Question

Compare the alkali metals and alkaline earth metals with respect to (i) ionisation enthalpy (ii) basicity of oxides and (iii) solubility of hydroxides.

Short answer

Alkali metals have lower ionisation enthalpy, more basic oxides, and more soluble hydroxides compared to alkaline earth metals.

Step-by-step solution

Understanding Ionisation Enthalpy

Ionisation enthalpy refers to the energy required to remove the outermost electron from an isolated gaseous atom. In the context of alkali metals (Group 1) and alkaline earth metals (Group 2), alkali metals have lower ionisation enthalpy compared to alkaline earth metals. This is because alkali metals have a single electron in their outermost shell, which is easier to remove compared to the two electrons in the outer shell of alkaline earth metals.

Analyzing Basicity of Oxides

The basicity of oxides is determined by their ability to form hydroxide ions when dissolved in water. Alkali metal oxides (e.g., Na2O) are more basic than alkaline earth metal oxides (e.g., MgO). This is because the oxides of alkali metals form strong bases like NaOH, while alkaline earth metal oxides form weaker bases, as they do not dissociate as readily.

Exploring Solubility of Hydroxides

Solubility of hydroxides in water decreases as you move down from top to bottom in both groups. However, alkali metal hydroxides (e.g., NaOH) are generally more soluble than alkaline earth metal hydroxides (e.g., Mg(OH)2). This is partly due to the larger atomic size and lower charge density of alkali metals, which results in stronger ion-dipole interactions with water and therefore greater solubility.

Key concepts

Ionisation Enthalpy

Ionisation enthalpy is an important concept in chemistry, especially when comparing alkali metals with other groups on the periodic table. It refers to the amount of energy needed to remove the outermost electron from an atom in its gaseous state. This is an endothermic process because energy input is required to overcome the attraction between the electron and the nucleus.

For alkali metals, which belong to Group 1 of the periodic table, the ionisation enthalpy is relatively low. Each alkali metal atom has just one electron in its outermost shell, making it much easier to remove. As such, these elements readily lose this single electron to form a stable, positively charged ion. On the other hand, alkaline earth metals, located in Group 2, have two electrons in their outer shell. The presence of this additional electron makes removing one of them slightly more difficult, hence a higher ionisation enthalpy is observed.

To summarize:

• Alkali metals have lower ionisation enthalpy compared to alkaline earth metals.
• This is due to the single electron in their outer shell which is easier to remove.
• Higher ionisation enthalpy in alkaline earth metals is due to the extra electron requiring more energy to remove.

Basicity of Oxides

The basicity of a compound, especially oxides, is an important indicator of its chemical properties and behavior in reactions. Basicity in oxides refers to the ability of a compound to produce hydroxide ions (OH-) when dissolved in water.

Alkali metal oxides, such as sodium oxide (Na₂O), are highly basic. When dissolved in water, they readily form strong alkaline solutions, like sodium hydroxide (NaOH). This readiness to dissociate into hydroxide ions is a characteristic feature of the oxides of alkali metals, contributing to their notable basicity.

In contrast, oxides of alkaline earth metals, like magnesium oxide (MgO), are less basic. Although they also form hydroxides when dissolved, they do not dissociate as completely or readily as alkali metal oxides. This results in the formation of weaker bases and less alkaline solutions.

Key points:

• Alkali metal oxides are more basic than alkaline earth metal oxides.
• They dissociate readily to form strong bases, unlike alkaline earth metal oxides.
• This is due to alkali metals forming highly soluble oxides, further enhancing their basicity.

Solubility of Hydroxides

The solubility of a compound in water, such as hydroxides, affects how these compounds participate in chemical reactions. Generally, the solubility of hydroxides decreases as you move down a group in the periodic table. This trend can be observed in both alkali metals and alkaline earth metals.

Alkali metal hydroxides, like those formed from sodium (NaOH) and potassium (KOH), are often highly soluble in water. This is because of their larger atomic size and lower charge density, which lead to strong ion-dipole interactions with water molecules. These strong interactions help these hydroxides dissolve readily, allowing them to form concentrated solutions.

Conversely, alkaline earth metal hydroxides like magnesium hydroxide (Mg(OH)₂) are less soluble. The increased charge density and smaller atomic size make interactions with water less effective, resulting in lower solubility. This means these hydroxides form less concentrated solutions when dissolved.

In summary:

• Alkali metal hydroxides are generally more soluble than those of alkaline earth metals.
• Larger atomic size and lower charge density in alkali metals aid in their solubility.
• Less soluble alkaline earth metal hydroxides have reduced interaction with water due to higher charge density.