Intermolecular Forces

Question 1

Dipole-dipole Attractions

Answer 1

Intermolecular Forces are the forces between molecules, intramolecular are forces that hold atoms together within a molecule. A permanent dipole is a permanent partial charge separation along a bond.

Dipole-dipole attractions result when two polar molecules interact with each other. When this occurs, the partially negative part of one of the polar molecules is attracted to the partially positive part of the second polar molecule.

Question 2

Hydrogen Bonding

Answer 2

Hydrogen bonding is a particularly strong type of dipole-dipole attraction. It occurs when the polar bond has a particularly large dipole. This means that the intermolecular force is particularly strong.

Hydrogen bonds are only formed when hydrogen is bonded directly to a fluorine, oxygen or a nitrogen. This is because of a large difference in electronegativity for these elements as Fl, O and N are the most electronegative but still covalent, and visa versa with hydrogen.

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Question 3

Comparing dipole-dipole attractions and hydrogen bonds

Answer 3

The stronger the intermolecular force, the more energy it takes to break them. Therefore, the stronger the intermolecular forces, the higher the melting and boiling points. Hydrogen bonding provides quirky characteristics to it’s molecules.

Question 4

Dispersion Forces (van der Waals force/Londen forces)

Answer 4

Non-polar molecules also have intermolecular forces, even without a permanent dipole. Instead of a permanent dipole, non-polar molecules form temporary dipoles. These are a result of random movement of electrons in the molecule. These are weaker than dipole-dipole attractions and hydrogen bonding, but the strength of the dispersion forces depends on the total number of electrons in that molecule.

SInce all molecules have electrons, all molecules have disperson forces. However, dispersion forces may not be the dominant intermolecular force.

Question 5

Identifying the dominant intermolecular forces

Answer 5

1. Are molecules polar (YES = 2) (NO = dispersion forces)
2. Does molecule contain H bonded to O, F, N? (YES = Hydrogen Bonding) (NO = Dipole-dipole attractions)

For molecules with similar number of electrons, hydrogen forces are the strongest, then dipole-dipole attractions and dispersion forces are weakest.