Problem 66 - 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

H3PO3 is a diprotic acid because it can release two protons due to the presence of two -OH groups bonded to the phosphorous atoms. Nitric acid is a strong acid because it ionizes completely in aqueous solutions, while phosphoric acid is a weak acid, as it partially dissociates in water. Phosphate rock is an ineffective phosphate fertilizer due to its insolubility in water. Phosphorus exists in various allotropes at room temperature but not as diatomic molecules, while nitrogen forms diatomic molecules due to strong nitrogen-nitrogen bonds. Solutions of Na3PO4 are quite basic because phosphate ions react with water to form hydroxide ions, increasing the pH of the solution.

Step 1: (a) H3PO3 is a diprotic acid.

A diprotic acid is an acid that can donate two protons (H+ ions) per molecule during the process of dissociation. H3PO3, known as phosphorous acid, is a diprotic acid as it can release two protons and comprises of two acidic protons. The structure of H3PO3 is \[H-O-PO(OH)_2\] with the presence of two \(-OH\) groups bonded to the atoms of phosphorous. When it dissociates in water, it releases two protons helping in the formation of two hydronium ions, as shown below: 1. \[H3PO3(aq) \rightarrow H^+(aq) + H_2O\] 2. \[H2PO3^-(aq) \rightarrow H^+(aq) + HPO3^{2-}(aq)\]

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

A strong acid is one that completely dissociates in water, releasing all its protons, while a weak acid partially dissociates, releasing only a small fraction of its protons. Nitric acid (HNO3) is a strong acid because it ionizes completely in aqueous solutions to form hydrogen ions and nitrate ions \[(HNO_3 \rightarrow H^+ + NO_3^-)\]. In contrast, phosphoric acid (H3PO4) is a weak acid as it partially dissociates in water, which means not all its protons are released in solution. Phosphoric acid undergoes three ionization steps, with each step resulting in the release of only one proton, as shown below: 1. \(H_3PO_4(aq) \rightleftharpoons H^+(aq) + H_2PO_4^-(aq)\) 2. \(H_2PO_4^-(aq) \rightleftharpoons H^+(aq) + HPO_4^{2-}(aq)\) 3. \(HPO_4^{2-}(aq) \rightleftharpoons H^+(aq) + PO_4^{3-}(aq)\) Each dissociation constant for these steps is much lower than that of nitric acid, displaying the weak acidic nature of phosphoric acid.

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

Phosphate rock is mainly composed of minerals like apatite (calcium phosphate) and other impurities. It is insoluble in water and hence, cannot provide phosphorus required for plant growth in a readily available form. As a result, it is ineffective as a phosphate fertilizer. For utilization, phosphate rock needs to be treated with a chemical process to convert it into more soluble and readily available forms like single superphosphate (SSP) or triple superphosphate (TSP).

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

Phosphorus is a nonmetal and exists in various allotropes. At room temperature, it primarily exists as a white, red or black phosphorus, with each allotrope having a distinct molecular structure (P4). However, none of these structures have diatomic molecules (P2). On the other hand, nitrogen (N2) exists as diatomic molecules at room temperature due to the triple bond between the two atoms, making it very stable and unreactive, as the bond is difficult to break. Nitrogen's stability as a diatomic molecule is the result of its smaller size, hence enabling the formation of strong nitrogen-nitrogen bonds.

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

Sodium phosphate (Na3PO4) is a salt derived from phosphoric acid (a weak acid) and sodium hydroxide (a strong base). When Na3PO4 dissolves in water, it dissociates into its constituent ions: \[Na_3PO_4(aq) \rightarrow 3Na^+(aq) + PO_4^{3-}(aq)\] Since phosphoric acid is a weak acid, phosphate ion (PO4^3-) behaves as a weak conjugate base. In an aqueous solution, it reacts with water (H2O) to form hydroxide ion (OH-) and HPO4^2-. This reaction is shown below: \[PO_4^{3-}(aq) + H_2O(l) \rightleftharpoons HPO_4^{2-}(aq) + OH^-(aq)\] The presence of hydroxide ions increases the pH of the solution, making it quite basic.