Problem 119 - Chemistry An Atoms First Approach

Question

Consider the compounds butanoic acid, pentanal, \(n\) -hexane, and \(1-\) pentanol. The boiling points of these compounds (in no specific order) are \(69^{\circ} \mathrm{C}, 103^{\circ} \mathrm{C}, 137^{\circ} \mathrm{C},\) and \(164^{\circ} \mathrm{C} .\) Match the boiling points to the correct compound.

Step-by-Step Solution

Answer

The boiling points of the given compounds are as follows: n-hexane has a boiling point of \(69^{\circ} \mathrm{C}\), due to weak van der Waals forces. Pentanal has a boiling point of \(103^{\circ} \mathrm{C}\) because of dipole-dipole interactions. 1-pentanol has a boiling point of \(137^{\circ} \mathrm{C}\) due to hydrogen bonding, and butanoic acid has the highest boiling point, \(164^{\circ} \mathrm{C}\), because of both hydrogen bonding and its higher molecular weight.

Step 1: Determine the molecular weight of each compound

Find the molecular weight of butanoic acid, pentanal, n-hexane, and 1-pentanol. Molecular weight can be an indication of boiling point, as heavier molecules typically require more energy to break the intermolecular forces. - Butanoic acid (\(C_4H_8O_2\)): \(4(12.01) + 8(1.01) + 2(16.00) = 88.11 \, g/mol\) - Pentanal (\(C_5H_{10}O\)): \(5(12.01) + 10(1.01) + 1(16.00) = 86.16 \, g/mol\) - n-hexane (\(C_6H_{14}\)): \(6(12.01) + 14(1.01) = 86.18 \, g/mol\) - 1-pentanol (\(C_5H_{12}O\)): \(5(12.01) + 12(1.01) + 1(16.00) = 88.15 \, g/mol\)

Step 2: Examine the intermolecular forces in the compounds

Determine the major types of intermolecular forces operating in each of the compounds. This information can help us predict boiling points, where stronger forces lead to higher boiling points. - Butanoic acid: hydrogen bonding (due to the presence of a carboxyl group) - Pentanal: dipole-dipole interactions (due to the presence of a carbonyl group) - n-hexane: weak van der Waals forces (due to its nonpolar nature) - 1-pentanol: hydrogen bonding (due to the presence of a hydroxyl group)

Step 3: Match boiling points to compounds

Based on molecular weight and intermolecular forces, determine which boiling point corresponds to each compound. - n-hexane: The boiling point should be the lowest, as it has weak van der Waals forces and a similar molecular weight to some of the other compounds: \(69^{\circ} \mathrm{C}\) - Pentanal: The boiling point should be slightly higher than n-hexane, as there are dipole-dipole interactions: \(103^{\circ} \mathrm{C}\) - 1-pentanol: The boiling point should be higher due to the presence of hydrogen bonding but lower than butanoic acid because it has a lower molecular weight. Therefore 1-pentanol corresponds to the boiling point of \(137^{\circ} \mathrm{C}\). - Butanoic acid: The boiling point should be highest, as it has hydrogen bonding in addition to a higher molecular weight than 1-pentanol, which also experiences hydrogen bonding. Thus, butanoic acid corresponds to the boiling point of \(164^{\circ} \mathrm{C}\). In conclusion, we have matched the boiling points to the correct compounds: - Butanoic acid: \(164^{\circ} \mathrm{C}\) - Pentanal: \(103^{\circ} \mathrm{C}\) - n-hexane: \(69^{\circ} \mathrm{C}\) - 1-pentanol: \(137^{\circ} \mathrm{C}\)

Key Concepts

Molecular Weight

Molecular weight, or molecular mass, refers to the mass of a single molecule of a substance and is typically expressed in atomic mass units (amu) or grams per mole (g/mol). It's determined by summing the atomic weights of each atom in the molecule. When considering the boiling point of a substance, molecular weight can be a useful indicator. Generally, the heavier a molecule, the higher the energy that must be applied to convert it from liquid to gas. This is because heavier molecules tend to have greater surface area and more electrons, leading to stronger van der Waals forces, which are a type of intermolecular force.
For instance, comparing substances with similar molecular structures but different molecular weights often reveals that the substance with the higher molecular weight has a higher boiling point. However, this is not a standalone criterion, as other factors, such as the type of intermolecular forces present, also play a significant role in determining boiling points.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles: atoms, molecules, or ions. They are much weaker than the intramolecular forces, such as covalent or ionic bonds, within molecules. Yet, these forces are crucial for dictating the physical properties of a substance, including boiling point, melting point, and solubility.
There are different types of intermolecular forces, including:

• Dipole-dipole interactions: Occur between polar molecules
• London dispersion forces: Present in all molecules, especially significant in nonpolar molecules
• Hydrogen bonding: A special type of dipole-dipole interaction that occurs when hydrogen is bonded to electronegative atoms like nitrogen, oxygen, or fluorine

Each type of force has a different impact on the boiling point. Dipole-dipole interactions and hydrogen bonds, for example, significantly increase the boiling point, because extra energy is needed to overcome these stronger interactions.

Hydrogen Bonding

Hydrogen bonding is one of the strongest types of intermolecular forces and has a profound effect on the boiling point of compounds. This specific force occurs when a hydrogen atom is covalently bonded to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, and is attracted to another electronegative atom in a neighboring molecule.
In the context of boiling point determination, molecules capable of hydrogen bonding often exhibit significantly higher boiling points than those that cannot. This is because a considerable amount of energy is required to break the hydrogen bonds for the phase transition from liquid to gas to occur. As seen in the solution for butanoic acid and 1-pentanol, both of which can form hydrogen bonds, these compounds have higher boiling points compared to those that form weaker intermolecular forces.

Van der Waals Forces

Van der Waals forces refer to a group of intermolecular forces that include dipole-dipole interactions, dipole-induced dipole interactions, and London dispersion forces. Among these, London dispersion forces are present in all molecules, regardless of their polarity, and are caused by the correlated movements of the electrons in interacting molecules.
These forces are typically much weaker than hydrogen bonds, therefore, compounds that exhibit primarily London dispersion forces will generally have lower boiling points, assuming other factors like molecular weight and structure are comparable. In the example of n-hexane, its nonpolar nature and the absence of significant forces other than van der Waals forces make its boiling point the lowest amongst the given compounds.