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Intermolecular forces for OCR A-level Chemistry

Intermolecular forces

This page covers the following topics:

1. Permanent dipole interactions
2. London dispersion forces
3. Hydrogen bonding
4. Strength of intermolecular forces

Permanent dipole interactions are electrostatic forces between permanent dipoles with regions of both positive and negative partial charges. The partial charges in molecules arise because of non-polar covalent bonds that commonly include atoms with high electronegativities like chlorine and oxygen. Therefore, compounds that have higher differences in electronegativities and are not 3D symmetric form stronger permanent diplole interactions.

Permanent dipole interactions

London dispersion forces, also known as van der Waals dispersion forces or induced diplo-dipole interactions, are intermolecular interactions caused by electrostatic forces which arise due to temporary fluctuating dipoles in all molecules and atoms. Naturally electrons fluctuate in particles, this way forming temporary dipoles that continuously change. When two particles get close to each other, the temporary dipoles may become induced dipoles with ฮด+ sides attracting ฮดโˆ’, resulting in an intermolecular interaction. Individually they're weak, but their strength increases with number.

Polarisability is a measure of how readily the electron distribution in an atom or molecule can be distorted, to form a dipole. A larger particle usually has a higher number of electrons; and thus, a higher polarisability. The higher the polarisability, the stronger London dispersion forces are formed. Thatโ€™s why larger molecules like Pโ‚„ and Sโ‚ˆ have higher melting and boiling points than Clโ‚‚, Oโ‚‚ or Ne.

London dispersion forces

Hydrogen bonding is an intermolecular interaction between a hydrogen polarised by an electronegative atom in one molecule and a lone pair of electrons of an electronegative atom in another molecule. Molecules participating in hydrogen bonding have clear partial negative and partial positive charges. Common examples of the electronegative atoms include N, O, F. Water molecules have each 0 hydrogen bonds in steam, 2 in liquid water and 4 in ice, which results in ice having a lower density than liquid water due to spaced out lattice.

The stronger the hydrogen bonding, the higher energy is needed for molecules to escape a solid lattice or a liquid or move around or to dissolve in water; and respectively, the higher melting point, boiling point, viscosity and solubility in water are. In fact, hydrogen bonding is one of the major causes of higher melting and boiling points of ammonia, water and hydrogen fluoride compared to some other compounds.

Hydrogen bonding

The main intermolecular interactions in an increasing strength are London dispersion forces, permanent dipole-dipole interactions, hydrogen bonding. Permanent dipole interactions occur between permanent dipoles, whereas London dispersion forces occur between temporary fluctuating dipoles.

Intermolecular interactions are weak in comparison to the strength of interatomic bonds, but collectively they contribute greatly to the physical properties of a molecule, like boiling and melting points. Therefore, compounds that break covalent bonds to melt or boil (silicon, carbon, sand) need much more energy, which results in higher melting or boiling points. In contrast, materials that only break weak intermolecular interactions while melting or boiling, require less energy and have lower melting/boiling points.

Strength of intermolecular forces

1

Define permanent diplole interactions.

Permanent dipole interactions are electrostatic forces between permanent dipoles with regions of both positive and negative partial charges.

electrostatic forces between permanent dipoles

Define permanent diplole interactions.

2

Why does phosphorus have a lower melting point than sulphur?

A larger Sโ‚ˆ molecule has a higher number of electrons than Pโ‚„; and thus, a higher polarisability. The higher the polarisability, the stronger London dispersion forces are formed resulting in higher energy needed for molecules to leave a solid lattice and for the material to melt. Sulphur molecules also have a higher mass.

Sโ‚ˆ has higher mass, more electrons, higher polarisability, stronger London dispersion forces, and more energy needed for molecules to leave solid lattice than Pโ‚„.

Why does phosphorus have a lower melting point than sulphur?

3

Define hydrogen bonding.

Hydrogen bonding is an intermolecular interaction between a hydrogen polarised by an electronegative atom in one molecule and a lone pair of electrons on an electronegative atom in another molecule.

intermolecular interaction between a hydrogen in one molecule and an electronegative atom in another one

Define hydrogen bonding.

4

Water molecules can form hydrogen bonds with each other as well as dissolved compounds. Draw a hydrogen bonding that occurs between two water molecules.

Hydrogen bonding is an intermolecular interaction between a hydrogen polarised by an electronegative atom in one molecule and a lone pair of electrons on an electronegative atom in another molecule. Molecules participating in hydrogen bonding have clear partial negative and partial positive charges. Common examples of the electronegative atoms include N, O, F.

image

Water molecules can form hydrogen bonds with each other as well as dissolved compounds. Draw a hydrogen bonding that occurs between two water molecules.

5

Explain why oxygen has a higher boiling point than neon.

A larger Oโ‚‚ molecule has a higher number of electrons than Ne atom; and thus, a higher polarisability. The higher the polarisability, the stronger London dispersion forces are formed resulting in higher energy needed for particles to leave a liquid lattice and for the material to boil. Oxygen molecules also have a higher mass than neon atoms.

Oโ‚‚ has higher mass, more electrons, higher polarisability, stronger London dispersion forces, and more energy needed for molecules to leave a liquid than Ne.

Explain why oxygen has a higher boiling point than neon.

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