Tutorial (Structure and bonding)
1. Rank the following functional groups: hydroxyl, carbonyl, carboxyl, phenol
and ether in the order of decreasing polarity. Give reasons. (10 marks)
 Two marks for correct arrangement
Carboxyl:

Two oxygen atom and one hydrogen atom available for hydrogen
bonding.

Electron delocalisation between the two oxygen atoms stabilises the
carboxylate ion. (pKa 4.76)
Phenol:

One oxygen and one hydrogen atom available for hydrogen bonding

Electron delocalisation confers stability to phenoxide ion (pKa 9.95)
Hydroxyl:

Oxygen available for hydrogen bonding (acceptor)

Hydrogen bonded to oxygen available for hydrogen bonding (donor)
Carbonyl:

Only one oxygen atom for hydrogen bonding, no ions possible
Ether:

Steric factors inhibit hydrogen bonding
2. Calculate the ΔH°comb for 1 mol of methanol. Methanol burns in oxygen
according to the equation below.
(6 marks)
2 CH3OH + 3 O2 → 2 CO2 + 4 H2O
Methanol
The bond energy of the different bonds are as follows:
C−O
O=O
C−H
O−H
C=O
358 kJ/mol
498 kJ/mol
413 kJ/mol
467 kJ/mol
799 kJ/mol
CH3OH + 1.5 O2 → CO2 + 2 H2O
3 C−H bonds: 3(413 kJ/mol)
= 1239 kJ
1 C−O bonds: 1(358 kJ/mol)
= 358 kJ
1 O−H bonds: 1(467 kJ/mol)
= 467 kJ
1.5 O=O bonds: 1.5(498 kJ/mol) = 747 kJ
∑ ΔH°disassociation
= 2811 kJ
2 C=O bonds: 2(−799 kJ/mol) = −1598 kJ
4 O−H bonds: 4(−467 kJ/mol) = −1868 kJ
∑ ΔH°formation = −3466 kJ
ΔH°comb = ∑ ΔH°disassociation + ∑ ΔH°formation
= 2811 + (−3466) = −655 kJ/mol
3. Ionic compounds tend to have a ∆EN >1.7. Based on their electronegativity,
explain why Group 4 compounds (Carbon family) tend to be covalent rather
than ionic. (3 marks)
 Group 4 elements have moderate EN ranging from 1.8 to 2.5.
 They are unable to form anions with most alkali metals that have low
electronegativity (EN ~0.9)
 They are unable to form cations with most halogens except for fluorine
(EN 4.0)
4. Arrange the following diatomic halogen molecules in terms of increasing bond
length: F2, I2, Cl2 and Br2. Briefly explain the difference in bond length.
(2 marks)
 Increasing bond length: F2 < Cl2 < Br2 < I2,
 Atomic size increases down a group
5. Do substances with stronger bonds or weaker bonds release more energy when
burned? Briefly explain. (3 marks)
 Substances with weaker bonds release more energy
 Less energy would be required to break the molecular bonds
 This would result in a more exothermic enthalpy change when products are
formed