Problem 35 - Chemistry and Chemical Reactivity

Question

Rank the following compounds in order of increasing molar enthalpy of vaporization: \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{C}_{2} \mathrm{H}_{6}, \mathrm{HCl}.\)

Step-by-Step Solution

Answer

\( \mathrm{C}_{2} \mathrm{H}_{6} < \mathrm{HCl} < \mathrm{CH}_{3} \mathrm{OH} \)

Step 1: Understanding Molar Enthalpy of Vaporization

The molar enthalpy of vaporization is the energy required to vaporize one mole of a substance at its boiling point. It is influenced by the strength of intermolecular forces present in the liquid phase.

Step 2: Identifying Intermolecular Forces

Identify the intermolecular forces for each compound: - **Methanol (\( \text{CH}_3\text{OH} \))**: Exhibits hydrogen bonding as well as dipole-dipole and London dispersion forces.- **Ethane (\( \text{C}_2\text{H}_6 \))**: Only exhibits London dispersion forces, which are weak.- **Hydrogen chloride (\( \text{HCl} \))**: Exhibits dipole-dipole interactions and London dispersion forces.

Step 3: Analyzing Relative Strength of Intermolecular Forces

Compare the strength of intermolecular forces: - **Methanol**: Due to hydrogen bonding, has the strongest intermolecular forces, leading to a high enthalpy of vaporization. - **Hydrogen chloride**: Has dipole-dipole forces, weaker than hydrogen bonding but stronger than London dispersion forces. - **Ethane**: Only has weak London dispersion forces, thus will have the lowest enthalpy of vaporization.

Step 4: Ranking Compounds by Molar Enthalpy of Vaporization

Based on the analysis of intermolecular forces, rank the compounds from lowest to highest molar enthalpy of vaporization:1. **Ethane (\( \text{C}_2\text{H}_6 \))**2. **Hydrogen chloride (\( \text{HCl} \))**3. **Methanol (\( \text{CH}_3\text{OH} \))**

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These forces are crucial in determining the physical properties of compounds, such as boiling points and molar enthalpy of vaporization. These forces are generally weaker than the bonds that hold atoms together within a molecule. Intermolecular forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces, each varying in strength.

• **Hydrogen Bonding:** The strongest type of intermolecular force, occurring in molecules where hydrogen is directly bonded to a highly electronegative element like oxygen, nitrogen, or fluorine. It significantly raises the boiling point and molar enthalpy of vaporization.
• **Dipole-Dipole Interactions:** Medium strength intermolecular forces that occur between polarized molecules. The positive end of one molecule attracts the negative end of another.
• **London Dispersion Forces:** The weakest of the three, caused by momentary changes in electron distribution in a molecule. Present in all molecules, but dominant in non-polar substances.

Understanding intermolecular forces helps determine why certain substances have higher molar enthalpies of vaporization than others.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction found in molecules where hydrogen is connected to an electronegative atom, typically nitrogen, oxygen, or fluorine. In methanol (\( ext{CH}_3 ext{OH}\)), this occurs between the hydrogen of one molecule and the oxygen of another molecule.

These bonds are much stronger than typical dipole-dipole interactions. When hydrogen bonding is present, substances will have higher boiling points and higher molar enthalpies of vaporization because more energy is required to separate the molecules during a phase change.

Hydrogen bonds are responsible for many unique properties of water, such as its high surface tension and ability to dissolve many compounds. They allow intermolecular attractions to be strong in substances like water and methanol, leading to their classification as one of the strongest types of intermolecular forces.

London Dispersion Forces

London dispersion forces are the weakest intermolecular forces and result from temporary fluctuations in electron density within a molecule. These forces are present in all molecules, regardless of their polarity, but they are the only type of intermolecular force in nonpolar compounds, like ethane (\( ext{C}_2 ext{H}_6\)).

• **Transient Attraction:** London dispersion forces are caused by fleeting charges that form as electrons move around the nucleus.
• **Cumulative Effect:** While individually weak, they can collectively create significant attraction in large molecules.

The strength of London dispersion forces increases with larger electron clouds, as these can create more significant temporary dipoles. Hence, larger nonpolar molecules will exhibit stronger dispersion forces. However, compared to hydrogen bonding and dipole-dipole forces, London dispersion forces contribute the least to the total intermolecular attraction in a substance, resulting in lower molar enthalpies of vaporization.