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Chapter 27: Problem 16

In terms of intermolecular forces, why does the boiling point of a particular type of compound (for example, an alkane) increase as the molecular weight increases?

Short Answer

Expert verified
The boiling point of a compound like an alkane increases with molecular weight due to the increase in strength of London dispersion forces. Heavier molecules have more electrons and therefore a larger electron cloud, which is more easily polarized, leading to stronger London forces. As these intermolecular forces strengthen, more energy (higher temperature) is needed to overcome them, hence the boiling point increases.

Step by step solution

01

Understanding Intermolecular Forces

Intermolecular forces are forces of attraction between molecules. These forces influence various physical properties of substances, including boiling points, melting points, and solubilities. There are three major types of intermolecular forces: Dipole-dipole forces, hydrogen bonding, and London dispersion forces. In alkanes, the primary intermolecular force is the London dispersion force.
02

Understanding London Dispersion Forces

London Dispersion Forces (also known as Van der Waals forces) are weak, temporary attractions between molecules. They are caused by momentary changes in electron density in a molecule, which create temporary poles. These temporary poles induce similar polarity in nearby molecules, creating a temporary attraction. In case of alkanes, which are non-polar, these are the predominant intermolecular forces.
03

Relation between Molecular Weight and London Dispersion Forces

The strength of London dispersion forces is determined by how easily the electron cloud of a molecule can be distorted to induce a temporary polarization. This usually increases with the size of the molecule. A larger or heavier molecule has more electrons and thus a larger electron cloud, which can be more easily distorted. This means more polarizable and thus stronger London forces.
04

Connecting Boiling Point with Intermolecular Forces

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. This requires enough energy to overcome the intermolecular forces holding the molecules together. Therefore, stronger intermolecular forces result in higher boiling points. So, due to the increase in London dispersion forces with molecular weight, the boiling point also increases.

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Most popular questions from this chapter

In the alkanes: a) what type(s) of intermolecular force is (are) present? b) what is the strongest intermolecular force present?

In the ketones: a) what type(s) of intermolecular force is (are) present? b) what is the strongest intermolecular force present?

When water evaporates, are any bonds between \(\mathrm{H}\) atoms and \(\mathrm{O}\) atoms within \(\mathrm{a}\) molecule broken?

Is the strength of intermolecular forces determined by the bond strengths within the individual molecules? Explain your reasoning.

What is the difference between intramolecular bonds and intermolecular forces?
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