stpm 2 ( Group 2 Period 3)

1. Na2O, Al2O3 and SO3 have melting points of 1275oC, 2072oC and 17oC respectively.
a) Relate the melting points with the structure and bonding present in these oxides. [9]
b) Give an account of the solubility and reactions of these oxides with water.
Give the pH for the solutions formed and write relevant equations. [6]

Answer:

a)
Na2O and Al2O3 have giant ionic structures which are held together by ionic bonds. Strong electrostatic forces of attraction exist between the ions. Therefore Na2O and Al2O3 have high melting points.

Of the two oxides, Al2O3 has a higher melting point than Na2O. This is because the ionic bond between Al3+ ions and O2- ions is stronger than that between Na+ and O2-. The charge on the Al3+ ion is larger than that of the Na+ ion. The lattice energy of Al2O3 > Na2O.

The attractive force between sulphur trioxide molecules, SO3 are weak van der Waals forces of attraction. Therefore, it has a very low melting point.

b)
Na2O dissolves easily in water.
Na2O(s) + H2O(l) ----> 2NaOH(aq)

The solution formed is strongly alkaline. The pH for a strong alkali is 14. The reaction between Na2O and water is exothermic.

Al2O3 is insoluble and does not react with water. However, SO3 dissolves easily in water.
SO3 (g) + H2O(l) -----> H2SO4(aq)

The solution formed is a strong acid. The pH of a strong acid is 1. The reaction between SO3 and water is exothermic.

stpm 1 (Free Radical Reaction)

2.
Each chlorine atom that is produced from chlorofluoroalkane in the stratosphere can destroy almost a hundred thousand ozone molecules. Explain. [5]

Answer:

Chorofluoroclkanes, such as Freon can initiate a free radical chain reaction.

Initiation step: CF2Cl2 + light (UV) ---> CF2Cl● + Cl●

Propagation step: Cl● + O3 --> ClO● + O2
ClO● + O3 --> Cl● + 2O2

In the propagation step, ozone is destroyed and the chlorine radical Cl● is regenerated. Therefore, one chlorine atom ( in the form of a radical) can destroy thousands of ozone molecules. The chlorine radical can only be annihilated by colliding with another chlorine radical.

Cl● + Cl● -> Cl2