Question

The solubilities of carbonates decrease down the magnesium group due to a decrease in: (a) Inter ionic interaction (b) Lattice energies of solids (c) Entropy of solution formation (d) Hydration energies of cations

Short answer

(b) Lattice energies of solids

Step-by-step solution

Understand Solubility and Lattice Energy

When substances dissolve, the energy changes involved include lattice energy (energy required to separate ions in a solid lattice) and hydration energy (energy released when ions become surrounded by water molecules). The solubility of a compound is heavily influenced by these energies.

Analyze Cations in the Magnesium Group

The magnesium group consists of magnesium, calcium, strontium, and barium. As you go down the group, the size of cations increases. Larger cations require less energy to remove from the lattice due to weaker ion-ion forces.

Decreasing Lattice Energy Trend

Lattice energy typically decreases as you move down the group because the distance between the ions increases (bond length increases), making it easier for the ions to separate. This suggests a less favorable or weaker lattice.

Relationship with Solubility

The decrease in lattice energy would actually increase solubility if not balanced by a decrease in hydration energy. However, it is the decrease in lattice energy that primarily influences the solubility trend of carbonates in this group.

Key concepts

Lattice Energy

Lattice energy is the energy needed to separate one mole of an ionic solid into its gaseous ions. It’s a measure of the strength of the forces holding the ions together in the crystal lattice. The higher the lattice energy, the harder it is to break those bonds and dissolve the compound. In the context of carbonates in the magnesium group, lattice energy plays a significant role.

As you move down the magnesium group from magnesium to barium, the lattice energy decreases. This happens because the ionic size increases, leading to a greater distance between the ions in the lattice. Greater distances mean weaker attractions. Hence, less energy is needed to break the lattice apart, resulting in a lower lattice energy. Low lattice energy usually suggests easier solubility, yet in the magnesium group, this decrease is offset by other factors.

• Lattice energy is crucial in determining solubility.
• Higher lattice energy indicates stronger ion-ion attraction.
• The decrease in lattice energy down the group signifies weaker ion-ion forces.

Hydration Energy

Hydration energy is the energy released when ions in a compound get surrounded by water molecules, forming a solution. It essentially helps determine how favorable the dissolution of an ionic compound is.

For the carbonates in the magnesium group, hydration energy decreases down the group. As the metal cations become larger, the positive charge is spread over a greater volume. This means the cations have less attraction to the water molecules. With larger ions, less energy gets released upon hydration, making the dissolution process less favorable.

• Hydration energy contributes significantly to solubility.
• Higher hydration energy usually indicates better solubility.
• In the magnesium group, decreasing hydration energy counteracts decreasing lattice energy, reducing solubility.

Magnesium Group

The magnesium group, also known as the alkaline earth metals, includes magnesium, calcium, strontium, and barium. These elements share similar properties because they all have two electrons in their outer shell. Additionally, their ionic size increases as we move down the group. This results in interesting trends regarding their compounds' solubility.

In the context of carbonate solubility, as with their general reactivity, trends are linked to cation size. These metals react to form carbonates, and as the cation size increases, other aspects like lattice and hydration energies also influence solubility.

• The ionic size increases down the group.
• Trends in properties are consistent across the magnesium group compounds.
• Larger cations in the group become less soluble in water due to interactions with both lattice and hydration energies.
• Understanding the magnesium group is essential for evaluating carbonate solubility trends.

Cation Size

Cation size affects many properties of ionic compounds, including their solubility. In the magnesium group, as you move from magnesium to barium, cation size increases. Larger cations have several implications:

They exert weaker attraction to both the anions in the solid lattice and the water molecules in solution. This means that

• Larger cations require less energy to overcome the lattice energy, hence weakening their overall hold within the structure.
• With weaker cation-water attractions, the hydration process becomes less energy-releasing, impacting the solubility.

Therefore, as the cation size increases, the combined effect of lower lattice energy (good for solubility) and significantly lower hydration energy (bad for solubility) tends to make these carbonates less soluble. Understanding how cation size influences these energies is key to comprehending their solubility behavior.

Ion-Ion Forces

Ion-ion forces are the electrostatic forces between positively charged cations and negatively charged anions within an ionic compound. These forces are what give ionic compounds like carbonates their crystalline structure and relatively high melting points.

In the magnesium group, the strength of ion-ion forces decreases as you move down the group. This is primarily due to the increase in the distance between ions caused by larger cation sizes. The further apart the ions, the weaker the attraction, which impacts the lattice energy directly.

• Ion-ion forces are vital to the stability and solubility of ionic compounds.
• Weakened ion-ion forces lead to lower lattice energies.
• Understanding these forces helps explain the solubility trend of carbonates. As ion-ion forces weaken down the group, compounds become less stable but not necessarily more soluble due to other competing factors.