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SolutionsPhysical Properties

1. Order by boiling point

Arrange in order of increasing boiling point: hexane, 2,2-dimethylbutane, pentane and butane.

Butane (−0.5 °C) < pentane (36 °C) < 2,2-dimethylbutane (50 °C) < hexane (69 °C).

The boiling point rises with the number of carbons and, for the same number of carbons, falls with branching.

2. Isomers and contact surface

The C5H12 isomers boil at 36.1 °C (pentane), 27.8 °C (2-methylbutane) and 9.5 °C (2,2-dimethylpropane). Explain this trend.

The three molecules attract one another only through dispersion forces, which depend on the contact surface. The linear zigzag chain of pentane has the largest surface; branched molecules are more compact, almost spherical, touch less and separate with less energy.

3. Which intermolecular forces?

State the most important intermolecular force between the molecules of each substance:

  1. Octane
  2. Chloromethane, CH3Cl
  3. Methanol, CH3OH
  4. Water
  1. Dispersion (London) forces, the only ones possible between alkanes.
  2. Dipolar forces: the C–Cl bond is polar.
  3. Hydrogen bonds, thanks to the O–H group.
  4. Hydrogen bonds.

4. Alkanes and water

Why does an alkane such as hexane not mix with water and float on it?

Hexane is nonpolar and cannot form hydrogen bonds with water; dissolving it would mean breaking hydrogen bonds between water molecules and getting nothing in return. Besides, its density (≈0.66 g/mL) is lower than that of water (1 g/mL), so it floats.

5. True or false?

  1. When an alkane boils, its C–C bonds break.
  2. Alkanes with 1 to 4 carbons are gases at room temperature.
  3. No alkane reaches the density of water.
  1. False. The molecules do not break: they only stop attracting one another.
  2. True. From methane to butane they are gases; from pentane onwards, liquids.
  3. True. Even alkanes with many carbons stay below 0.8 g/mL.

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