Figure P10.4 shows representations of the molecules pentane, C5 H12, and decane, C10 H22 . Which

step 1 In this problem, we have two particular compounds that are considered hydrocarbons. Hydrocarbons

Figure P10.4 shows representations of the molecules pentane,...

Figure P10.4 shows representations of the molecules pentane, $\mathrm{C}_{5} \mathrm{H}_{12},$ and decane, $\mathrm{C}_{10} \mathrm{H}_{22} .$ Which substance has the lower freezing point? Explain your answer.

Key Concepts

Molecular Size and Chain Length

The size of a molecule, particularly its carbon chain length, plays a significant role in its physical properties. Longer chains, such as in decane, have more surface area for intermolecular interactions, which influences how the molecules pack together and transition from liquid to solid. As a molecule increases in length, its overall molecular mass and contact area tend to increase.

Intermolecular Forces (London Dispersion Forces)

Intermolecular forces, particularly London dispersion forces, affect the boiling and freezing points of substances. Larger molecules with longer carbon chains have stronger London dispersion forces because they have more electrons and a larger area over which the forces can act. In this context, decane exhibits stronger dispersion forces than pentane, meaning it generally has a higher freezing point.

Phase Transition and Freezing Point

The freezing point is the temperature at which a liquid becomes a solid, a process that is highly influenced by the strength of intermolecular forces. Substances with stronger intermolecular attractions, such as decane with its greater chain length and mass, require higher temperatures to overcome the forces that hold them in a liquid state. Consequently, pentane, with its weaker intermolecular forces, has a lower freezing point compared to decane.

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