Learning Outcomes
i. Describe the solubility trends of hydroxides, sulphates, and carbonates of Group II elements.
ii. Explain the influence of lattice energy and hydration energy on the solubility of these compounds.
iii. Analyze the factors that contribute to the solubility trends observed in hydroxides, sulphates, and carbonates.
iv. Apply the concept of solubility trends to predict the relative solubility of Group II element compounds in various aqueous solutions.
Introduction
The solubility of a compound is its ability to dissolve in a solvent, such as water. Solubility is an important property of compounds, as it affects their reactivity and behavior in various chemical reactions. In this lesson, we will explore the solubility trends of hydroxides, sulphates, and carbonates of Group II elements. These compounds are formed by the reactions of Group II elements with water, oxygen, and carbon dioxide, respectively.
i. Solubility Trends of Hydroxides
The solubility of hydroxides of Group II elements generally decreases down the group. This means that beryllium hydroxide (Be(OH)2) is more soluble than magnesium hydroxide (Mg(OH)2), which is more soluble than calcium hydroxide (Ca(OH)2), and so on. The trend can be explained by the increasing lattice energy and decreasing hydration energy as we move down the group.
Lattice energy is the energy required to break apart a crystal lattice into its constituent ions. A higher lattice energy indicates a stronger attraction between the ions, making it more difficult for the compound to dissolve. Hydration energy is the energy released when ions dissolve in water and are surrounded by water molecules. A higher hydration energy indicates a stronger interaction between the ions and water molecules, making the compound more soluble.
In the case of hydroxides, the lattice energy increases down the group due to the increasing size of the cations. The larger cations have a stronger attraction to the hydroxide anions, resulting in a higher lattice energy. The hydration energy, on the other hand, decreases down the group due to the decreasing size of the cations. The smaller cations are less effective at attracting water molecules, resulting in a lower hydration energy.
Therefore, the balance between lattice energy and hydration energy determines the solubility of hydroxides. For beryllium hydroxide, the hydration energy is high enough to overcome the relatively low lattice energy, making it soluble. However, for barium hydroxide, the lattice energy is too high for the hydration energy to overcome, making it insoluble.
ii. Solubility Trends of Sulphates
The solubility of sulphates of Group II elements is generally high. This is because the lattice energy of sulphates is relatively low, and the hydration energy of the sulphate ion is high. The trend is similar to that of hydroxides, with beryllium sulphate (BeSO4) being more soluble than magnesium sulphate (MgSO4), which is more soluble than calcium sulphate (CaSO4), and so on.
The lattice energy of sulphates is relatively low because the sulphate ion is a large and delocalized anion. The negative charge of the sulphate ion is spread over the entire ion, reducing the attraction between the sulfate ion and the cations. The hydration energy of the sulphate ion is high because the sulphate ion is able to form strong hydrogen bonds with water molecules.
iii. Solubility Trends of Carbonates
The solubility of carbonates of Group II elements generally decreases down the group. This means that beryllium carbonate (BeCO3) is more soluble than magnesium carbonate (MgCO3), which is more soluble than calcium carbonate (CaCO3), and so on. The trend can be explained by the increasing lattice energy and decreasing hydration energy as we move down the group.
The lattice energy of carbonates is higher than that of hydroxides because the carbonate ion is a larger and more polarizable anion. The negative charge of the carbonate ion is concentrated on the oxygen atoms, making it more attractive to the cations. The hydration energy of the carbonate ion is lower than that of the sulphate ion because the carbonate ion is not as effective at forming hydrogen bonds with water molecules.
The solubility trends of hydroxides, sulphates, and carbonates of Group II elements can be explained by the interplay of lattice energy and hydration energy. Understanding these trends is crucial for predicting the behavior of these compounds in various chemical reactions.
