Hydrocarbons

1. How many moles of water would be produced by the complete combustion o

How many moles of water would be produced by the complete combustion of one mole of natural gas, CH₄, in excess of oxygen ?

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This question was previously asked in
UPSC CISF-AC-EXE – 2020
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The complete combustion of natural gas, which is primarily methane (CH₄), in excess oxygen is represented by the balanced chemical equation:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g or l)
The coefficients in the balanced equation represent the relative number of moles of reactants and products. According to the equation, one mole of methane (CH₄) reacts with two moles of oxygen (O₂) to produce one mole of carbon dioxide (CO₂) and two moles of water (H₂O).
The stoichiometry of a balanced chemical equation gives the mole ratios of reactants and products. For the combustion of methane, 1 mole of CH₄ produces 2 moles of H₂O.
Combustion is a rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light. Complete combustion occurs when there is sufficient oxygen, yielding carbon dioxide and water as products for hydrocarbons like methane. Incomplete combustion occurs with insufficient oxygen, producing carbon monoxide and/or carbon. The question specifies “complete combustion in excess of oxygen”, ensuring the reaction proceeds as shown in the balanced equation.

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2. Which among the following is correct with respect to bond formation in

Which among the following is correct with respect to bond formation in an ethyne molecule ?

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Carbon-Carbon single bond
Carbon-Carbon double bond
Carbon-Carbon triple bond
Carbon-Hydrogen double bond
This question was previously asked in
UPSC CAPF – 2024
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Ethyne is an organic compound with the chemical formula C₂H₂. It belongs to the alkyne family of hydrocarbons, which are characterized by the presence of a carbon-carbon triple bond. The structure of ethyne is H-C≡C-H. Each carbon atom is bonded to one hydrogen atom via a single covalent bond and to the other carbon atom via a triple covalent bond. Alkanes have single bonds (C-C), alkenes have double bonds (C=C), and alkynes have triple bonds (C≡C) between carbon atoms.
Hydrocarbons are classified based on the types of bonds between carbon atoms. Alkanes have only single bonds, alkenes have at least one double bond, and alkynes have at least one triple bond. Ethyne is the simplest alkyne.
A carbon-carbon triple bond consists of one sigma bond and two pi bonds. The carbon atoms in ethyne are sp-hybridized, resulting in a linear geometry around the triple bond. Ethyne is also commonly known as acetylene and is used as a fuel in welding and cutting.

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3. Hydrogenation of alkenes can be carried out in the presence of

Hydrogenation of alkenes can be carried out in the presence of

copper
zinc
aluminium
nickel
This question was previously asked in
UPSC CAPF – 2022
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Hydrogenation is a chemical reaction between molecular hydrogen ($\text{H}_2$) and another compound, usually in the presence of a catalyst. Hydrogenation of alkenes involves adding hydrogen across the carbon-carbon double bond, converting the alkene into a saturated alkane. This reaction typically requires a catalyst to lower the activation energy, as the $\text{H-H}$ bond in hydrogen is strong.
Common catalysts used for the hydrogenation of alkenes are transition metals, particularly from Group 8, 9, and 10 of the periodic table. These metals, such as platinum ($\text{Pt}$), palladium ($\text{Pd}$), rhodium ($\text{Rh}$), and nickel ($\text{Ni}$), adsorb both the alkene and hydrogen molecules onto their surface, facilitating the reaction.

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Looking at the options provided:
A) Copper: Copper can catalyze some hydrogenation reactions, but it is generally less active than Ni, Pt, or Pd for alkene hydrogenation.
B) Zinc: Zinc is not a typical catalyst for the hydrogenation of alkenes.
C) Aluminium: Aluminium is not used as a catalyst for the hydrogenation of alkenes; it is a reactive metal itself.
D) Nickel: Nickel is a very common and widely used catalyst for the hydrogenation of alkenes, especially in industrial processes (e.g., hydrogenation of vegetable oils to margarine – the Sabatier-Senderens process often uses Ni).

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Therefore, nickel is a standard catalyst for the hydrogenation of alkenes.

– Hydrogenation of alkenes is the addition of H₂ to the double bond.
– This reaction requires a catalyst.
– Transition metals like Ni, Pt, Pd, and Rh are common hydrogenation catalysts.
– Nickel is a common and cost-effective catalyst for alkene hydrogenation.
The catalytic hydrogenation of alkenes is a heterogeneous catalytic process where the reactants and catalyst are in different phases (alkenes/hydrogen are gases or liquids, catalyst is a solid metal). The reaction proceeds via a mechanism where both the alkene and hydrogen are adsorbed onto the catalyst surface. Raney nickel is a porous form of nickel alloy commonly used for hydrogenation.

4. The hydrogen atoms present in acetylene molecule are :

The hydrogen atoms present in acetylene molecule are :

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acidic
basic
both acidic and basic
neutral
This question was previously asked in
UPSC CAPF – 2015
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Acetylene has the chemical formula C₂H₂ and a linear structure H-C≡C-H. The carbon atoms in acetylene are sp hybridized. sp hybridized orbitals have higher s-character (50%) compared to sp² (33.3%) or sp³ (25%) hybridized orbitals. Higher s-character means the electrons in the C-H bond are held more closely to the carbon nucleus due to the greater penetration of the s orbital. This makes the carbon atom more electronegative than sp² or sp³ hybridized carbons, and consequently, it pulls electron density away from the hydrogen atom. This makes the C-H bond weaker and the hydrogen slightly positive, allowing it to be removed as a proton (H⁺) by a strong base. Therefore, the hydrogen atoms in acetylene are acidic.
The acidity of terminal alkynes (like acetylene) is due to the high electronegativity of the sp-hybridized carbon atom to which the hydrogen is attached. This polarizes the C-H bond, making the proton relatively easy to remove.
Acetylene is a weak acid, much weaker than carboxylic acids or mineral acids, but significantly more acidic than alkanes or alkenes. It can react with very strong bases, such as sodium amide (NaNH₂), to form acetylide salts (e.g., Na⁺ [C≡CH]⁻ or Na⁺ [C≡C]⁻ Na⁺). This acidic property is utilized in certain organic synthesis reactions.

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5. C 4 H 8 belongs to the homologous series of

C4H8 belongs to the homologous series of

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alkanes
alkenes
alkynes
cycloalkanes
This question was previously asked in
UPSC NDA-2 – 2022
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The correct answer is alkenes.
Homologous series are a series of compounds with the same general formula, usually differing by a single methylene (CH₂) unit. The general formula for alkanes is CnH2n+2, for alkenes is CnH2n (with one double bond), and for alkynes is CnH2n-2 (with one triple bond). For n=4, C₄H₈ fits the general formula CnH2n, which corresponds to alkenes. While cycloalkanes also have the formula CnH2n, alkenes are the primary open-chain homologous series with this formula.
C₄H₈ can represent various isomers, including but-1-ene, but-2-ene, 2-methylpropene (isobutylene), and cyclobutane or methylcyclopropane. Among the given options for homologous series, alkenes are the direct fit for the formula CnH2n in the context of typical classification alongside alkanes and alkynes.

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6. The number of structural isomers of pentane is

The number of structural isomers of pentane is

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5
4
2
3
This question was previously asked in
UPSC NDA-2 – 2022
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The correct answer is 3.
Structural isomers are compounds with the same molecular formula but different arrangements of atoms. Pentane has the molecular formula C₅H₁₂. The three structural isomers of pentane are:
1. n-pentane (a straight chain of five carbon atoms).
2. Isopentane (2-methylbutane, a four-carbon chain with a methyl group attached to the second carbon).
3. Neopentane (2,2-dimethylpropane, a three-carbon chain with two methyl groups attached to the central carbon).
As the number of carbon atoms increases in alkanes, the number of possible structural isomers grows rapidly. For example, hexane (C₆H₁₄) has 5 isomers, and heptane (C₇H₁₆) has 9 isomers.

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7. To weld metals together, high temperature is required. Such a high tem

To weld metals together, high temperature is required. Such a high temperature is obtained by burning:

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Acetylene in oxygen
LPG in oxygen
Methane in oxygen
Acetylene in nitrogen
This question was previously asked in
UPSC NDA-2 – 2015
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The correct option is A, Acetylene in oxygen.
Welding requires a high-temperature flame to melt and fuse metals. The combustion of acetylene (C₂H₂) with oxygen (O₂) produces an oxy-acetylene flame, which can reach temperatures of around 3500 °C, making it suitable for welding. Other fuel gases like LPG and methane produce lower temperatures when burned with oxygen. Burning acetylene in nitrogen would not sustain combustion as nitrogen is inert and would not provide the necessary oxidant.
Oxy-acetylene welding is a common process used for welding, cutting, and heating metals. The high temperature is achieved by using pure oxygen as the oxidant instead of air, which contains about 78% nitrogen and would limit the flame temperature. Different ratios of acetylene to oxygen can produce oxidizing, neutral, or carburizing flames, used for various applications.

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8. Which of the hydrocarbons are arranged as per the increasing order of

Which of the hydrocarbons are arranged as per the increasing order of their boiling points ?

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Methane, Butane, Propane, Heptane
Propane, Butane, Pentane, Octane
Propane, Butane, Heptane, Methane
Octane, Ethane, Methane, Propane
This question was previously asked in
UPSC NDA-1 – 2023
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The correct answer is B) Propane, Butane, Pentane, Octane.
For a homologous series of hydrocarbons like alkanes, the boiling point generally increases with increasing molecular weight, which corresponds to an increasing number of carbon atoms. This is because larger molecules have stronger van der Waals forces (London dispersion forces), requiring more energy (higher temperature) to overcome them and transition from liquid to gas phase.
The number of carbon atoms in the given hydrocarbons are: Methane (C1), Ethane (C2), Propane (C3), Butane (C4), Pentane (C5), Heptane (C7), Octane (C8). Arranging them by increasing carbon number gives the expected order of increasing boiling points: C1 < C2 < C3 < C4 < C5 < C6 < C7 < C8. Option B lists hydrocarbons in the order: Propane (C3), Butane (C4), Pentane (C5), Octane (C8), which corresponds to increasing carbon numbers (3 < 4 < 5 < 8), and thus increasing boiling points.

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9. What is the number of mole(s) of H₂(g) required to saturate one mole b

What is the number of mole(s) of H₂(g) required to saturate one mole benzene ?

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This question was previously asked in
UPSC NDA-1 – 2016
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The correct answer is C) 3.
– Benzene (C₆H₆) is a cyclic organic compound with a hexagonal ring structure containing alternating single and double bonds (represented by resonance). Effectively, there are three double bonds in the benzene ring.
– Saturation of an unsaturated hydrocarbon involves adding hydrogen (H₂) to break the double or triple bonds, converting them into single bonds.
– The reaction for the complete hydrogenation of benzene to cyclohexane (C₆H₁₂) is:
C₆H₆ + 3 H₂ → C₆H₁₂
– Each mole of H₂ reacts with one mole of a double bond. Since benzene has three double bonds, 3 moles of H₂ are required to saturate one mole of benzene.
This reaction is typically carried out at high pressure and temperature in the presence of a catalyst like Nickel, Platinum, or Palladium. The saturation of aromatic compounds like benzene is more difficult than that of simple alkenes or alkynes.

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10. Which one of the following carbon compounds will not give a sooty fl

Which one of the following carbon compounds will not give a sooty flame ?

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Benzene
Hexane
Naphthalene
Anthracene
This question was previously asked in
UPSC NDA-1 – 2016
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Hexane will not give a sooty flame.
Sooty flames are a result of incomplete combustion, where there is insufficient oxygen to fully oxidize the fuel. This often occurs with hydrocarbons that have a relatively high carbon content compared to hydrogen. Aromatic hydrocarbons (like Benzene, Naphthalene, Anthracene) and unsaturated aliphatic hydrocarbons have a higher carbon-to-hydrogen ratio and tend to burn with sooty flames due to incomplete combustion and the formation of carbon particles (soot). Saturated aliphatic hydrocarbons (alkanes like Hexane, C6H14) have a lower carbon-to-hydrogen ratio and typically undergo more complete combustion, burning with a clean, blue flame, assuming sufficient oxygen is available.
Benzene (C6H6), Naphthalene (C10H8), and Anthracene (C14H10) are all aromatic hydrocarbons with high carbon content. Hexane (C6H14) is a saturated alkane. The general formula for alkanes is CnH2n+2, while aromatics have proportionally less hydrogen for the same number of carbon atoms. This difference in composition affects how they burn.

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