Forces Between Molecules
Melting and boiling a simple molecular substance change attractions between molecules while leaving their covalent bonds intact. Boiling points depend on dispersion, molecular polarity and hydrogen bonding together.
What a learner can do afterwards
- Distinguish the energy needed to separate molecules from the energy needed to break a covalent bond
- Predict how electron-cloud size affects boiling points when dispersion is the only relevant attraction, and recognise why force names alone do not give a universal ranking
- Explain why water boils far above the temperature its molecular mass alone would suggest
1 · Read
You have met polar bonds. Covalent bonds join atoms within a molecule. Intermolecular forces attract separate molecules. Melting rearranges molecules; boiling separates them into a gas. Their covalent bonds stay intact. Vaporising one mole of liquid HCl takes about 17 kJ. Splitting the H-Cl bonds in one mole of gaseous HCl takes about 430 kJ, roughly 25 times as much energy.
Every molecule has dispersion forces, caused by temporary uneven electron distributions. Larger, more easily distorted electron clouds usually give stronger dispersion attractions. Shape matters too, because it affects contact between molecules. At room temperature and atmospheric pressure, F2 and Cl2 are gases, Br2 is liquid and I2 is solid. Their increasing electron-cloud sizes help explain this trend.
Polar molecules also have dipole-dipole attractions between opposite partial charges. In the hydrogen bonding studied here, H bonded to N, O or F attracts a lone pair on N, O or F in a nearby molecule. This is a particularly strong dipole-dipole attraction. For small molecules of similar size and shape, hydrogen bonding usually gives the strongest attractions of these three types. The comparison is not universal.
Water boils at 100°C at standard atmospheric pressure. This is unusually high for such a small molecule because water molecules form hydrogen bonds with several neighbours. Attraction between molecules of the same substance is called cohesion. Cohesion produces surface tension, which helps a brimful glass hold a small dome of water. The covalent O-H bonds do not stretch into a skin.
Compare boiling points at the same pressure. Identify all the attractions, then compare electron-cloud size and molecular shape. For similar-sized molecules, polarity and hydrogen bonding often raise boiling points. Among similarly shaped nonpolar molecules, a larger, more easily distorted electron cloud usually raises the boiling point. A large nonpolar molecule can have stronger total attractions than a much smaller molecule that hydrogen-bonds.
Boiling separates intact molecules. Compare their combined attractions, including dispersion, polarity and hydrogen bonding, at the same pressure.
Written for this dataset using OpenStax Chemistry 2e and Khan Academy as factual references. Their prose was not adapted.
Both readings were written for this dataset. OpenStax Chemistry 2e and Khan Academy supplied factual background, not adapted wording.
More to read
Separating molecules
Water vapour contains H2O molecules, just as liquid water does. Boiling changes the spacing and movement of molecules without splitting their atoms apart. Breaking covalent O-H bonds would be a different process.
All molecules attract through dispersion forces. Their electrons can have temporary uneven distributions, which affect neighbouring electron clouds. Clouds that are more easily distorted usually produce stronger dispersion attractions. The shape of a molecule also affects how much contact it can make with its neighbours.
Polar molecules have lasting partial positive and negative regions. Opposite regions on neighbouring molecules attract. Hydrogen bonding is a particularly strong interaction involving hydrogen attached to N, O or F and a suitable nearby lone pair.
A boiling-point prediction needs all these effects together. Compare substances at the same pressure. When size and shape are similar, polarity and hydrogen bonding often raise boiling points. When nonpolar molecules have similar shapes, increasing electron-cloud size usually strengthens dispersion attractions.
The force names do not give an ordering that works for every substance. A large nonpolar molecule can have strong total attractions. Water's hydrogen bonding explains why its boiling point is unusually high for its small molecular size.
Water touching water and glass
A small amount of water can form a dome above a full glass's rim. The liquid surface resists an increase in its area. This effect is called surface tension.
Attractions between water molecules produce surface tension. Attraction between molecules of the same substance is called cohesion. Hydrogen bonding contributes to water's cohesion, but the surface is not a separate sheet of material. The O-H bonds inside water molecules do not stretch into a rubber skin.
Adhesion describes attraction between different materials. Water can adhere to glass. In a narrow glass tube, adhesion to the walls and cohesion within the water together help the water rise. The curved top of the liquid is a meniscus.
These words name different contacts. Water attracting water is cohesion. Water attracting glass is adhesion. Both help explain the shape and behaviour of liquid water.
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