When these electrons return to their ground states, radiation is
emitted. For many Groups I and II elements, the emitted
radiation falls into the visible light region of the
electromagnetic spectrum. Since the amount of energy of the
emitted radiation is quantized, the flame colour is a
characteristic property of the element.
Solubility: When an ionic solid dissolves in water, two
processes are taking place. They are the breakdown of the
ionic lattice, and the subsequent stabilization of the ions by
water molecules ( this process is called hydration ).
When an ionic solid dissolves in water, there must be
energetically favourable interactions between the water
molecules and the dissolved ions. These interactions
compensate for the breaking of ionic bonds present in the
ionic lattice. This can be considered from the point of view of
energetics. The first process involves a release of energy
when the ions are hydrated, that is , when new bonds are
formed between the ions and water molecules.
5. Answers to Question
5.1 Account for the underlying principle of flame test.
Since the outermost shell electrons of atoms of both
Groups I and II elements are weakly held by the nucleus, the electrons are easily excited to higher energy levels upon heating.
When these electrons return to their ground states, radiation is emitted. For many Groups I and II elements, the emitted radiation falls into the visible light region of the electromagnetic spectrum. Since the amount of energy of the emitted radiation is quantized, the flame colour is a characteristic property of the element.
5.2 Based on the observed trend, explain the
solubility of selected Group II metal
sulphates and hydroxides in terms of
lattice enthalpy and hydration enthalpy.
For the sulphates(VI) of Group II metals, the cations are
much smaller than the anions. The H lattice is mainly
determined by the reciprocal of the sum of cationic and anionic radii ( i.e. 1/(r+ + r-) ). The large ionic radius of the anionic makes the sizes of the much smaller cations relatively insignificant in contributing to the sum of r+ and r-. Therefore, going down the group, the increase in size of the cations does not cause a significant change in the H lattice. However, the increase in size of the cations does cause the H hyd to become less negative down the group. That is to say, the decrease in H hyd is more significant than the decrease in H lattice. As a result, the
H soln becomes less negative, and hence the solubility of the sulphates(VI) of Group II metals decreases down the group.
For the hydroxides of Group II metals, the sizes of anions and cations are of the same order of magnitude. Again, the H lattice is proportional to 1/(r+ + r-). Going down the group, less energy is required to break down the ionic lattice (i.e. the H lattice becomes less negative ) as the cationic size increases, and the change in H hyd is comparatively small. That is to say, the decrease in
Hsoln becomes more negative, and hence the solubility of the hydroxides of Group II metals increases down the group.
6. Conclusion
The flame colour of selected s – block metal chlorides are different since the amount of energy of emitted radiation is quantized. At the same time, the solubility of selected Group II metal sulphates decreases down the group. However, the solubility of selected Group II metal hydroxides increases down the group.
