Hydrogen bonding is a relatively strong intermolecular attraction that occurs when hydrogen is covalently bonded to a highly electronegative atom such as fluorine (F), oxygen (O), or nitrogen (N) and is attracted to a lone pair of electrons on an electronegative atom of a neighboring molecule.
Hydrogen bonding has a significant effect on the physical properties of molecular substances, particularly their melting points and boiling points. This is because additional energy is required to overcome these intermolecular attractions.
Effect on Boiling Point
The boiling point of a substance is the temperature at which its liquid changes into a gas. During boiling, molecules must separate sufficiently to enter the gas phase.
When hydrogen bonds are present between molecules, these molecules are held together more strongly. Consequently, more thermal energy is required to overcome the intermolecular attractions and convert the liquid into a gas.
Thus, hydrogen bonding generally results in an increase in boiling point.
Example: Water
Water molecules form extensive hydrogen bonds with one another. These strong intermolecular attractions require considerable energy to overcome, giving water a relatively high boiling point of 100 °C at 1 atm pressure.
In comparison, molecules of similar size that cannot form hydrogen bonds generally have much lower boiling points when other relevant intermolecular forces are comparable.
Effect on Melting Point
The melting point is the temperature at which a solid changes into a liquid. In a solid, molecules are arranged closely and are held together by intermolecular forces.
Hydrogen bonding can strengthen the interactions within the solid and make the solid structure more stable. More thermal energy may therefore be required to disrupt the intermolecular arrangement and produce a liquid.
As a result, hydrogen bonding can lead to a higher melting point. However, the effect on melting point is more complicated than the effect on boiling point because the melting point also depends strongly on the shape, symmetry, packing, and crystal structure of the molecules.
Examples of Hydrogen Bonding and Boiling Points
1. Water (H2O): Water molecules form extensive hydrogen-bond networks. This contributes to its relatively high boiling point compared with many molecules of similar molecular mass.
2. Ethanol (C2H5OH): Ethanol molecules can form hydrogen bonds through their hydroxyl (-OH) groups. Therefore, ethanol has a higher boiling point than hydrocarbons of comparable molecular size that cannot form hydrogen bonds.
3. Ammonia (NH3): Ammonia can form hydrogen bonds because nitrogen is electronegative and the molecule contains N-H bonds. However, its hydrogen bonding is less extensive than that in water, contributing to differences in their physical properties.
Why Hydrogen Bonding Raises Boiling Point
The effect can be summarized in the following sequence:
Hydrogen bonding → stronger intermolecular attraction → more energy required to separate molecules → higher boiling point.
Hydrogen Bonding and Melting Point: An Important Qualification
Although hydrogen bonding often raises melting points, it does not always produce a simple increase. The melting point of a molecular solid depends not only on the strength of intermolecular forces but also on how efficiently molecules pack in the crystal lattice.
For example, two compounds may both form hydrogen bonds but have different melting points because their molecular shapes and crystal packing are different. Therefore, hydrogen bonding should be considered together with molecular symmetry, shape, size, and crystal packing when comparing melting points.
Boiling-Point Comparison
The following table compares substances with and without hydrogen bonding. It shows how stronger intermolecular attractions can lead to higher boiling points.
| Substance | Formula | Hydrogen Bonding | Boiling Point |
|---|---|---|---|
| Methane | CH4 | No | −161.5 °C |
| Ammonia | NH3 | Yes | −33.3 °C |
| Ethanol | C2H5OH | Yes | 78.4 °C |
| Water | H2O | Yes, extensive | 100 °C |
Observation: Hydrogen bonding contributes to higher boiling points because additional energy is needed to overcome the intermolecular attractions between molecules. The exact boiling point also depends on molecular mass, shape, polarity, and other intermolecular forces.
Summary
- Hydrogen bonding is a relatively strong intermolecular attraction.
- It increases the attraction between neighboring molecules.
- More energy is generally required to separate hydrogen-bonded molecules.
- Hydrogen bonding generally increases the boiling point of a molecular substance.
- It can also increase the melting point, although melting points are strongly influenced by crystal packing and molecular symmetry.
- Water and ethanol are common examples of substances whose physical properties are strongly influenced by hydrogen bonding.