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Chapter 11: Problem 23

Butane and 2 -methylpropane, whose space-filling models are shown at the top of the next column, are both nonpolar and have the same molecular formula, \(\mathrm{C}_{4} \mathrm{H}_{10}\), yet butane has the higher boiling point \(\left(-0.5^{\circ} \mathrm{C}\right.\) compared to \(\left.-11.7^{\circ} \mathrm{C}\right)\). Explain.

Short Answer

Expert verified
Although both butane and 2-methylpropane are nonpolar and have the same molecular formula \(\mathrm{C}_{4}\mathrm{H}_{10}\), the difference in their boiling points can be attributed to their difference in molecular structures. Butane's linear structure has more surface area, leading to stronger London dispersion forces among its molecules, and therefore has a higher boiling point, compared to the branched structure of 2-methylpropane.

Step by step solution

01

Define Boiling Point

Boiling point is the temperature at which a substance changes from a liquid to a gas phase. It is influenced by intermolecular forces, molecular size, molecular weight, and molecular shape.
02

Examine the Molecular Models

Let's look at the molecular models of butane and 2-methylpropane to understand their structures: Butane: \(\phantom{-}CH_{3}−CH_{2}−CH_{2}−CH_{3}\) 2-methylpropane: \(\phantom{-}CH_{3}−C(CH_{3})_{3}\) As we can see, butane is a linear molecule, while 2-methylpropane has a branched structure.
03

Analyze Intermolecular Forces

Both butane and 2-methylpropane are nonpolar molecules, which means they have no net charge separation. As a result, they do not experience strong dipole-dipole interactions. However, they do experience weak London dispersion forces (LDFs) as their primary intermolecular force. LDFs occur due to the temporary fluctuations in electron distribution, creating temporary dipoles that cause molecules to attract each other.
04

Relate LDFs to Molecular Structures

The strength of London dispersion forces depends on the surface area of the molecule and the number of electrons. Generally, larger surface area and more electrons lead to stronger LDFs and thus higher boiling points. Butane, being a linear molecule, has a larger surface area than the more compact, branched structure of 2-methylpropane. This increased surface area leads to stronger LDFs between adjacent butane molecules, resulting in a higher boiling point compared to 2-methylpropane.
05

Explain the Difference in Boiling Points

To summarize, although both butane and 2-methylpropane are nonpolar and have the same molecular formula \(\mathrm{C}_{4}\mathrm{H}_{10}\), the difference in their boiling points can be attributed to their difference in molecular structures. Butane's linear structure has more surface area, leading to stronger London dispersion forces among its molecules, and therefore has a higher boiling point, compared to the branched structure of 2-methylpropane.

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Most popular questions from this chapter

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