Part A. Answer all twelve questions with a few sentences or equations (5 marks each).
1. State Dalton’s law of partial pressures.
Dalton’s law of partial pressures states that the total pressure of a mixture of gases is the sum of their partial pressures.
2. How does NO(g) act as a catalyst in the production of ozone at ground level? Show the relevant reactions.
NO(g) + ½ O2(g) → NO2(g)
NO2(g) + hν → NO(g) + O(g)
O(g) + O2(g) → O3(g)
Since NO(g) is destroyed in the first step but regenerated in the second step, it is acting as a catalyst.
3. Why does a bomb calorimeter actually measure (E, and not (H of a reaction?
In a bomb calorimeter, the volume is constant, so no work is done. (H is measured only under constant pressure conditions, like in a coffee cup calorimeter.
4. State Hund’s rule.
Hund’s rule states that electrons will occupy degenerate orbitals so as to be as unpaired as possible.
5. Why is a substance with a large bandgap likely to be transparent?
If the bandgap is large, then lower energy photons, such as those in the visible portion of the spectrum, do not have enough energy to excite electrons from the HOMO to the LUMO. These visible photons are therefore not absorbed, and are transmitted by the substance, causing it to be transparent (clear).
6. Why does CO2 have a much lower boiling point than NO2?
CO2 is a linear molecule, and is not polar, and therefore experiences only dispersion forces with other CO2 molecules. NO2 is bent and polar, and therefore interacts with other NO2 molecules via a relatively strong dipole-dipole force. This means that it will require more thermal energy to break free from these intermolecular forces, i.e. the boiling point will be higher.
7. Why is hexane (C6H14) not soluble in water?
Hexane has only dispersion forces. Water has H-bonds. The two types of forces are very different, and simply put,...