physical properties - unit 4 AOS 1

Question 1

temporary dipoles (dispersion forces)

Answer 1

In any molecule, electrons can momentarily be distributed unevenly within the molecules, inducing a temporary dipole.

Neighbouring molecules with similar temporary dipoles are attracted weakly to each other. This results in weak dispersion forces between the molecules.

The strength of the dispersion forces is affected by the size and shape of molecules.

Therefore, dispersion force strength increases with and increase in carbon chain length, reduced branching, increased saturation.

Question 1

permeant dipoles

Answer 1

The partial positive charge on one molecule is electrostatically attracted to the partial negative charge on a neighbouring molecule.

Permanent dipole-dipole attractions only occur between polar molecules (which
contain permanent dipoles, and not between symmetrical non-polar
molecules with a polar bond.

Dipole–dipole attractions are stronger intermolecular forces than dispersion forces.

Question 2

hydrogen bonding

Answer 2

When hydrogen forms a bond with one of fluorine, oxygen or nitrogen (highly electronegative atoms), its electrons move slightly towards that atom.

The molecule that forms is a dipole. Hydrogen bonding occurs between this dipole and another molecule that must also contain an electronegative atom, such as oxygen or nitrogen.

Question 3

relative strength of each force

Answer 3

hydrogen bonding > dipole-dipole attractions > dispersion forces

• hydrogen bonds are the strongest due to the the larger dipole moment that exists within these molecules, and because of the small size of the hydrogen atom involved.
• This allows the molecules to get closer to each other than in dipole–dipole attractions, thus increasing the force of attraction

Question 4

boiling point and malting point

Answer 4

The stronger the intermolecular forces, the higher the boiling point and melting point

The presence of different functional groups increases melting points because of the potential hydrogen bonding or dipole–dipole attractions.

As the number of carbon atoms increases (i.e. the mass increases) there are more dispersion forces between the molecules, which may have a greater effect than the hydrogen bonding and dipole–dipole attractions.

The closer the molecules can be arranged in solids, the greater the dispersion forces, so compounds with longer-chain molecules will have higher melting temperatures than branched molecules of the same mass.

Similarly, more symmetrical molecules can pack more closely, which also results in a higher melting point.

Question 5

viscosity

Answer 5

Viscosity is the resistance to flow of a liquid, and is affected by intermolecular forces and the shapes of the molecules present. Honey has high viscosity and water has low viscosity

The increased number of intermolecular forces in larger molecules, together with the possibility of branched molecules becoming tangled, results in higher viscosity

Viscosity decreases as temperature increases because the molecules attain enough energy to overcome the forces holding them together.

non-polar substances that mainly experience dispersion forces, like cooking oil, tend to be less viscous because their intermolecular forces are weaker, allowing them to flow more readily in comparison.