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.

Short answer

H3PO3 is a diprotic acid as it can donate two protons due to the presence of two acidic hydrogens. Nitric acid is a strong acid because of its complete ionization and the stability of the nitrate ion, while phosphoric acid is weak due to incomplete ionization. Phosphate rock is an ineffective fertilizer due to the low solubility of its minerals. Phosphorus doesn't exist as diatomic molecules at room temperature due to the higher energy needed to form P-P bonds compared to its allotropes. Solutions of Na3PO4 are basic due to the presence of hydroxide ions formed when PO43- reacts with H2O.

Step-by-step solution

Observation (a): H3PO3 is a diprotic acid

H3PO3, also known as phosphorous acid, is a diprotic acid because it can donate two protons (hydrogen ions, H+) in aqueous solution, undergoing two ionization steps. Its structure contains three hydrogens bonded to the central phosphorus (P) atom and two of them are bonded directly to oxygens (O), which makes these two hydrogens acidic. The third hydrogen is bonded directly to P and this bond is not considered acidic as its bond is relatively strong. Hence, H3PO3 can lose two protons in two ionization steps, making it a diprotic acid.

Observation (b): Nitric acid is a strong acid, whereas phosphoric acid is weak

Nitric acid (HNO3) is a strong acid because it completely ionizes (dissociates) in aqueous solution, releasing hydrogen ions (H+) and nitrate ions (NO3-). This complete ionization is due to the high electronegativity of oxygen (O) and the presence of a resonance in the nitrate ion, which stabilizes the ion and makes it a better leaving group. Phosphoric acid (H3PO4), on the other hand, is a weak acid because it does not ionize completely in aqueous solution. In its first ionization step, phosphoric acid releases a hydrogen ion (H+) to form the dihydrogen phosphate ion (H2PO4-). After that, the ionization of the remaining hydrogen ions is less complete, due to electrostatic repulsion between the negative charges on the ion and the oxygen atom holding the hydrogen. As a result, a significant amount of H3PO4 remains in the solution, making it a weak acid.

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

Phosphate rock is primarily composed of calcium phosphate minerals, such as apatite. These minerals are very insoluble in water, meaning they do not readily dissolve and release phosphate ions (PO43-) that plants need for growth. For a phosphate fertilizer to be effective, it needs to provide a readily available source of phosphate ions for plant uptake. Since phosphate rock is insoluble and does not dissolve well in water, it cannot provide sufficient amounts of phosphate ions for plants, making it an ineffective phosphate fertilizer.

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

Phosphorus is found in nature as a solid, existing in several allotropic forms. It does not exist as diatomic molecules (P2) at room temperature because of the higher energy needed to form the P-P bond compared to the bonds in its allotropes, which are more stable. Instead, phosphorus usually forms a more complex structure, such as white phosphorus (P4). Nitrogen (N2), on the other hand, exists as a diatomic molecule (N2) at room temperature due to the strong triple bond between its two nitrogen atoms. This bond is very stable and requires a lot of energy to break. Besides, no other configuration provides greater stability. As a result, nitrogen is found in nature as diatomic molecules.

Observation (e): Solutions of Na3PO4 are quite basic

Sodium phosphate (Na3PO4) is the sodium salt of the weak acid phosphoric acid (H3PO4). When Na3PO4 is dissolved in water, it forms sodium ions (Na+) and phosphate ions (PO43-). Due to the basicity of the phosphate ion, it can react with water (H2O) to form hydroxide ions (OH-) and H2PO4-, according to the following reaction: $$P{O}_4^{3-}+H_{2}O\rightleftharpoons H_{2}P{O}_4^{-}+O{H}^{-}$$ The presence of hydroxide ions (OH-) makes the solution basic. Since phosphoric acid is a weak acid, its conjugate base (PO43-) is a relatively strong base, leading to a basic solution containing sodium phosphate (Na3PO4).

Key concepts

Ionization of Acids

Understanding the ionization of acids is critical for explaining why certain substances, like H₃PO₃, display particular acidic or basic properties. Ionization refers to the process by which an acid releases hydrogen ions (H+) when dissolved in water, forming its conjugate base. Diprotic acids, such as phosphorous acid (H₃PO₃), undergo this process in two distinct steps. In the first ionization step, the acid loses one hydrogen ion, and in the second step, it loses a second hydrogen ion. The ease of this ionization, and consequently the strength of an acid, can be affected by factors such as the stability of the conjugate base and the presence of resonance structures. In contrast to strong acids, which ionize completely in solution, a weak acid like phosphoric acid (H₃PO₄) does not fully ionize, contributing to higher concentrations of the acid remaining intact in the solution.

Phosphate Fertilizers

The application and efficacy of phosphate fertilizers play a pivotal role in agricultural sciences and plant nutrition. Plants require phosphate ions (PO₄^3-) for their growth, but the source of these ions must be soluble in water. Phosphate rock is typically an inefficient fertilizer due to its poor solubility, meaning that it fails to adequately provide the necessary phosphate to plants through the soil. An effective fertilizer would be one that readily dissolves in soil moisture to deliver these essential nutrients. This is why treatments are often applied to phosphate rocks to increase their solubility, such as treatment with sulfuric acid to create more soluble phosphate forms like mono- or diammonium phosphate, which are more plant-accessible.

Phosphorus Allotropes

Delving into the world of chemical elements, the phosphorus allotropes represent the different physical forms in which the element phosphorus exists. Allotropes differ in their molecular structure and properties. For example, the most common allotrope of phosphorus is white phosphorus (P₄), which consists of P₄ tetrahedra. This form is highly reactive and less stable at room temperature compared to the diatomic molecule of nitrogen (N₂), which is very stable due to the strong triple bond between the nitrogen atoms. The stability of these allotropes, or lack thereof, can influence their natural occurrence and practical uses in industries such as in making matches or in military applications.

Acid and Base Strength

The acid and base strength determines how well acids and bases dissociate in water. It's quintessential to recognize this strength to forecast the behavior of substances in a variety of chemical reactions. Strong acids, such as nitric acid (HNO₃), ionize completely in aqueous solutions, releasing all of their hydrogen ions. This complete ionization typically occurs due to the high stability of the resultant ions in solution. Conversely, weak acids like phosphoric acid (H₃PO₄) do not release all of their hydrogen ions and exist in an equilibrium between the ionized and non-ionized forms. Base strength is akin to acid strength but refers to the tendency of a base to accept hydrogen ions. Hence, strong bases are more readily to accept hydrogen ions compared to weak bases.

Molecular Stability

The molecular stability of a compound is an indication of how energetically favorable its structure is, playing a significant role in its chemical properties and reactions. Factors such as bond strength, resonance stabilization, and electron delocalization contribute to the overall stability of a molecule. In the context of acids and bases, for instance, a strong bond within a molecule can mean less tendency to release H+ ions, as seen in the third hydrogen of H₃PO₃. On the other hand, the strong triple bond between nitrogen atoms in N₂ provides it with incredible stability, explaining why nitrogen is found as a diatomic molecule in nature, unlike phosphorus, which forms more stable allotropes due to the preferred energy state.