Problem 23 - Chemistry The Central Science In Si Units

Question

Butane and 2 -methylpropane, whose space-filling models are shown here, 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.

Step-by-Step Solution

Answer

Butane has a higher boiling point because its straight-chain structure results in stronger dispersion forces, requiring more energy to change state.

Step 1: Understanding Molecular Structure

Both butane and 2-methylpropane have the same molecular formula, \(\mathrm{C}_4\mathrm{H}_{10}\), but their structures differ. Butane is a straight-chain alkane, while 2-methylpropane is a branched structure.

Step 2: Interaction Between Molecules

Because butane is a straight-chain molecule, it has a larger surface area in contact with other butane molecules, leading to slightly stronger dispersion forces compared to the more compact, branched structure of 2-methylpropane.

Step 3: Boiling Point Correlation

Stronger intermolecular forces in butane require more energy (in the form of heat) to overcome when changing from a liquid to a gas, resulting in a higher boiling point compared to 2-methylpropane.

Key Concepts

Molecular Structure

When we talk about molecular structure, we're referring to the arrangement of atoms within a molecule. Even if two substances have the same molecular formula, their structure can make a significant difference. In the case of butane and 2-methylpropane, both share the same molecular formula \( ext{C}_4 ext{H}_{10} \). But it's their structural geometry that distinguishes them. Butane is a linear molecule, which means it forms a line-like shape, while 2-methylpropane is branched.

This structural difference affects how closely these molecules pack together. Straight-chain molecules like butane often have more contact with each other compared to branched molecules. This is important when we consider how these molecules interact at the molecular level.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are a type of intermolecular force. These are weak attractive forces that arise due to temporary dipoles induced when atoms shift in electron density. This effect is universal and occurs in all atoms and molecules, but it is particularly relevant in nonpolar substances like butane and 2-methylpropane.

In butane, the linear configuration offers a more extended surface area for these temporary dipoles to interact. This results in increased dispersion forces between butane molecules compared to the more compact and less accessible structure of 2-methylpropane. Supporting stronger interactions among the molecules, these forces increase the energy needed to separate them, leading to higher boiling points.

Intermolecular Forces

Intermolecular forces are forces of attraction between molecules, which can greatly affect a substance's physical properties. Boiling point is one of these properties influenced by intermolecular forces. The main types of intermolecular forces are London dispersion forces, dipole-dipole interactions, and hydrogen bonds.

For butane and 2-methylpropane, which are both nonpolar, the most significant intermolecular force at play is the London dispersion force. Since butane provides an elongated shape, it affords stronger dispersion forces compared to the more compact 2-methylpropane. This means that as butane molecules attract each other more strongly, it takes more heat energy to convert them from a liquid state to a gaseous state. Therefore, butane has a higher boiling point than 2-methylpropane due to stronger intermolecular attractions.