Question

How would you explain the fact that \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) is quite soluble in water but \(\mathrm{AlPO}_{4}\) is essentially insoluble?

Short answer

The difference in solubility between Na3PO4 and AlPO4 can be attributed to the difference in ionic charges and lattice energies of these compounds. Na3PO4, composed of Na+ and PO4^3- ions, has a lower lattice energy and is quite soluble in water due to the lower charge on its cations. In contrast, AlPO4, composed of Al^3+ and PO4^3- ions, has a higher lattice energy and larger charge on the aluminum ions, making it essentially insoluble in water.

Step-by-step solution

I. Identify the ions in the compounds

Both Na3PO4 and AlPO4 are ionic compounds. In Na3PO4, we have sodium ions (Na+) and phosphate ions (PO4^3-), while in AlPO4, we have aluminum ions (Al^3+) and phosphate ions (PO4^3-).

II. Compare the ionic charges

In Na3PO4, the sodium ion (Na+) has a +1 charge, and the phosphate ion (PO4^3-) has a -3 charge. In AlPO4, the aluminum ion (Al^3+) has a +3 charge, and the phosphate ion (PO4^3-) also has a -3 charge. The aluminum ion (Al^3+) in AlPO4 has a higher charge than the sodium ion (Na+) in Na3PO4.

III. Discuss the lattice energy

Lattice energy is the energy required to separate a mole of an ionic compound into its gaseous ions. The lattice energy increases with the charge of ions and decreases with increasing ionic size. Since aluminum ion (Al^3+) in AlPO4 has a larger charge than the sodium ion (Na+) in Na3PO4, the lattice energy of AlPO4 would be greater than that of Na3PO4. The higher lattice energy means it is more difficult to separate the ions in AlPO4 than in Na3PO4.

IV. Solubility in water

To dissolve an ionic compound in water, the energy released by solvation, where the ions interact with water molecules and form a solution, should be strong enough to overcome the lattice energy of the compound. The solvation energy depends on the charge of the ions and the size of the ions. In the case of Na3PO4, the lower charge of sodium ions makes the solvation energy high enough to overcome the lattice energy, resulting in a soluble compound. However, in AlPO4, the larger charge and higher lattice energy make it more difficult for solvation energy to overcome, and hence AlPO4 is practically insoluble in water. In conclusion, the difference in solubility between Na3PO4 and AlPO4 can be attributed to the difference in the ionic charges and lattice energies of these compounds. Na3PO4 is quite soluble in water due to the lower charge on its cations and lower lattice energy, while AlPO4 is essentially insoluble because of its higher lattice energy and larger charge on the aluminum ions.

Key concepts

Ionic Compounds

Understanding the nature of ionic compounds is crucial in grasping why they behave differently in water. These compounds are composed of positive and negative ions held together by the strong electrostatic forces known as ionic bonds. The attraction occurs between ions with opposite charges, for example, a metal cation like sodium (Na+) and a non-metal anion like phosphate (PO4^3-).

When ionic compounds come into contact with water, their ions can separate and interact with water molecules if the conditions are right. The water molecules, which are polar and have a partial negative charge near the oxygen atom and a partial positive charge near the hydrogen atoms, surround the separated ions, stabilizing them in solution. This is why substances like sodium phosphate (Na3PO4) can be quite soluble in water under the right circumstances.

Lattice Energy

Lattice energy is an essential concept when discussing the solubility of ionic compounds. It represents the energy required to break apart the ionic lattice, the orderly arrangement of ions within a solid, turning it into gaseous ions. The more substantial the charges on the ions and the smaller their ionic radius, the higher the lattice energy, due to stronger attractions between them.

It takes a considerable amount of energy to disrupt the lattice of AlPO4 because of the high charge on the aluminum ion (Al^3+), and hence it is much harder for water molecules to overcome this lattice energy and dissolve the compound. In contrast, Na3PO4 with a smaller charged sodium ion (Na+) has a lower lattice energy, which water can more easily disrupt, resulting in better solubility.

Solvation Energy

The process by which solvent molecules, such as water, surround and interact with solute ions is known as solvation. The energy released during this process is called solvation energy. When an ionic compound dissolves, water molecules work to separate and surround each ion, mitigating their strong ionic bonds. This stabilization results from the electrostatic interactions between the partial charges on the water molecules and the full charges on the ions.

In compounds with lower charged ions like Na3PO4, the solvation energy is typically high enough to overcome the lattice energy, allowing them to dissolve. However, in compounds like AlPO4, where the ions are highly charged, solvation energy is insufficient to overcome the high lattice energy, hindering their solubility.

Ionic Charges

The ionic charges within compounds significantly influence their physical properties, including solubility. In the context of Na3PO4 and AlPO4, the difference in charge between sodium (Na+) and aluminum (Al^3+) has a direct effect on their interactions with water. Sodium ions have a single positive charge, whereas aluminum ions carry a triple positive charge.

The higher charge of an ion, the stronger it will attract electrons and nearby negative ions, increasing the compound's lattice energy. High lattice energies typically correspond to low solubility in water, because the ions are so strongly held together that water molecules cannot easily break them apart. The lower charge on sodium compared to aluminum means that Na3PO4 has a lower lattice energy and can more easily interact with water, hence its higher solubility compared to AlPO4.