Question

Which has the stronger intermolecular forces: \(\mathrm{N}_{2}\) or \(\mathrm{H}_{2} \mathrm{O} ?\) Explain.

Short answer

H2O has stronger intermolecular forces than N2. This is because H2O experiences hydrogen bonding, which is a stronger force than the London dispersion forces present in N2.

Step-by-step solution

Identify the type of intermolecular force in N2

N2 is a diatomic molecule composed of two nitrogen atoms. It is a nonpolar molecule because the electronegativity difference between the nitrogen atoms is zero. This means it only experiences London dispersion forces, which are the weakest type of intermolecular forces.

Identify the type of intermolecular force in H2O

H2O, or water, is a polar molecule because the oxygen atom is more electronegative than the hydrogen atoms. This creates a dipole moment in the molecule. As a result, H2O primarily experiences hydrogen bonding, which is a stronger intermolecular force than London dispersion forces.

Compare intermolecular forces and strengths

Comparing the intermolecular forces between N2 and H2O, we see that N2 has London dispersion forces, while H2O has hydrogen bonding. Hydrogen bonding is significantly stronger than London dispersion forces.

Determine which molecule has stronger intermolecular forces

Since H2O experiences hydrogen bonding, which is a stronger intermolecular force than the London dispersion forces experienced by N2, the molecule with the stronger intermolecular forces is H2O.

Key concepts

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular forces and are present in all molecules, whether polar or nonpolar. These forces arise due to the instantaneous fluctuations in electron distribution within molecules. Even in a nonpolar molecule like nitrogen ( N_2 ), transient dipoles can form momentarily. This is due to the electrons moving around the nucleus of the atoms, leading to temporary regions of partial positive and negative charges.

This temporary dipole in one molecule can induce a dipole in a nearby molecule, leading to a weak attraction between the two. Since these forces are dependent on the ease of polarizing the electron cloud, larger molecules with more electrons generally have stronger London dispersion forces.
However, in small molecules like ( N_2 ), these forces are weak and contribute much less to the overall intermolecular attraction compared to other forces like hydrogen bonding.

In summary:

• London dispersion forces occur in all molecules.
• They originate from temporary dipoles due to electron movement.
• Their strength increases with larger, more polarizable electron clouds.
• They are weaker than most other intermolecular forces.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs in molecules where hydrogen is bonded to highly electronegative elements such as oxygen, fluorine, or nitrogen. The classic example of hydrogen bonding is found in water ( H_2O ).

In ( H_2O ), the oxygen atom is significantly more electronegative than the hydrogen atoms. This causes a dipole moment, where the oxygen atom has a partial negative charge, and the hydrogen atoms have partial positive charges. The hydrogen atom in one water molecule is attracted to the oxygen atom in another, forming a hydrogen bond.
These hydrogen bonds are responsible for many of the unique properties of water, such as its high boiling and melting points compared to other molecules of similar size.

Characteristics of hydrogen bonding:

• Present in molecules with hydrogen bonded to F, O, or N.
• Contributes to higher boiling and melting points.
• Stronger than other dipole-dipole interactions.
• Critical for biological molecules like DNA and proteins.

Polarity of Molecules

The concept of molecular polarity is fundamental in understanding the behavior and properties of molecules. Polarity arises due to the difference in electronegativity between atoms bonded together. When electrons are not shared equally, a dipole moment is created.

In polar molecules, like water ( H_2O ), this unequal sharing of electrons leads to one side of the molecule having a slightly negative charge, while the other side has a slightly positive charge. This makes polar molecules interact strongly with other polar substances, influencing their solubility and boiling points.
In contrast, nonpolar molecules like nitrogen ( N_2 ) have equal sharing of electrons between their atoms, resulting in no permanent dipole moment. Therefore, they only experience weaker intermolecular forces such as London dispersion forces.

Key points of molecular polarity:

• Caused by differences in electronegativity.
• Affects boiling/melting points and solubility.
• Polar molecules have permanent dipole moments.
• Nonpolar molecules do not have permanent dipoles.