Problem 16 - Chemistry Matter and Change

Question

Evaluate Which of the molecules listed below can form hydrogen bonds? For which of the molecules would dispersion forces be the only intermolecular force? Give reasons for your answers. $$\text { a. }\mathrm{H}_{2} \quad \text { b. } \mathrm{H}_{2} \mathrm{S} \quad \text { c. } \mathrm{HCl} \quad \text { d. } \mathrm{HF}$$

Step-by-Step Solution

Answer

HF can form hydrogen bonds. H2 exhibits only dispersion forces.

Step 1: Understanding Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative element (such as fluorine, oxygen, or nitrogen) and is in proximity to another electronegative atom. Identify which molecules have this capability.

Step 2: Analyze Dispersion Forces

Dispersion forces, also known as London dispersion forces, occur in all molecules but are the only intermolecular forces in nonpolar molecules. Evaluate which molecule fits this description.

Step 3: Evaluate Molecule H2

Hydrogen (H2) is a nonpolar molecule consisting of two identical atoms. It cannot form hydrogen bonds because there are no electronegative atoms involved, and dispersion forces are the only intermolecular forces present.

Step 4: Evaluate Molecule H2S

In H2S, although sulfur is more electronegative than hydrogen, it is not electronegative enough (as are N, O, or F) to form hydrogen bonds. Hence, H2S cannot form hydrogen bonds, but can exhibit dispersion forces and dipole-dipole interactions.

Step 5: Evaluate Molecule HCl

HCl is a polar molecule where hydrogen is bonded to chlorine, an electronegative atom, but still not enough like F, O, or N to create hydrogen bonds. Hence, HCl exhibits dipole-dipole interactions besides dispersion forces.

Step 6: Evaluate Molecule HF

HF has hydrogen directly bonded to fluorine, a highly electronegative atom, which allows the formation of strong hydrogen bonds. Thus, HF can form hydrogen bonds alongside the dispersion forces.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction. This occurs when there is a hydrogen atom covalently bonded to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F). These electronegative atoms have strong tendencies to attract electrons toward themselves, which leads to a highly polar covalent bond. The result is a strong attraction between the positively charged hydrogen and other electronegative atoms in nearby molecules.

• For example, in HF (hydrogen fluoride), the hydrogen is directly linked with fluorine, making it capable of forming hydrogen bonds due to fluorine's high electronegativity.
• Materials like water ( H_2O) and ammonia ( NH_3) are other classic examples where hydrogen bonding plays a crucial role.

Hydrogen bonds significantly influence the physical properties of a substance. They contribute to the high boiling point of water and enable DNA to maintain its helical structure through the hydrogen bonds between base pairs. Without them, many biological structures would not be able to hold their shape.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest intermolecular forces. They are present between all atoms and molecules regardless of polarity. These forces arise from temporary fluctuations in the electron distribution within atoms or molecules, which create an instantaneous dipole. This instantaneous dipole then influences neighboring atoms, inducing further dipoles, resulting in weak attractions.

• Nonpolar molecules like H_2 (hydrogen gas) solely rely on dispersion forces as their intermolecular force.
• Though weak, dispersion forces can significantly increase in strength with the size of the molecule or the number of electrons as seen in larger atoms or long-chain hydrocarbons.

As they are the only forces present in nonpolar molecules, they solely dictate the boiling and melting points for such substances, making them crucial for understanding molecular behavior and interactions in gases and liquids.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that possess permanent dipoles. A permanent dipole arises when there is a significant difference in electronegativity between two bonded atoms, leading to a partial positive and partial negative charge within the molecule. These opposite charges on different molecules attract each other.

• For instance, in HCl (hydrogen chloride), the chlorine atom is more electronegative than hydrogen, resulting in a polar molecule with dipole-dipole interactions.
• Similarly, H_2S demonstrates dipole-dipole interactions due to the polarity between hydrogen and sulfur, though weaker compared to hydrogen bonding.

This kind of intermolecular interaction is generally stronger than dispersion forces but weaker than hydrogen bonds. They play a significant role in determining the properties of many polar compounds, affecting their boiling points, solubility, and other physical properties.