Final answer:
Hydrocarbons can be arranged in order of decreasing boiling points based on the length of the carbon chain and the molecule's structure, with larger or more branched molecules generally having higher boiling points due to stronger van der Waals forces. For a series of alkanes, the boiling point increases with the chain length.
Step-by-step explanation:
To arrange hydrocarbons in order of decreasing boiling point (highest to lowest), we consider the molecular size and structure of each hydrocarbon. As the length of the carbon chain increases or the molecule becomes more branched, the boiling point usually increases. This is due to the fact that larger molecules or more branched structures have greater surface areas, leading to stronger van der Waals forces or London dispersion forces, thus requiring more energy to separate the molecules during the boiling process.
For linear alkanes such as C₂H₆ (ethane), C₃H₈ (propane), and C₄H₁₀ (butane), the boiling points increase with the number of carbons in the chain. The order from highest to lowest boiling points will then be C₄H₁₀ > C₃H₈ > C₂H₆. For branched isomers such as isobutane, the boiling point is typically lower than its linear counterpart due to a reduced surface area that affects the strength of intermolecular forces. Therefore, for n-butane, propane, 2-methylpropane (isobutane), and n-pentane, the order from highest to lowest boiling points would be n-pentane > n-butane > propane > 2-methylpropane.