Question

How would you explain the following observations? (i) $\mathrm{BeO}$ is almost insoluble but ${\mathrm{BeSO}}_4$ is soluble in water, (ii) $\mathrm{BaO}$ is soluble but ${\mathrm{BaSO}}_4$ is insoluble in water, (iii) LiI is more soluble than KI in ethanol.

Short answer

Solubility depends on the balance between lattice and hydration energies; compounds with lower lattice energies or higher hydration energies are more soluble.

Step-by-step solution

Understanding Solubility Trends

When considering solubility, we must think about the lattice energy and hydration energy. Lattice energy is the energy required to break the solid lattice into gaseous ions, while hydration energy is the energy released when ions in solution are surrounded by water molecules. The compound dissolves if the hydration energy overcomes the lattice energy.

Analyzing BeO and BeSO4 Solubility

(i) BeO is almost insoluble in water because it has a high lattice energy due to its ionic and covalent character, which is not overcome by the hydration energy of the water. On the other hand, BeSO4 is soluble since the sulfate ion has a much greater ability to stabilize the charge through distribution among its atoms, leading to a significant decrease in the lattice energy, which allows the hydration energy to dominate.

Evaluating BaO and BaSO4 Solubility

(ii) BaO is soluble in water because it has lower lattice energy than BeO, and the hydration energy is sufficient to dissolve it. However, BaSO4 is insoluble because the sulfate ion forms an extremely stable solid lattice with barium ions, leading to very high lattice energy that the hydration energy in water cannot overcome.

Comparing LiI and KI Solubility in Ethanol

(iii) LiI is more soluble in ethanol than KI because the ionic bond in LiI is less strong than in KI due to the smaller size and high polarization of the lithium ion compared to potassium. Ethanol, being a polar solvent but less polar than water, dissolves LiI better because the interactions between ethanol and LiI are strong enough to overcome its lattice energy.

Key concepts

Lattice energy

When discussing solubility, a key factor is lattice energy. Lattice energy is defined as the amount of energy required to separate one mole of a solid ionic compound into its gaseous ions. In simpler terms, it measures how tightly the ions are held together in a crystalline structure. A high lattice energy means that the ions are strongly attracted to each other, resulting in a more stable solid. This often makes the compound less soluble because breaking apart these tightly bound ions would require significant energy. Lattice energy depends on two primary factors:

• The charge of the ions: Greater ion charge leads to higher lattice energy.
• The size of the ions: Smaller ions result in a more compact lattice and thus higher energy.

In general, a compound with high lattice energy will need a substantial amount of opposing energy to dissolve in water or another solvent. Understanding lattice energy is crucial in explaining the solubility differences, such as why $\mathrm{BeO}$ is less soluble than ${\mathrm{BeSO}}_4$.

Hydration energy

Hydration energy comes into play when an ionic compound dissolves in water. It's the energy released when ions are surrounded by water molecules in a solution. The process of hydration provides energy that helps to counteract the lattice energy of a compound. The hydration energy is influenced by:

• The size of the ions: Smaller ions have higher hydration energy because they are more effectively surrounded by water molecules.
• The charge of the ions: Higher charged ions attract water molecules strongly, resulting in higher hydration energy.

For a compound to be soluble, its hydration energy should ideally be greater than its lattice energy. This concept helps explain why ${\mathrm{BeSO}}_4$ is soluble in water. The hydration energy it generates is sufficient to overcome its relatively lower lattice energy, in contrast to ${\mathrm{BaSO}}_4$, where the hydration energy is insufficient to break its strong lattice bonds.

Ionic and covalent character

The ionic and covalent character of a bond significantly affect the solubility of a compound. Compounds with high ionic character generally dissolve more readily in polar solvents like water because polar solvents can stabilize the separated ions. The bond character is determined by:

• Electronegativity difference: Larger differences often mean more ionic character.
• Polarizability: Small cations and large anions may lead to partial covalent character due to distortion in electron clouds.

For instance, $\mathrm{BeO}$ has a significant covalent character because of the small, highly polarizable beryllium ion. This results in a high lattice energy which the hydration energy cannot easily overcome, making it insoluble in water. On the flip side, $\mathrm{LiI}$ is more soluble than $\mathrm{KI}$ in ethanol, a less polar solvent, due to the greater polarizability of the lithium ion creating weaker lattice interactions.

Polar solvents

Polar solvents, like water, play a crucial role in dissolving ionic compounds. The polarity of the solvent leads to partial charges on its molecules, which can stabilize the ions formed from dissolving an ionic compound. This interaction helps to lower the energy barrier for solubility.Here are some characteristics of polar solvents:

• High dielectric constant: This reduces the electrostatic forces between ions, aiding dissolution.
• Ability to form hydrogen bonds: This further helps stabilize dissolved ions.

In the case of ethanol, although it is polar, it is less so than water. Yet, it has enough solvent power to dissolve certain ionic compounds like $\mathrm{LiI}$. Ethanol's structure allows it to interact effectively with $\mathrm{LiI}$ compared to $\mathrm{KI}$, dictating the solubility trends seen with these specific compounds.