Question

Which of the following statements is not true about alkali metals? (a) All alkali metals form oxo salts such as carbonates, sulphates and nitrates. (b) The basic character of oxides increases down the group. (c) Carbonates and sulphates of lithium are stable and their stability decreases down the group. (d) Solubility of carbonates and sulphates increases down the group.

Short answer

Statement (d) is not true; the solubility of carbonates and sulphates decreases down the group, not increases.

Step-by-step solution

Assess statement (a)

Evaluate if all alkali metals form oxo salts such as carbonates, sulphates, and nitrates. This is true. All alkali metals react with oxygen to form oxides, which can further react to form the listed oxo salts.

Assess statement (b)

Check if the basic character of oxides increases down the group. This statement is also true, as moving down the group, the metallic character increases, making the oxides more basic.

Assess statement (c)

Determine the stability of carbonates and sulphates of lithium compared to those of other alkali metals. It is accurate to say that lithium's carbonates and sulphates are more stable than those of heavier alkali metals, which decompose more readily upon heating.

Assess statement (d)

Investigate the solubility trend of carbonates and sulphates within the alkali metals group. This statement is false. The solubility of carbonates and sulphates actually decreases down the group due to their increasing lattice energy outweighing hydration energy.

Key concepts

Oxo Salts of Alkali Metals

Alkali metals, located in Group 1 of the periodic table, are known for their reactivity, especially with oxygen. They form a variety of compounds known as oxo salts, which include carbonates (M₂CO₃), sulphates (M₂SO₄), and nitrates (MNO₃).

Oxo salts are a result of the reaction between alkali metal oxides and the respective acidic oxoacid. For example, when sodium reacts with oxygen, sodium oxide is formed, which can react with carbon dioxide to yield sodium carbonate, a common oxo salt. These salts have a wide range of applications, from detergents to glass production, and their study is essential in inorganic chemistry.

Understanding the formation and properties of these salts is crucial. For instance, they typically display high solubility in water due to the ionic nature of alkali metals and their relatively low lattice energies, which are overcome by the energy released when their ions are solvated in water.

Basic Character of Oxides

The oxides formed by alkali metals are known to be basic. This means that these compounds react with acids to form salt and water, a typical property of bases. With increasing atomic number down the group, the metallic character of the elements increases, which in turn enhances the basicity of their oxides.

For instance, lithium oxide (Li₂O) reacts with water to form lithium hydroxide (LiOH), which is less basic than potassium hydroxide (KOH) formed from the reaction of potassium oxide (K₂O) with water. This trend in the increasing basic character is attributed to the larger size of the metal ions and their decreasing ionization energy as we move down the group. This property impacts various industrial processes and the environmental impact of these compounds.

Stability of Carbonates and Sulphates

When talking about the stability of carbonates and sulphates of alkali metals, it's important to note that lithium's compounds are more stable compared to those of the heavier alkali metals. This is inversely related to the size of the alkali metal ion; as the size increases, the polarizing power decreases, thus reducing the stability of the carbonate and sulphate.

The stability factor is linked to the thermal decomposition of these compounds; those of lithium require a higher temperature to decompose, whereas those of heavier alkali metals decompose more readily. For example, the lithium carbonate (Li₂CO₃) decomposition temperature is higher than that of potassium carbonate (K₂CO₃). This concept is especially important when considering the manufacture of materials that must withstand varying temperatures without breaking down chemically.