Problem 17 - Chemistry: An Atoms-Focused Approach

Question

Concept Review Why does a branched alkane have a lower boiling point than a straight-chain alkane of the same molar mass?

Step-by-Step Solution

Answer

Answer: Branched alkanes have lower boiling points than straight-chain alkanes with the same molar mass due to their reduced surface area available for contact between the molecules. This results in weaker London dispersion forces holding the branched alkane molecules together, which require less energy to break, leading to their lower boiling points compared to straight-chain alkanes.

Step 1: Understanding Alkanes and Intermolecular Forces

Alkanes are hydrocarbons with the general formula CnH2n+2. They can be classified into straight-chain and branched alkanes. Straight-chain alkanes have carbon atoms arranged in a continuous chain, while branched alkanes have one or more carbon atoms connected to other carbon atoms, deviating the linear chain. Boiling points of these alkanes are dependent on intermolecular forces, primarily the London dispersion forces or van der Waals forces, which are influenced by the size, shape, and mass of molecules.

Step 2: Comparing the Molecular Size and Shape

For straight-chain and branched alkanes with the same molar mass, the number of carbon atoms (C) and hydrogen atoms (H) in both alkane types would be the same. However, their structural arrangement differs. The straight-chain alkanes have a more extended structure, allowing more points of contact between the molecules, whereas branched alkanes are more compact due to the branching, which reduces the surface area available for contact between the molecules.

Step 3: Effect of Molecular Size and Shape on London Dispersion Forces

The London dispersion forces increase with molecular size and surface area. For straight-chain alkanes, the extended structure increases the contact area between the molecules, thus increasing the London dispersion forces between them. In contrast, branched alkanes have fewer points of contact due to their compact structure, which reduces the magnitude of London dispersion forces between the molecules.

Step 4: Relating London Dispersion Forces to Boiling Points

The boiling point of a substance depends on the strength of the intermolecular forces between its molecules. Higher intermolecular forces require more energy to break, resulting in a higher boiling point. Since straight-chain alkanes have stronger London dispersion forces than branched alkanes of the same molar mass, they will require more energy to break the intermolecular forces, leading to higher boiling points.

Step 5: Conclusion

A branched alkane has a lower boiling point than a straight-chain alkane of the same molar mass because of the reduced surface area available for contact between the molecules in branched alkanes. This results in weaker London dispersion forces holding the molecules together, which require less energy to break, leading to lower boiling points for branched alkanes compared to their straight-chain counterparts.

Key Concepts

Intermolecular Forces

Intermolecular forces determine how molecules interact with each other and have significant effects on the properties of substances. In simple terms, they are the forces of attraction or repulsion which act between neighboring particles like atoms, molecules, or ions. These forces play a crucial role in defining characteristics such as boiling and melting points. There are various types of intermolecular forces, including:

• London dispersion forces
• Dipole-dipole interactions
• Hydrogen bonds

In alkanes, the intermolecular forces at play are mainly London dispersion forces. These are the weakest type among the intermolecular forces but are notably significant in non-polar substances like alkanes. As the main factor, they influence the energy required to change the physical state from liquid to gas, directly impacting boiling points.

London Dispersion Forces

London dispersion forces are a type of van der Waals force and are present in all molecules, whether they are polar or non-polar. They arise due to temporary fluctuations in electron density that create instantaneous dipoles, leading to an attraction between neighboring molecules. These forces are notably significant in alkanes because they don't have dipoles or hydrogen bonding.
The strength of London dispersion forces depends on several factors:

• Molecular size: Larger molecules have more electrons, leading to stronger dispersion forces.
• Shape of the molecule: Long, linear molecules can align closer to each other, enhancing the forces, compared to more compact, branched molecules.

In the case of alkanes, the straight-chain varieties have a larger surface area in contact with each other, thus incurring stronger London dispersion forces than their branched counterparts. This is why straight-chain alkanes exhibit higher boiling points.

Boiling Points

Boiling points are determined largely by the strength of intermolecular forces acting between molecules in a liquid. The stronger these forces, the more energy is required to separate the molecules, leading to a higher boiling point. For alkanes, boiling points can vary significantly depending on their molecular structure.
For instance, straight-chain alkanes have higher boiling points compared to branched alkanes of the same molar mass, due to stronger London dispersion forces. The decreased contact area in branched alkanes results in weaker forces, requiring less energy to transition from liquid to gas state. Hence, the understanding of boiling points in relation to molecular structure and intermolecular forces is crucial for predicting and explaining the physical properties of alkanes.