Question

In each of the following group of compounds, identify (using the corresponding lowercase Roman numeral) the one with the:
For alkane names with more than 10 carbons click here.

a) hexane(i), octane(ii), 2-methylpentane(iii)

largest heat of combustion
  smallest heat of combustion
  highest boiling point

b) hectane(i), hexadecane(ii), tricosane(iii)

  largest heat of combustion
  smallest heat of combustion
lowest boiling point

c) heptane(i), 3-methylhexane(ii), 2,2,3-trimethylbutane(iii)

  largest heat of combustion
  smallest heat of combustion
  lowest boiling point

Answer 1

alkane names with more than 10 carbons

undecane(11 C)

dodecane(12)

tridecane(13)

tetradecane(14)

pentadecane(15)

hexadecane(16)

Heptadecane(17)

octadecane(18)

nonadecane(19)

eicosane(20)

Triacontane(30)

Tetracontane(40)

Pentacontane(50)

a) hexane(i) C₆H₁₄, octane(ii)C₆H₁₈, 2-methylpentane(iii) C₆H₁₄

largest heat of combustion : octane(highest -ve value)
  smallest heat of combustion :2-methylpentane
  highest boiling point : octane

b) hectane(i) C₁₀₀H₂₀₂, hexadecane(ii) C₁₆H₃₄, tricosane(iii)C₂₃H₄₈

largest heat of combustion :hectane
  smallest heat of combustion : hexadecane
lowest boiling point :hexadecane

c) heptane(i), 3-methylhexane(ii), 2,2,3-trimethylbutane(iii)-----C₇H₁₆

  largest heat of combustion :heptane
  smallest heat of combustion :2,2,3-trimethylbutane
  lowest boiling point :2,2,3-trimethylbutane

Explanation: Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure.Combustion is always exothermic.With the increase of number of c the heat of combustion increases(-ve value increases) . For alkanes with same number of c atoms with increase of brunch heat of combustion decreases(-ve value decreases).

Boiling Point (BP) also increase with increasing molecular weight AND For same molecular weight decrease with increase of brunch. in case of alkane the main attraction force is van der waal attraction which decrease with brunching.