What does the solubility of ionic solid depend upon and factors favour a strong ionic bond in a crystal lattice?
The solubility of an ionic solid depends on the following factors:
- Lattice energy: The lattice energy is the amount of energy required to break apart the crystal lattice of an ionic solid. The higher the lattice energy, the less soluble the ionic solid will be.
- Ionization energy: The ionization energy is the amount of energy required to remove an electron from an atom. The higher the ionization energy, the less soluble the ionic solid will be.
- Hydration energy: The hydration energy is the amount of energy released when an ion is surrounded by water molecules. The higher the hydration energy, the more soluble the ionic solid will be.
The factors that favor a strong ionic bond in a crystal lattice are:
- Large charges on the ions: The larger the charges on the ions, the stronger the electrostatic attraction between them, and the stronger the ionic bond.
- Smaller ions: The smaller the ions, the closer they can get together, and the stronger the electrostatic attraction between them.
- Complete octets: Ionic compounds are more stable when the ions have complete octets of electrons. This means that the ions are less likely to interact with other ions or molecules, and the ionic bond is stronger.
In general, ionic solids with high lattice energies and low hydration energies are less soluble. Ionic solids with low lattice energies and high hydration energies are more soluble.
Here are some examples of ionic solids and their solubilities:
- NaCl: NaCl has a high lattice energy and a low hydration energy. It is therefore a relatively insoluble ionic solid.
- KCl: KCl has a lower lattice energy than NaCl but a higher hydration energy. It is therefore more soluble than NaCl.
- LiF: LiF has a very high lattice energy and a very low hydration energy. It is therefore the least soluble of the three ionic solids.
As lattice energy becomes larger, solubility decreases. Both these factors oppose each other, and the resultant of these factors determines the solubility of an ionic compound in water. If the lattice enthalpy has a larger value, the compound is less soluble. Solubility of an ionic solid depends upon temperature, nature of solute and solvent, complex formation and H-bonding. It may show exothermic or endothermic nature.The solubility of a substance is the maximum amount of a solute that can dissolve in a given quantity of solvent; it depends on the chemical nature of both the solute and the solvent and on the temperature and pressure. The solubility of most substances depends strongly on the temperature and, in the case of gases, on the pressure. The solubility of most solid or liquid solutes increases with increasing temperature. Ionic compounds dissociate in solvents and are completely soluble in water, but as the covalent character increase the solubility of the compound decreases. Melting Point and Boiling Point: Covalent bonds are weaker than ionic bonds. Increasing the temperature will therefore increase the solubility of the solute. An example of a solute whose solubility increases with greater temperature is ammonium nitrate, which can be used in first-aid cold packs. Ammonium nitrate dissolving in solution is an endothermic reaction. For many solids dissolved in liquid water, the solubility increases with temperature. The increase in kinetic energy that comes with higher temperatures allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions. On increasing temperature the kinetic energy of the solvent particles increases and they are able to break apart effectively. The solute particles are held effectively between the solvent molecules thus increasing the solubility. If a solute is a solid or liquid, increasing the temperature increases its solubility. As, more sugar can dissolve in hot water than in cold water. If a solute is a gas, increasing the temperature decreases its solubility. This model emphasizes two main factors that contribute to the lattice energy of an ionic solid: the charge on the ions, and the radius, or size, of the ions. The effect of those factors is: as the charge of the ions increases, the lattice energy increases. as the size of the ions increases, the lattice energy decreases.
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