Properties And Trends Of Compounds Of S And P Block Elements - Session 1

We know that solubility is the ability of a compound whether in solid, liquid or gas phase to dissolve in a particular solvent. Salts formed by s block elements are mostly soluble in water. This is due to high electro-positive characteristics of s block elements. When these salts are dissolved in water, they ionize into individual ions. Then these ions are surrounded by water molecules. Surrounding of ions by water molecules is called hydration. Energy released during this process is called hydration energy. The enthalpy change when an ionic crystal is formed from its ions is called lattice enthalpy.

We know that when an acid combines with a base, it forms salt and water. If hydration enthalpy is greater than lattice enthalpy of a salt, then it is soluble in water. For example, the chloride salts formed by s block elements are sodium chloride, potassium chloride, Beryllium chloride, Magnesium chloride, Calcium chloride, Strontium chloride and Barium chloride. Some of these salts are soluble in water. But the extent of their solubility of these salts varies.

For example, Sodium chloride is more soluble in water as compared to Potassium chloride. This is due to smaller size of sodium ion compared to potassium ion. The hydration energy of Sodium ion is also greater than Potassium ion. Solubility of chloride salts of alkaline earth metals decrease downwards the group from Beryllium to Barium.

Beryllium chloride is more soluble in water as compared to Barium chloride. This due to large size of Barium ion as compared to Beryllium ion. Also, the hydration energy of Beryllium ion is greater than Barium ion. The decreasing order of solubility of alkaline earth metal chlorides is illustrated here.

Similarly, the solubility of Iodide salts and bromide salts of alkaline earth metals decrease downwards the group. Sodium bromide is more soluble in water as compared to potassium bromide. Which one will be more soluble in water? Sodium iodide or Potassium iodide? Now let’s discuss the solubility of sulphate salts of s block elements. The solubility of sulphate salts of s block elements also decreases from top to bottom in a group.

For example, Magnesium sulphate is more soluble in water as compared to Barium sulphate. This is due to fact that from top to bottom in a group, the decrease in hydration energy is more as compared to decrease in lattice in energy. Due to this reason Barium sulphate is less soluble in water as compared to Magnesium sulphate. The decreasing order of solubility of alkaline earth metal sulphate salts in water is illustrated.

Sodium and potassium carbonate are soluble in water. Beryllium carbonate is also soluble in water. Magnesium carbonate, calcium carbonate, strontium carbonate and barium carbonate are insoluble in water. This is because their hydration energy is less than their lattice energy. Solubility of carbonate and bicarbonate salts of alkaline earth metals decrease downwards in a group due to decrease in hydration energy. The decreasing order of solubility of carbonate and bicarbonate salts of alkaline earth metals is illustrated here.

Nitrite salts of s block elements are all soluble in water. Their solubility decreases downwards in a group due to decrease in hydration enthalpy of cations from to top to bottom in a group. For example, Beryllium nitrite is more soluble in water as compared to Barium nitrite. Solubility of sulphite salts of s block elements also decrease downwards in a group. This is due to decrease in hydration enthalpy of cations from top to bottom in a group.

Sulfides of all the metals except calcium, cesium, magnesium and barium are sparingly soluble in water. This is due to instability of sulfide ions. Sulfide ions undergo hydrolysis. Lithium sulfide is soluble in water. Sodium sulfide is also easily soluble in water. While Potassium sulfide is moderately soluble in water.

We shall now discuss the thermal stability of carbonate, bicarbonate and nitrate salts formed by s block elements. Thermal decomposition is the splitting of compound by heating it. We shall first discuss the thermal stability of nitrates of s block elements. Nitrates of group one and group two elements decompose on heating to give metal oxide, nitrogen dioxide gas and oxygen gas. Decomposition reaction is given in the illustration.

High thermal stability means the compound will less likely decompose on heating. Thermal stability of nitrates of group one and group two elements rises from top to bottom in a group. This is due to ionic nature of cations. For example, Barium Nitrate is thermally more stable as compared to Magnesium Nitrate. This is because of small cationic size of Magnesium as compared to Barium. Both ions have same a +2.

But due to small size of the Magnesium ion, the charge density is more concentrated on Magnesium ion. Because of this, it draws electron density of highly electronegative oxygen atom in nitrate more towards itself. Thus, the Nitrate ion becomes polarized. In other words we can say that bond between nitrogen atom and oxygen atom in nitrate ion becomes more polarized. If the anion is more polarized, then less heat is required for decomposition. As a result bond between nitrogen and oxygen atom is broken easily.

In case of Barium Nitrate, due to large cationic size of Barium Ion, the charge density is not concentrated and it does not draw the electron density of oxygen atom in nitrate ion towards itself. As a result the Nitrate Ion is less polarized. This means more heat will be required to break the bond between nitrogen and oxygen atom in a Nitrate Ion. This makes the Barium Nitrate thermally more stable.

We shall now discuss the thermal stability of carbonate salts of s block elements. Carbonate salts of Group I and Group II elements decompose on heating to give metal oxide and carbon dioxide gas. Decomposition reaction is given in the illustration. Thermal stability of carbonates of Group I and Group II elements rises from top to bottom in a group. This is due to ionic nature of cation.

For example, Barium Carbonate is thermally more stable as compared to Magnesium carbonate. This is because of small cationic size of Magnesium as compared to Barium. Both ions have same +2. But due to small size of Magnesium Ion, the charge density is more concentrated on Magnesium Ion. Because of this it draws electron density of highly electronegative oxygen atom in carbonate ion more towards itself. Carbonate ion becomes polarized.

As we know if the anion is more polarized, then less heat is required for decomposition. As a result bond between carbon and oxygen atom is broken easily. Meanwhile in case of Barium Carbonate, due to large cationic size of Barium Ion the charge density is not concentrated and it does not draws the electron density of oxygen atom in nitrate ion more towards itself. As a result carbonate ion is less polarized. This means that more heat will be required to break the bond between carbon and oxygen atom in carbonate ion. This makes the Barium carbonate thermally more stable.

Bicarbonates of Group I and Group II metals decompose on heating to give metal carbonate, carbon dioxide and water. Thermal stability of bicarbonates rises downwards in a group due to decrease in polarizing power of metal cation. Thermal stability of carbonate and bicarbonate salts rise downwards in a group. Among Calcium bicarbonate and and barium bicarbonate, which one is thermally more stable and why?.