Final answer:
Straight-chained molecules exhibit stronger dispersion forces than branched-chained molecules due to their greater contact area. This is reflected in higher boiling points for straight-chained alkanes when compared to their branched counterparts.
Step-by-step explanation:
Between straight-chained and branched-chained molecules, straight-chained molecules will have more dispersion forces. This is because straight-chain alkanes have a longer, more cylindrical shape, allowing for greater contact area between molecules and consequently stronger London dispersion forces. In contrast, branched molecules tend to be more spherical, resulting in less surface area for contact between molecules and therefore fewer dispersion forces. The larger surface area of straight-chained molecules, such as n-pentane, allows for stronger intermolecular forces when compared to the more compact branched isomers like neopentane, which exhibit weaker dispersion forces and lower boiling points.
An example demonstrating this concept is the comparison of the boiling points for the isomers n-pentane, isopentane, and neopentane, which are 36 °C, 27 °C, and 9.5 °C respectively. The differences in boiling points among these isomers with the same chemical formula (C5H12) suggest that dispersion forces vary with molecular shape, being strongest for n-pentane with its extended shape and weakest for the compact neopentane.
The concept can be related to the interaction of VELCRO® brand fasteners: just as a larger contact area leads to a stronger connection, a larger molecular surface area results in stronger dispersion forces between molecules.