Question

Which substance has the highest boiling point? Why? They are all nonpolar. (a) \(\mathrm{CH}_{4}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\)

Short answer

Substance (d) CH3(CH2)3CH3 will have the highest boiling point because it has the largest size, which results in the strongest London dispersion forces.

Step-by-step solution

Identify the Types of Intermolecular Forces

Since all substances mentioned are nonpolar, the primary intermolecular force to consider is London dispersion forces (LDFs). These forces increase with the size and shape of the molecule, meaning larger and more elongated molecules will have stronger LDFs and a higher boiling point.

Examine Molecular Size

The more atoms in the molecule, the larger the molecule. Larger molecules have more electrons which can contribute to temporary dipoles that increase the strength of London dispersion forces. Hence, bigger molecules will generally have higher boiling points.

Compare the Molecular Sizes of Given Substances

Comparing the number of carbon and hydrogen atoms in each compound: (a) CH4 has 1 carbon and 4 hydrogens.(b) CH3CH3 has 2 carbons and 6 hydrogens.(c) CH3CH2CH3 has 3 carbons and 8 hydrogens.(d) CH3(CH2)3CH3 has 5 carbons and 12 hydrogens.With increasing carbon and hydrogen content, the molecular size increases.

Determine the Substance with the Highest Boiling Point

Since London dispersion forces get stronger with increasing molecular size, and molecule (d) has the most carbons and hydrogens, it will have the strongest dispersion forces and therefore the highest boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction and repulsion between molecules. They are vital for understanding the physical properties of substances, including the boiling point. For nonpolar substances like the hydrocarbons listed in the exercise, with no significant electrical charges from polar bonds, the type of intermolecular force that comes into play is primarily the London dispersion force.

The strength of these forces affects how molecules interact with each other, and it is reflected in properties such as melting and boiling points. In general, substances with stronger intermolecular forces will have higher boiling points because more energy is required to overcome these forces during the phase change from liquid to gas.

London Dispersion Forces

London dispersion forces (LDFs) are a type of intermolecular force that occur even in nonpolar molecules. They are also known as Van der Waals forces, after Johannes Diderik van der Waals, who first postulated their existence. These forces are the result of momentary dipoles that occur due to the random movement of electrons within molecules.

Understanding Momentary Dipoles

As electrons move about, they can create temporary regions of positive and negative charge within the molecule. These momentary dipoles can induce dipoles in neighboring molecules, leading to an attraction between them.

The strength of LDFs depends on several factors, the main one being the size of the molecules. Bigger molecules have more electrons, which means that greater temporary dipoles can occur, leading to stronger London dispersion forces.

Molecular Size

Molecular size is significant when considering boiling points of nonpolar substances because it directly impacts the strength of London dispersion forces. Larger molecules have more surface area and are able to have more points of interaction with other molecules.

The Larger, The Stronger

As a molecule gets larger, it has more electrons that can form temporary dipoles. This increases the strength of London dispersion forces and, consequently, increases the boiling point of the substance.

In the exercise, we can determine the molecular size by counting the number of carbon and hydrogen atoms. A larger number of atoms generally indicates a larger molecule, which means stronger London dispersion forces and a higher boiling point, underlining the reason why the substance with the highest molecular size amongst the given options will have the highest boiling point.