forms of carbon because it consists of distinct molecules made solely of carbon atoms, which can be isolated and purified.
Charcoal, coke, and carbon black are amorphous or paracrystalline forms of carbon often produced by incomplete combustion or carbonization of organic matter. They typically contain impurities and consist of irregular aggregates of carbon structures. Fullerenes, like C₆₀, are molecular allotropes with cage-like structures composed only of carbon atoms, and they can be synthesized and purified to a high degree.
Other very pure forms of carbon include diamond (a crystalline allotrope) and carbon nanotubes and graphene (which can be produced with very high purity). Among the options listed, Fullerene best fits the description of a pure form.

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19. Which one of the following properties is NOT true for graphite ?

Which one of the following properties is NOT true for graphite ?

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Hybridisation of each carbon atom is sp³
Hybridisation of each carbon atom is sp²
Electrons are delocalized over the whole sheet of atoms
Each layer is composed of hexagon rings
This question was previously asked in
UPSC NDA-1 – 2018
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The statement that the hybridisation of each carbon atom in graphite is sp³ is NOT true.
Graphite has a layered structure. Each carbon atom within a layer is bonded to three other carbon atoms in a hexagonal lattice. These three bonds are formed using sp² hybrid orbitals. The remaining unhybridized p-orbital on each carbon atom overlaps with adjacent p-orbitals, creating a delocalized pi electron system across the layer. sp³ hybridization occurs in diamond, where each carbon atom forms single bonds with four other carbon atoms in a tetrahedral structure.
The delocalized electrons in graphite layers allow it to conduct electricity. The layers are held together by weak van der Waals forces, which explains its softness and use as a lubricant.

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20. How is carbon black obtained ?

How is carbon black obtained ?

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By heating wood at high temperature in absence of air
By heating coal at high temperature in absence of air
By burning hydrocarbons in a limited supply of air
By heating coal at high temperature in presence of air
This question was previously asked in
UPSC NDA-1 – 2018
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Carbon black is obtained by the incomplete combustion or thermal decomposition of hydrocarbons in a limited supply of air.
Methods like the furnace black process or channel black process involve burning hydrocarbon feedstock (like natural gas or oil) with insufficient oxygen, leading to the formation of fine carbon particles (soot), which is collected as carbon black.
Heating wood or coal at high temperatures in the absence of air are processes like pyrolysis or carbonization, which yield charcoal and coke, respectively. These are different forms of carbon and processes.

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