Bonding exam q

Question 1

Explain why pentan-2-ol has a higher boiling point than pent-1-ene.

Answer 1

M1idea that pentan-2-ol has stronger intermolecular forces
M1 idea that hydrogen bonds are stronger than van
M2pent-1-ene has van der Waals’ forces (only)
M3pentan-2-ol (also) has hydrogen bonds

Question 2

The melting point of XeF4 is higher than the melting point of PF3
Explain why the melting points of these two compounds are different.
In your answer you should give the shape of each molecule, explain why each
molecule has that shape and how the shape influences the forces that affect the
melting point.

Answer 2

Stage 1 electron pairs
1a XeF4 4BP and 2LP around Xe
1b PF3 3BP and 1LP around P

Stage 2 explanation of shapes
2a XeF4 is square planar
2b PF3 is pyramidal (allow tetrahedral)
2c Electron pairs repel as far as possible or Lone pair repels more than bonding
pairs

Stage 3 IMF
The relative strength of the intermolecular forces in the molecules must be
explained to gain maximum marks.
3a XeF4 has vdw forces and PF3 has dipole-dipole forces (and vdw)
3b Stronger/more intermolecular forces in XeF4
3c Due to larger Mr or more electrons or larger molecules or packs more closely
together

Question 3

Sodium fluoride contains sodium ions (Na+
) and fluoride ions (F–
).
Na+ and F– have the same electron configuration.
Explain why a fluoride ion is larger than a sodium ion

Answer 3

Fluoride ion has (two) fewer protons/lower nuclear charge
Weaker attraction between nucleus and (outer) electrons

Question 4

Explain, in terms of structure and bonding, why the melting point of sodium
fluoride is high.

Answer 4

(Electrostatic) forces of attraction between
oppositely charged ions
Lots of energy needed to overcome/break forces