6.4 - Hydrogen Bonding

Question 1

What is a hydrogen bond?

Answer 1

A hydrogen bond is a special type of permanent dipole-dipole interaction found between molecules containing:

1. An electronegative atom with a lone pair of electrons (e.g., oxygen, nitrogen, fluorine).
2. A hydrogen atom attached to an electronegative atom (e.g., H-O, H-N, H-F).

Question 2

How does a hydrogen bond form?

Answer 2

A hydrogen bond forms between:

A lone pair of electrons on an electronegative atom in one molecule.

A hydrogen atom attached to an electronegative atom in another molecule.

Question 3

How is a hydrogen bond represented?

Answer 3

It is shown by a dashed line.

Question 4

What is the shape around the hydrogen atom involved in a hydrogen bond?

Answer 4

The shape around the hydrogen atom is linear.

Question 5

What role do lone pairs play in hydrogen bonding?

Answer 5

Lone pairs on the electronegative atoms (e.g., oxygen in water or nitrogen in ammonia) are crucial in forming hydrogen bonds.

Question 6

What are common examples of molecules that form hydrogen bonds?

Answer 6

Molecules like water (H₂O) and ammonia (NH₃) form hydrogen bonds due to the presence of highly electronegative atoms and hydrogen atoms.

Question 7

Why is solid ice less dense than liquid water?

Answer 7

In ice, hydrogen bonds hold water molecules in an open lattice structure with molecules farther apart than in liquid water, making ice less dense.

Question 8

What happens to water molecules in the ice lattice?

Answer 8

1. The water molecules are held apart in a tetrahedral structure with holes in the lattice.
2. Each water molecule forms four hydrogen bonds, resulting in an open structure.

Question 9

Why does ice float on water?

Answer 9

Ice is less dense than liquid water due to its open lattice structure, allowing it to float.

Question 10

What happens when ice melts?

Answer 10

The lattice collapses, and water molecules move closer together, increasing density.

Question 11

How does floating ice benefit aquatic life?

Answer 11

Ice forms an insulating layer on ponds and lakes, preventing the water below from freezing solid, which is essential for aquatic life survival.

Question 12

Why does water have a high melting and boiling point?

Answer 12

Water molecules have hydrogen bonds in addition to London forces. These hydrogen bonds require significant energy to break, resulting in a much higher melting and boiling point than expected.

Question 13

What would happen to water without hydrogen bonds?

Answer 13

Without hydrogen bonds:

1. Water would boil at around -75°C, existing as a gas at room temperature.
2. Liquid water would not exist in most places on Earth, making life as we know it impossible.

Question 14

What happens to hydrogen bonds when ice melts or water boils?

Answer 14

1. When ice melts: The rigid hydrogen bond arrangement is broken.
2. When water boils: Hydrogen bonds break completely.

Question 15

What other unusual properties of water are caused by hydrogen bonds?

Answer 15

Hydrogen bonds contribute to water’s:

1. High surface tension: Water forms droplets that are not easily wet.
2. High viscosity: Allows insects to walk on water surfaces.

Question 16

How do detergents affect water’s surface tension?

Answer 16

Detergents reduce surface tension, making water “wetter.”

Question 17

What anomalous properties of water are you required to know?

Answer 17

1. Density (ice is less dense than water).
2. Melting point.
3. Boiling point.