Unit 1 - Structure and Bonding: Intermolecular Forces

Question 1

Intermolecular Forces

Answer 1

The noble gases do not form covalent or ionic bonds between their atoms and yet all of the noble gases will condense to liquids and eventually form solids, provided the temperature is sufficiently low.

**The fact that it is possible to liquefy and solidify the noble gases implies that attractive forces exist between atoms to hold them together in the liquid and solid state. **

Attractive forces also exist between molecular elements and compounds. The intermolecular forces acting between molecules are known as van der Waals forces.

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Question 2

London Dispersion Forces

Answer 2

London dispersion forces are forces of attraction that can operate between all atoms and molecules and are the weakest of the van der Waals forces.

London dispersion forces are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.

London dispersion forces are cause by the uneven distribution of the constantly moving electrons around the nuclei of the atoms. This causes temporary dipoles on the atoms. The atoms then attract each other. The dipoles are constantly changing but there are always some in existence.

Question 3

Strength of LDFs

Answer 3

The strenght of London dispersion forces is related to the number of electrons within an atom or molecule.

As the number of electrons within the atoms increases, the strength of the London dispersion forces increases. As the strength of the London dispersion forces increases, the boiling point of the noble gases, halogens and alkanes increases.

Question 4

Polar Molecules

Answer 4

A molecule is polar if it has a permanent dipole. Some molecules containg polar covalent bonds end up with an overall polarity (permanent dipole) because the spatial arrangement of polar covalent bonds (bonds not arranged symmetrically) can result in a molecule being polar.

Question 5

Polar Molecule Requirements

Answer 5

For a molecule to be polar, it must have polar covalent bonds and when a line is drawn through the middle, all the atoms on one side are slightly positive and slightly negative on the other side.

other molecules have a symmetrical arrangement of polar bonds and the polarity cancels out. These molecules are described as non-polar.

Polar liquids are deflected by an electric field.

Question 6

Permanent Dipole - Permanent Dipole Interactions

Answer 6

The intermolecular forces between non-polar molecules are the weak London dispersion forces. PD-PD interactions are additional electrostatic forces of attraction between the permanent dipoles of polar molecules.

Permanent dipole-permanent dipole interactions are stronger than London dispersion forces for molecules with similar numbers of electrons (equivalent size).

Question 7

Hydrogen Bonding

Answer 7

Hydrogen atoms have a very low electronegativity (2.2). As a result, bonds consisting of a hydrogen atom covalently bonded to an atom of a strongly electronegative element such as flurone, oxygen of nitrogen are highly polar. Molecules containing O-H or N-H covalent bonds are always polar.

The pull of electrons away from the hydrogen results in an unusually strong permanent dipole capable of interacting with similar molecules.

A hydrogen bond is slightly stronger than other types of intermolecular forces but much weaker than covalent bonds.

Question 8

Forces of Attraction - Order of Strength

Answer 8

Covalent bond - 100-500 kJ mol-1
Hydrogen Bond - 40 kJ mol-1
Permanent dipole-permanent dipole - 30 kJ mol-1
London dispersion forces - 1-20 kJ mol-1