Question

6\. What is the strongest intermolecular interaction between \(\mathrm{H}_2 \mathrm{O}\) and \(\mathrm{CH}_3 \mathrm{OH}\) molecules? 1\. Dipole-dipole 2\. Dipole-induced dipole 3\. H-bonding 4\. Dispersion

Short answer

The strongest intermolecular interaction between water (H₂O) and methanol (CH₃OH) molecules is hydrogen bonding. Both molecules are polar and contain -OH groups, which involve hydrogen atoms bonded to oxygen atoms, allowing for hydrogen bonding to occur between them. So, the correct answer is \(3. H-bonding\).

Step-by-step solution

Identify molecular structures and dipoles of water and methanol

Water (H₂O) is a bent molecule with an oxygen atom at the center surrounded by two hydrogen atoms at an angle of 104.45°. Methanol (CH₃OH) is a polar molecule with a hydroxyl (-OH) group bonded to a methyl (CH₃) group. Both water and methanol are polar molecules, meaning they have a positive and a negative end (dipole).

Compare dipole-dipole interaction

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule. Since both water and methanol are polar molecules, they will have dipole-dipole interactions between them. However, we need to check if there are any stronger interactions present.

Compare dipole-induced dipole interaction

Dipole-induced dipole interactions occur when a polar molecule induces a temporary dipole in a nonpolar molecule by influencing the electron distribution in the nonpolar molecule. However, since both water and methanol are polar molecules, dipole-induced dipole interactions are not relevant in this case.

Compare hydrogen bonding interaction

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) in one molecule and interacts with a highly electronegative atom in another molecule. Both water and methanol contain -OH groups, which involve hydrogen atoms bonded to oxygen atoms. Thus, hydrogen bonding can occur between water and methanol molecules.

Compare dispersion forces

Dispersion forces, also known as London dispersion forces or van der Waals forces, are weak interactions that occur between all molecules due to temporary fluctuations in electron distribution. However, these forces are typically weaker than other types of intermolecular interactions, including hydrogen bonding.

Determine the strongest intermolecular interaction

Based on our comparisons, hydrogen bonding is the strongest intermolecular interaction between water and methanol molecules, as it is a more significant and specific type of interaction compared to general dipole-dipole and dispersion forces. Therefore, the correct answer is: 3. H-bonding

Key concepts

Intermolecular Interactions

Intermolecular interactions are forces that act between molecules, determining many of their physical properties such as boiling point, melting point, and solubility. These forces vary in strength and can be broadly categorized into several types, including hydrogen bonding, dipole-dipole interactions, and dispersion forces.

Every substance around us, from the water in our glass to the air we breathe, is influenced by these interactions. The understanding of intermolecular interactions helps us explain why substances are solid, liquid, or gas at room temperature, and why some materials mix well while others separate. For example, oil doesn't mix with water primarily because of differences in the intermolecular interactions within each substance.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction, but it's worthy of its category because it's so much stronger than other dipole-dipole forces. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, is attracted to another electronegative atom in a different molecule.

Key Characteristics of Hydrogen Bonds:

• Directional nature, often resulting in organized structures like the DNA double helix or the lattice of ice crystals.
• Significantly influence boiling and melting points, making water liquid at room temperature.
• Crucial in biological molecules, determining the structure of proteins and nucleic acids.

Understanding hydrogen bonding is critical, especially when looking at the properties of water and its unparalleled ability to support life due to its hydrogen bonding-induced high surface tension, heat capacity, and solvent properties.

Dipole-Dipole Interactions

When molecules are polar, meaning they have a region with a slight positive charge and another with a slight negative charge, they exhibit dipole-dipole interactions. These are attractive forces where the positive end of one polar molecule is drawn to the negative end of another polar molecule.

These interactions play a significant role in the physical properties of compounds. For instance, the boiling point of a substance can be higher when dipole-dipole interactions are present, since more energy is needed to overcome these interactions for a molecule to escape into the gaseous state.

Examples:

• Sulfur dioxide (SO2), with its polar molecules, has a higher boiling point than similarly sized but nonpolar carbon dioxide (CO2).

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest of all intermolecular interactions. They arise from temporary fluctuations in the electron distribution within molecules or atoms, leading to a momentary dipole that induces a dipole in a neighboring particle.

These forces are universal, meaning they occur between all particles, whether they are polar or nonpolar. Their strength is influenced by the size and shape of the molecules; the larger the molecule, the stronger the dispersion forces.

Role in Everyday Materials:

• They are behind the ability of nonpolar gases like neon to be liquefied.
• Explain why heavier noble gases like xenon have higher boiling points than lighter ones like helium.