Problem 62 - Chemistry The Central Science

Question

Account for the following observations: (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a diprotic acid. (b) Nitric acid is a strong acid, whereas phosphoric acid is weak. (c) Phosphate rock is ineffective as a phosphate fertilizer. (d) Phosphorus does not exist at room temperature as diatomic molecules, but nitrogen does. (e) Solutions of \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) are quite basic.

Step-by-Step Solution

Answer

(a) H3PO3 is a diprotic acid because it can donate two protons (H+ ions) when dissolved in water, forming H+ ions and HPO3^2-. (b) Nitric acid is a strong acid as it ionizes completely, while phosphoric acid is a weak acid due to its partial ionization. (c) Phosphate rock is ineffective as a phosphate fertilizer because it is insoluble in water and must be treated, usually by acid treatment, to create a soluble phosphate source for plants. (d) Phosphorus exists as tetratomic molecules (P4) at room temperature, while nitrogen forms diatomic molecules (N2) due to their chemical properties and bonding preferences. (e) Solutions of Na3PO4 are quite basic because the phosphate ion reacts with water molecules, acting as a Bronsted-Lowry base, and produces hydroxide ions (OH-).

Step 1: (a) H3PO3 is a diprotic acid

H3PO3, also known as phosphorous acid, is a diprotic acid which means it can donate two protons (H+ ions) to the solution. The structure of H3PO3 has three hydrogen atoms and one phosphorus atom. When dissolved in water, two of its hydrogen atoms dissociate, resulting in the formation of two H+ ions and the remaining molecule HPO3^2-. Thus, it acts as a diprotic acid.

Step 2: (b) Nitric acid (HNO3) is a strong acid, whereas phosphoric acid (H3PO4) is weak

A strong acid is an acid that ionizes completely in a solution, whereas a weak acid only partially ionizes. Nitric acid (HNO3) is a strong acid because it ionizes completely in a solution, yielding the hydronium ion (H+) and nitrate ion (NO3-). On the other hand, phosphoric acid (H3PO4) is a weak acid because it ionizes only partially in the solution, resulting in an equilibrium mixture of molecules and ions.

Step 3: (c) Phosphate rock is ineffective as a phosphate fertilizer

Phosphate rock is the natural source of phosphorus, mainly in the form of apatite minerals. It is ineffective as a phosphate fertilizer because it is not soluble in water, and plants require soluble phosphate ions (such as H2PO4- and HPO4^2-) for the uptake and transport of phosphorus inside the plant cells. Phosphate rock is an essential component of phosphate fertilizers, but it must be treated, usually by acid treatment, to create a soluble phosphate source that can be utilized by the plants.

Step 4: (d) Phosphorus does not exist at room temperature as diatomic molecules, but nitrogen does

Phosphorus and nitrogen have different molecular structures at room temperature due to their chemical properties and bonding preferences. Phosphorus exists as tetratomic molecules (P4) forming a tetrahedral structure at room temperature, where each phosphorus atom forms three covalent bonds with other phosphorus atoms. On the other hand, nitrogen exists as diatomic molecules (N2) at room temperature, where each nitrogen atom forms a triple covalent bond with another nitrogen atom.

Step 5: (e) Solutions of Na3PO4 are quite basic

Na3PO4, also known as trisodium phosphate (TSP), is a soluble phosphate salt. When Na3PO4 is dissolved in water, it dissociates into sodium ions (Na+) and phosphate ions (PO4^3-). The phosphate ion can react with water molecules, acting as a Bronsted-Lowry base, and accepting protons (H+) from the water molecules. As a result of this reaction, hydroxide ions (OH-) are produced, which increases the basicity of the solution. Therefore, solutions of Na3PO4 are quite basic.