Question 1

Why does branching lower the boiling point of pentane isomers?

Accepted Answer

A straight chain can make a larger surface contact with a neighbouring molecule. This produces stronger London dispersion forces, so more thermal energy is needed to separate the molecules. Branching reduces contact and therefore lowers the boiling point.

Question 2

What is the boiling-point order of the C5H12 isomers?

Accepted Answer

At 1 atm, n-pentane boils at about 36.1°C, 2-methylbutane at 27.8°C, and 2,2-dimethylpropane at 9.5°C. Thus, the boiling point decreases as branching increases.

Question 3

Does boiling break covalent bonds in a hydrocarbon?

Accepted Answer

No. Boiling mainly overcomes weak intermolecular attractions between separate molecules. The covalent carbon-carbon and carbon-hydrogen bonds within each molecule remain intact during boiling.

Question 4

Why is the melting point of neopentane higher than that of n-pentane?

Accepted Answer

Melting depends strongly on how efficiently molecules pack into a crystal. Neopentane is compact and highly symmetric, so it packs more efficiently than n-pentane and melts at a much higher temperature, around -17°C.

Question 5

Why do these hydrocarbons float on water?

Accepted Answer

Liquid hydrocarbon samples are non-polar and insoluble in polar water. Their density is lower than water's, so a liquid sample placed on water floats instead of dissolving. At 25°C and 1 atm, neopentane is already gaseous because its boiling point is only about 9.5°C.

Boiling Point and Physical Properties: Why Branching Lowers Boiling Point

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