AQA A-level Chemistry Intermolecular structure
This page covers the following topics:
1. Van der Waals forces
2. Permanent dipole interactions
3. London dispersion forces
4. Hydrogen bonding
5. Strength of intermolecular forces
Van der Waals forces are intermolecular interactions between covalent molecules. Larger molecules can have stronger van der Waals interactions, increasing the energy needed to separate them from other molecules. Therefore, the melting and boiling points of molecular compounds increase with the size of molecules.
Van der Waals forces are grouped into:
⋅ permanent dipole interactions
⋅ London dispersion forces
⋅ hydrogen bonding
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.
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 δ+ sides attracting δ−, 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. That’s why larger molecules like P₄ and S₈ have higher melting and boiling points than Cl₂, O₂ or Ne.
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. 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 high melting and boiling points of ammonia, water and hydrogen fluoride.
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.
Define van der Waals forces.
Define permanent diplole interactions.
Why does phosphorus have a lower melting point than sulphur?
Define hydrogen bonding.
Which of the following (1, 2, 3, 4) are van der Waals forces?
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