Problem 6 - Chemistry and Chemical Reactivity

Question

Rank the following in order of increasing strength of intermolecular forces in the pure substances. Which substance exists as a gas at \(25^{\circ} \mathrm{C}\) and 1 atm? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (butane) (b) \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol) (c) He

Step-by-Step Solution

Answer

Rank: He < Butane < Methanol. Helium is a gas at \(25^{\circ} \mathrm{C}\) and 1 atm.

Step 1: Identify Intermolecular Forces

First, identify the types of intermolecular forces present in each substance. \(\text{He}\) has only London dispersion forces. \(\text{Butane (C}_4\text{H}_{10})\) also has London dispersion forces, as it is a non-polar molecule. \(\text{Methanol (CH}_3\text{OH)}\) has hydrogen bonding due to the \(\text{OH}\) group, in addition to London dispersion forces and dipole-dipole interactions.

Step 2: Analyze Relative Strengths

Compare the strength of the intermolecular forces. Hydrogen bonding is the strongest among the forces involved, followed by dipole-dipole interactions, and then London dispersion forces. Thus, \(\text{Methanol}\) has the strongest intermolecular forces.

Step 3: Rank the Substances

Rank the substances based on the strength of their intermolecular forces. Helium, with the weakest forces, comes first, followed by butane, and finally methanol with the strongest forces.

Step 4: Identify Gas at Given Conditions

Determine which substance exists as a gas at \(25^{\circ} \mathrm{C}\) and 1 atm. Helium is a gas under these conditions, as it has the weakest intermolecular forces allowing it to be a gas at room temperature.

Key Concepts

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are the weakest type of intermolecular forces. They arise due to temporary fluctuations in the electron distribution around an atom or molecule, creating an instantaneous dipole. This momentary dipole can then induce others in nearby atoms or molecules, resulting in a weak force of attraction.

These forces are present in all molecules, whether polar or non-polar, but they are the only type of intermolecular force in noble gases and non-polar molecules. For example, both helium ( He ) and butane ( C_4H_{10} ) exhibit London dispersion forces.

Factors Affecting Strength

• Molecular Size: Larger atoms or molecules have more electrons, leading to stronger London dispersion forces due to larger electron cloud distortions.
• Shape of Molecules: Linear molecules like butane have more surface area to interact, which can increase the strength of dispersion forces compared to more compact molecules.

Hydrogen Bonding

Hydrogen bonding represents a stronger type of dipole-dipole interaction but only occurs in specific situations. For a hydrogen bond to form, hydrogen must be covalently bonded to a small, highly electronegative atom like nitrogen (N), oxygen (O), or fluorine (F). The high electronegativity of these atoms creates a significant bond polarity, allowing for a strong attraction between the hydrogen and the lone pairs on adjacent N, O, or F atoms.

In methanol ( CH_3OH ), the presence of an OH group facilitates hydrogen bonding due to the O-H bond. As a result, methanol molecules are able to form networks of strong hydrogen bonds, dramatically increasing their boiling points relative to hydrocarbons of similar molecular weight.

Importance of Hydrogen Bonding

• Boiling Points: Compounds with hydrogen bonding tend to have higher boiling points due to the additional energy required to break these strong interactions.
• Solubility: Hydrogen bonding significantly influences solubility patterns in solvents capable of hydrogen bonding, contributing greatly to methanol's miscibility with water.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where partial positive charges on one molecule attract partial negative charges on another. These forces are relative in strength between hydrogen bonds and London dispersion forces.

Methanol exhibits dipole-dipole interactions due to its molecular structure. The C-O and O-H bonds within methanol create a permanent dipole, allowing these molecules to align with neighboring molecules' oppositely charged ends. This alignment results in a significant but not as strong as hydrogen bonding.

Factors Influencing Dipole-Dipole Strength

• Polarity: The greater the polarity of the molecule, the stronger the dipole-dipole interactions.
• Orientation: Interactions are maximized when molecules are appropriately aligned with their dipole moments matching their counterparts.

While dipole-dipole forces are stronger than London dispersion forces, they still pale in comparison to the robustness of hydrogen bonds, especially apparent in substances like methanol, where all three interactions coexist.