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# Hydrogen Bonds

## Key Questions

Hydrogen bonds are intermolecular forces; covalent and ionic bonds are intramolecular forces.

#### Explanation:

Ionic Bonds

Ionic bonds form when one atom transfers electrons to another atom.

The atom that loses an electron becomes a positive ion. The atom that gains an electron becomes a negative ion.

The electrostatic attraction between these ions is an ionic bond.

The ions in ionic solids are close to each other, so ionic attractions are strong.

Ionic bonds are intramolecular bonds, because the ions exert forces among the ions of the same compound.

Ionic bond strengths (lattice energies) range from 600 kJ/mol to 6000 kJ/mol.

Covalent Bonds

Covalent bonds form when two atoms share electrons.

The covalent bonds are intramolecular bonds because they hold the atoms together in a single molecule.

Covalent bond strengths range from 100 kJ/mol to 1100 kJ/mol.

Hydrogen Bonds

Hydrogen bonds are especially strong intermolecular forces.

They exist when you have a negative $\text{O}$, $\text{N}$, or $\text{F}$ atom in one molecule and a positive $\text{H}$ atom attached to an $\text{O}$, $\text{N}$, or $\text{F}$ atom in another molecule.

Water is the best-known compound that has hydrogen bonds.

Hydrogen bonds have strengths ranging from 5 kJ/mol to 50 kJ/mol.

In summary, hydrogen bonds are (relatively weak) intermolecular forces, while covalent and ionic bonds are (relatively strong) intramolecular forces.

Molecules that can hydrogen bond with water have a higher solubility in water.

#### Explanation:

Molecules which are capable of hydrogen bonds have hydrogen atoms which are covalently bonded to highly electronegative elements (O, N, F). The presence of hydrogen bonding between molecules of a substance indicates that the molecules are polar. This means the molecules will be soluble in a polar solvent such as water.

Some examples of polar molecules which can hydrogen bond are ammonia ($N {H}_{3}$) and methanol ($C {H}_{3} O H$). The polarity of these molecules indicates that they will dissolve in water.

$C {O}_{2}$ can form hydrogen bonds with water, but its linear shape makes it a nonpolar molecule. This means that carbon dioxide is less soluble in water than polar molecules are. The solubility of carbon dioxide is increased when the water is cold, and decreased greatly when the water is warm.

NOTE: Possibly confusing is a molecule like ${O}_{2}$, which has no bonds between O and H, but rather just a double bond between the two atoms of oxygen. It is also a linear molecule (O=O) which makes it non-polar and insoluble in water.

• Hydrogen bonds are not really bonds but intermolecular forces - weak forces which arise between molecules.

Hydrogen bonds are the strongest kind of intermolecular force, but are still much weaker than the covalent bonds that hold the atoms together inside a molecule - remember the intermolecular forces operate from one molecule to another.

To form a hydrogen bond we need molecules which contain a hydrogen atom which is bonded to one of the most electronegative elements (N, O or F). As a result of the electronegativity the hydrogen atom will have a partial positive charge and will therefore be electrostatically attracted to the lone pair of electrons on one of these electronegative atoms on a nearby molecule.

Hydrogen bonds formed in this way are responsible for some surprising properties: water has hydrogen bonding, which is why pond-skater insects can walk on its surface, why you can place about 40 drops of water on a single small coin (try it!), why ice floats when being solid we'd expect it to be more dense than water and sink, and why water has a high boiling point compared with the hydrides of other Group 6 elements.