Hydrocarbons

11. Directions : The following 05 (Five) items consist of two statements,

Directions : The following 05 (Five) items consist of two statements, Statement I and Statement II. Examine these two statements carefully and select the answers to these items using the code given below :
Code :

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  • 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.

51.
Statement I: Petroleum is a mixture of many different hydrocarbons of different densities.
Statement II: The grade of petroleum depends mainly on the relative proportion of the different hydrocarbons.

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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 – 2016
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The correct answer is A) Both the Statements are individually true and Statement II is the correct explanation of Statement I.
Statement I is true. Petroleum is a complex mixture primarily composed of hydrocarbons, which vary in size, structure, and density.
Statement II is also true. The physical and chemical properties, and thus the ‘grade’ (e.g., light/heavy crude, different viscosities), of petroleum depend heavily on the specific blend and relative proportions of the various hydrocarbons present. These proportions directly influence density, boiling point range, viscosity, etc., which define different petroleum grades. Statement II explains the significance of the composition mentioned in Statement I.
Hydrocarbons in petroleum can range from light gases (like methane) to heavy, complex molecules. The process of refining separates petroleum into various fractions (like gasoline, diesel, kerosene, lubricants) based on their different boiling points, which are determined by the types of hydrocarbons present.

12. The ozonolysis of which one of the following alkenes yields a mixture

The ozonolysis of which one of the following alkenes yields a mixture of propan-2-one and formaldehyde?

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Propene
Butene
2-Methylpropene
2-Methylbutene
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UPSC Geoscientist – 2023
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The correct answer is 2-Methylpropene.
Ozonolysis of an alkene involves the cleavage of the carbon-carbon double bond, yielding carbonyl compounds (aldehydes or ketones). The structure of the products dictates the structure of the original alkene. Propan-2-one has the structure (CH₃)₂C=O, and formaldehyde has the structure H₂C=O. For ozonolysis to yield these two products, the original alkene must have been (CH₃)₂C=CH₂.
(CH₃)₂C=CH₂ is 2-Methylpropene (also known as isobutylene). When the double bond is cleaved by ozonolysis, the carbon atom connected to two methyl groups forms a ketone (propan-2-one), and the CH₂ group forms an aldehyde (formaldehyde). Propene (CH₃CH=CH₂) yields acetaldehyde and formaldehyde. Butene can be but-1-ene (CH₃CH₂CH=CH₂, yields propanal and formaldehyde) or but-2-ene (CH₃CH=CHCH₃, yields acetaldehyde). 2-Methylbutene could be 2-methylbut-1-ene ((CH₃)₂C=CHCH₃, yields propan-2-one and acetaldehyde) or 2-methylbut-2-ene (CH₃C(CH₃)=CHCH₃, yields propan-2-one and acetaldehyde). Only 2-Methylpropene yields propan-2-one and formaldehyde.

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13. In which one of the following reactions, the maximum quantity of H₂ ga

In which one of the following reactions, the maximum quantity of H₂ gas is produced by the decomposition of 1 g of compound by H₂O/O₂?

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CH₄ + H₂O → CO + 3H₂
CO + H₂O → CO₂ + H₂
CH₄ + ½O₂ → CO + 2H₂
C₁₂H₂₄ + 6O₂ → 12CO + 12H₂
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The reaction in option A produces the maximum quantity of H₂ gas per gram of the reactant compound.
To find the maximum quantity of H₂ produced per gram of reactant, we need to calculate the moles of H₂ produced per gram of the principal reactant in each reaction. Let’s calculate the moles of H₂ per gram of the compound undergoing decomposition/reaction:
A) CH₄ + H₂O → CO + 3H₂: Molar mass of CH₄ is 16 g/mol. 1 mol CH₄ produces 3 mol H₂. Moles of H₂ per gram of CH₄ = (3 mol H₂) / (16 g CH₄) = 3/16 mol/g.
B) CO + H₂O → CO₂ + H₂: Molar mass of CO is 28 g/mol. 1 mol CO produces 1 mol H₂. Moles of H₂ per gram of CO = (1 mol H₂) / (28 g CO) = 1/28 mol/g.
C) CH₄ + ½O₂ → CO + 2H₂: Molar mass of CH₄ is 16 g/mol. 1 mol CH₄ produces 2 mol H₂. Moles of H₂ per gram of CH₄ = (2 mol H₂) / (16 g CH₄) = 2/16 = 1/8 mol/g.
D) C₁₂H₂₄ + 6O₂ → 12CO + 12H₂: Molar mass of C₁₂H₂₄ is 168 g/mol. 1 mol C₁₂H₂₄ produces 12 mol H₂. Moles of H₂ per gram of C₁₂H₂₄ = (12 mol H₂) / (168 g C₁₂H₂₄) = 12/168 = 1/14 mol/g.
Comparing the values: 3/16 (0.1875), 1/28 (~0.0357), 1/8 (0.125), 1/14 (~0.0714). The largest value is 3/16 mol/g, corresponding to reaction A.
These reactions represent different methods for producing hydrogen gas, often from hydrocarbon fuels or syngas (CO + H₂). Reaction A is a simplified steam reforming of methane. Reaction B is the water-gas shift reaction. Reaction C is partial oxidation of methane. Reaction D involves a larger hydrocarbon. The amount of hydrogen produced depends on the stoichiometry of the reaction and the molar mass of the starting material.

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14. Which of the following is the general formula for saturated

Which of the following is the general formula for saturated hydrocarbons?

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CnH₂n+2
CnH₂n-2
CnH₂n+1
CnH₂n-1
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UPSC CDS-2 – 2018
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Saturated hydrocarbons, also known as alkanes, are hydrocarbons where all carbon-carbon bonds are single bonds. The general formula for alkanes is CnH₂n+2, where ‘n’ represents the number of carbon atoms. This formula represents the maximum number of hydrogen atoms that can be bonded to ‘n’ carbon atoms in an acyclic structure with only single bonds.
– Saturated hydrocarbons are alkanes.
– They contain only single bonds between carbon atoms.
– The general formula represents the relationship between the number of carbon and hydrogen atoms in such molecules.
Unsaturated hydrocarbons include alkenes (at least one double bond, general formula CnH₂n) and alkynes (at least one triple bond, general formula CnH₂n-2). Cycloalkanes (cyclic saturated hydrocarbons) have the general formula CnH₂n.

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15. How many moles of CO can be obtained by reacting 2·0 mole of CH₄ with

How many moles of CO can be obtained by reacting 2·0 mole of CH₄ with 2·0 mole of O₂ according to the equation given below?
CH₄(g) + 1/2 O₂ → CO + 2H₂

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2·0
0·5
2·5
4·0
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UPSC CDS-2 – 2018
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2.0 moles of CO can be obtained by reacting 2.0 mole of CH₄ with 2.0 mole of O₂.
The balanced chemical equation is CH₄(g) + 1/2 O₂ → CO + 2H₂. We need to determine the limiting reactant based on the initial amounts (2.0 mol CH₄ and 2.0 mol O₂).
According to the stoichiometry, 1 mole of CH₄ reacts with 0.5 moles of O₂.
For 2.0 moles of CH₄, 2.0 * 0.5 = 1.0 mole of O₂ is required. We have 2.0 moles of O₂, which is more than required. Thus, CH₄ is the limiting reactant.
The amount of product is determined by the limiting reactant. From the equation, 1 mole of CH₄ produces 1 mole of CO. Therefore, 2.0 moles of CH₄ will produce 2.0 moles of CO.

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16. Which one of the following is known as cetane?

Which one of the following is known as cetane?

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Hexadecane
Heptadecane
Octadecane
Nonadecane
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UPSC CDS-1 – 2024
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Cetane is a term used in the context of diesel fuel quality.
Cetane is the common name for the alkane molecule hexadecane (C₁₆H₃₄). It is used as a reference standard with a high cetane number (defined as 100) to evaluate the ignition quality of diesel fuel.
The cetane number is a measure of a diesel fuel’s autoignition delay time. Fuels with higher cetane numbers ignite more readily under compression, leading to smoother engine operation.

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17. The correct order of octane number of butane, pentane, hexane and cycl

The correct order of octane number of butane, pentane, hexane and cyclohexane is

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”butane
”butane
”butane
”cyclohexane
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UPSC CDS-1 – 2024
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Octane number is a measure of a fuel’s resistance to knocking. Branched hydrocarbons and cyclic hydrocarbons have higher octane numbers than straight-chain hydrocarbons with the same number of carbon atoms. The octane number decreases with increasing chain length for straight-chain alkanes.
Comparing the standard (n-) forms:
n-Butane (C4H10) has a higher octane number than n-pentane (C5H12) and n-hexane (C6H14).
n-Pentane has a higher octane number than n-hexane.
Cyclohexane (C6H12), a cyclic alkane, has a higher octane number than the corresponding straight-chain alkane, n-hexane, and typically higher than n-pentane.
The typical order of RON (Research Octane Number) is approximately: n-Butane (~92) > Cyclohexane (~83) > n-Pentane (~62) > n-Hexane (~25).
Thus, the correct order of decreasing octane number is butane > cyclohexane > pentane > hexane.
Octane number is inversely related to chain length for n-alkanes and higher for branched/cyclic structures compared to straight chains.
Octane numbers are determined relative to two reference hydrocarbons: isooctane (2,2,4-trimethylpentane), assigned an octane number of 100 (highly resistant to knocking), and n-heptane, assigned an octane number of 0 (knocks readily). The octane number of a fuel is the percentage of isooctane in a mixture with n-heptane that has the same knocking characteristics as the fuel.

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18. Naphthalene burns with a yellow sooty flame. This is because

Naphthalene burns with a yellow sooty flame. This is because

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carbon to hydrogen ratio is low
there is incomplete combustion
there is excess supply of air
of presence of impurities of nitrogen and sulfur
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When a substance burns with a yellow, sooty flame, it indicates that combustion is incomplete. Combustion is a chemical process where a substance reacts rapidly with oxygen to produce heat and light. Complete combustion occurs when there is sufficient oxygen, producing primarily carbon dioxide and water. Incomplete combustion occurs when there is insufficient oxygen, leading to the formation of carbon monoxide and/or unburnt carbon particles (soot). The yellow colour of the flame is due to the incandescence (glowing due to heat) of these hot soot particles. Naphthalene (C₁₀H₈) is an aromatic hydrocarbon with a relatively high carbon-to-hydrogen ratio compared to simple alkanes. Compounds with higher carbon content require more oxygen for complete combustion and are more prone to incomplete combustion and soot formation, especially in open air where the oxygen supply may not be perfectly mixed.
– A yellow sooty flame is a characteristic sign of incomplete combustion.
– Incomplete combustion produces soot (unburnt carbon particles) which glow yellow when hot.
– Fuels with a high carbon-to-hydrogen ratio are more likely to produce sooty flames during incomplete combustion.
In contrast, complete combustion typically produces a clean, often blue, flame (as seen in a Bunsen burner with the air hole open or a gas stove flame). Factors affecting the completeness of combustion include the amount of oxygen available, the mixing of fuel and air, and the temperature.

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19. The number of saturated and unsaturated bonds in cyclohexane are :

The number of saturated and unsaturated bonds in cyclohexane are :

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9 and 0 respectively.
18 and 3 respectively.
18 and 0 respectively.
9 and 3 respectively.
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UPSC CDS-1 – 2022
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Cyclohexane (C6H12) is a cyclic alkane. Its structure consists of a six-membered ring of carbon atoms, with each carbon atom bonded to two other carbon atoms and two hydrogen atoms. All bonds in cyclohexane are single bonds. A single bond is considered a saturated bond in contrast to double or triple bonds which are unsaturated bonds. There are 6 carbon-carbon single bonds forming the ring and 12 carbon-hydrogen single bonds. Thus, there are a total of 6 + 12 = 18 saturated bonds (single bonds) and 0 unsaturated bonds (double or triple bonds).
In organic chemistry, molecules containing only single bonds between carbon atoms and single bonds to hydrogen atoms are considered saturated. The term “saturated bond” commonly refers to a single bond, while “unsaturated bond” refers to double or triple bonds.
The formula C6H12 matches the general formula for cycloalkanes (CnH2n) or alkenes. Cyclohexane is a cycloalkane. A six-carbon chain alkane (hexane) has the formula C6H14. The formation of a ring involves the loss of two hydrogen atoms and results in 6 C-C single bonds and 12 C-H single bonds.

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20. While burning hydrocarbon fuels, if we see a yellow flame with lots of

While burning hydrocarbon fuels, if we see a yellow flame with lots of black smoke, it means that the fuel is :

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made of saturated hydrocarbons.
made of unsaturated hydrocarbons.
burning completely.
wet.
This question was previously asked in
UPSC CDS-1 – 2022
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A yellow flame with lots of black smoke indicates incomplete combustion. This occurs when there is insufficient oxygen for the fuel to burn completely. Unsaturated hydrocarbons, which have a higher carbon-to-hydrogen ratio than saturated hydrocarbons, tend to undergo incomplete combustion more readily and produce sooty flames (yellow flame and black smoke) when the oxygen supply is limited.
Complete combustion produces a clean, often blue, flame and primarily carbon dioxide and water. Incomplete combustion produces a yellow, luminous, and sooty flame, along with carbon monoxide and unburnt carbon particles (soot), which appear as black smoke. The likelihood of incomplete combustion depends on the fuel type and the oxygen supply.
Saturated hydrocarbons (like alkanes) typically burn with cleaner, often blue, flames compared to unsaturated hydrocarbons (like alkenes, alkynes, and aromatic hydrocarbons) which have higher carbon content and are more prone to incomplete combustion leading to sooty, yellow flames.

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