Use the normal boiling points propane (C3H8) -42.1 °C butane (C4H10) -0.5 °C pentane (C5H12) 36.1 °C hexane (C6H14) 68.7 °C heptane (C7H16) 98.4 °C to estimate the normal boiling point of octane (C8H18). Explain the trend in the boiling points.

Key Concepts

Boiling Point and Intermolecular Forces

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure, allowing it to transition from liquid to gas. This property is significantly influenced by intermolecular forces, such as van der Waals forces, which increase with molecular size and surface area. As the molecular weight of hydrocarbons increases, so do the London dispersion forces, leading to higher boiling points.

Hydrocarbon Structure and Molecular Weight

Hydrocarbons are organic compounds consisting solely of hydrogen and carbon. The structure of these molecules, including the number of carbon atoms, directly affects their physical properties. As the number of carbon atoms increases, the molecular weight also increases, which typically results in stronger intermolecular forces and higher boiling points, as seen in the provided data for propane, butane, pentane, hexane, and heptane.

Trends in Boiling Points of Alkanes

In a series of straight-chain alkanes, the boiling points generally increase with the number of carbon atoms. This trend can be attributed to the increasing surface area and the resultant increase in van der Waals forces. For octane (C8H18), one can estimate its boiling point by observing the pattern in the boiling points of the preceding alkanes, suggesting it will be higher than that of heptane (98.4 °C) due to its larger molecular size.