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Question: The correct order of heat of combustion of the following hydrocarbon is (P) Pent-l-ene (Q) Pentane (...

The correct order of heat of combustion of the following hydrocarbon is (P) Pent-l-ene (Q) Pentane (R) Neopentane (S) Isopentane (A)

A

Pent-l-ene

B

Pentane

C

Neopentane

D

Isopentane

Answer

Q > S > R > P

Explanation

Solution

The heat of combustion is the amount of heat released when one mole of a substance undergoes complete combustion. For hydrocarbons, the heat of combustion depends on several factors:

  1. Number of Carbon Atoms: Generally, heat of combustion increases with the number of carbon atoms in the molecule because more C-C and C-H bonds are present to be oxidized. All the given hydrocarbons (P, Q, R, S) have 5 carbon atoms, so this factor doesn't differentiate them.

  2. Degree of Saturation: For hydrocarbons with the same number of carbon atoms, saturated hydrocarbons (alkanes) generally have a higher molar heat of combustion than unsaturated hydrocarbons (alkenes, alkynes). This is because alkanes have more hydrogen atoms per carbon atom (CnH2n+2 vs CnH2n for alkenes), meaning more C-H bonds are oxidized, contributing to a greater release of energy.

    • (P) Pent-1-ene is an alkene (C5H10).
    • (Q) Pentane, (R) Neopentane, and (S) Isopentane are alkanes (C5H12).

    Therefore, the alkanes (Q, R, S) will have higher heats of combustion than the alkene (P). This means P will have the lowest heat of combustion.

  3. Branching (for isomers): For isomeric hydrocarbons (compounds with the same molecular formula but different structures), increased branching leads to greater stability. A more stable molecule has lower potential energy. When a more stable molecule combusts, it releases less energy compared to its less stable isomer.

    The C5H12 isomers are:

    • (Q) Pentane (n-pentane): Straight-chain, least branched.

    • (S) Isopentane (2-methylbutane): One branch.

    • (R) Neopentane (2,2-dimethylpropane): Most branched.

    Order of stability: Neopentane (R) > Isopentane (S) > Pentane (Q).

    Therefore, the order of heat of combustion (magnitude of energy released) will be the reverse of stability:

    Pentane (Q) > Isopentane (S) > Neopentane (R).

Combining the observations:

  • Alkanes (Q, S, R) have higher heat of combustion than the alkene (P).
  • Among alkanes: Q > S > R.

Thus, the overall correct order of heat of combustion is:

Q (Pentane) > S (Isopentane) > R (Neopentane) > P (Pent-1-ene)