Intermolecular Forces (Secondary Bonding)

Question 1

Intermolecular Forces

Answer 1

Weaker attractive forces than primary bonding (covalent, ionic, metallic), but exist between molecules not within molecules. Intermolecular forces determine how a molecule compound exists at room temperature, as well as melting and boiling points which is dependent on the strength of the intermolecular forces.

Question 2

Dispersion Forces

Answer 2

Dispersion forces are the weakest intermolecular forces that exists between all molecules, both polar and non polar. Dispersion forces occur between instantaneous dipoles as electrons are constantly in motion and can create temporary dipoles. It is influenced by two factors:
1. the number of electrons in the molecules which increases strength of dispersion forces.
2. the shape of molecules, affects how closely the molecules may attract in solid and liquid states. The closer the molecules get = stronger attraction

Question 3

Dipole-dipole interactions

Answer 3

Occurs between polar molecules which cause simultaneous intermolecular attractions when the positive pole of one molecule attracts the negative pole of another. This is a stronger intermolecular force than dispersion forces and results in a higher boiling point if a substance is affected by both dispersion and dipole-dipole.

Question 4

Hydrogen Bonding

Answer 4

Hydrogen bonding occurs in polar molecules, it is the strongest type of intermolecular bonding. Forms only between molecules when hydrogen bonds to the atoms nitrogen or oxygen, or fluorine, which have a higher electronegativity, causing a positive and negative dipole. hey are weaker bonds than covalent bonds. Higher melting and boiling points are expected.

Question 5

Strongest to Weakest Intermolecular Forces

Answer 5

IMFS are used to predict relative boling points as the stronger the forces between the molecules, the more force is required to break these bonds when substances are converted from a liquid to gas.
Strength of intermolcular forces:
Ionic bonds>Hydrogen Bonding>Dipole-dipole>Dispersion Forces

Question 6

Intermolecular Forces and boiling points

Answer 6

Strong bonds such as ionic bonds are highly polarised as seen in high melting points. Hydrogen bonding is not as polar as ionic bonding but sitll possesses a dipole. Dipole-dipole forces tend to be weaker as electronegativity difference is more minimal. Dispersion forces are the weakest as attraction is temporary due to movement of electrons. Polarisability increases with atomic size therefore higher boiling point. Higher molecular weight also plays a role as dispersion forces increase in larger molecules, there is more surface area for dispersion forces.

Question 7

Explain the high boiling point of water

Answer 7

While a water molecule has a small size the high boiling point can be explained on the basis of H-bonding that takes place. A water molecule can form a maximum of 4 H- bonds so it forms many strong intermolecular bonds . This increases the effective size of water molecules too, which in turn increases the dispersion forces.

The two reasons for high boiling point of water are
1. Formation of H-bonds, which requires more energy to break
2. Increase in dispersion forces, therefore more energy must be supplied to break bonds

Question 8

Polarisability: ability to be polariased another molecule

Answer 8

1. Dependent on molecular size, so larger molecules have more polarisable electron clouds as electrons move more freely
2. Molecules with elongated shapes have larger more polarisable electron clouds, symmetrical molecules are more compact (less polarisable)
3. Number of electrons in electron clouds, molecules with more electrions are more likely to form temporary dipoles.

Question 9

Answer 9

Pentane has the higher boiling point as the long shape increases size and polarisability of electron clouds, therefore stronger dispersion forces.