Which of the hydrocarbons are arranged as per the increasing order of their boiling points?
- A.Methane, Butane, Propane, Heptane
- B.Propane, Butane, Pentane, Octane
- C.Propane, Butane, Heptane, Methane
- D.Octane, Ethane, Methane, Propane
Correct answer
Explanation
The correct answer is B, Propane, Butane, Pentane, Octane. In the alkane series the boiling point rises as the carbon chain gets longer, and this order goes from 3 to 4 to 5 to 8 carbons.
Alkanes are non-polar, so their molecules are held together only by weak van der Waals (London) forces. A longer chain has more electrons and more surface in contact, so these forces grow and more heat is needed to boil the liquid. Methane (C1) boils at about −162°C, propane (C3) at about −42°C, butane (C4) near 0°C, pentane (C5) at 36°C, heptane (C7) at 98°C and octane (C8) at about 126°C.
A is wrong because butane (C4) is placed before propane (C3).
C is wrong because methane, the lightest, is placed last.
D is wrong because it runs roughly from highest to lowest, starting with octane.
Exam tip: in a homologous series, more carbon atoms mean stronger van der Waals forces and a higher boiling point; branching lowers it.
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