Problem 60 - Chemistry: The Central Science in SI Units, Global Edition

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 phos- (c) Phosphate rock is ineffective as a phoric acid is weak. 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 due to having two ionizable hydrogen protons which dissociate in two separate reactions. Nitric acid (HNO3) is a strong acid because it almost completely dissociates in water, whereas phosphoric acid (H3PO4) is a weak acid that ionizes in steps. Phosphate rock is ineffective as a fertilizer due to low solubility in water but can be made effective by converting it to more soluble forms of phosphate. Phosphorus forms multi-atomic molecules, unlike nitrogen, which forms stable diatomic molecules due to larger atomic radii and lower electronegativity. Solutions of Na3PO4 are quite basic due to the PO4(3-) ions, which accept protons and generate excess hydroxide ions in the solution.

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

H3PO3, or phosphorous acid, is a diprotic acid because it has two ionizable hydrogen protons. In aqueous solution, it can lose two protons (H+) at different steps, creating two separate dissociation reactions: \(1^{st} \: dissociation: H_{3}PO_{3} \rightarrow H_{2}O \: + \: H_{2}PO_{3}^{-}\) \(2^{nd} \: dissociation: H_{2}PO_{3}^{-}\rightarrow H_{2}O \: + \: HPO_{3}^{2-}\) The third hydrogen atom in H3PO3 is bonded to the central phosphorus atom in a phosphorus-hydrogen (P-H) bond, which is much more difficult to ionize and thus does not contribute to the acid behavior.

Step 2: Observation (b): Nitric acid as a strong acid and phosphoric acid as a weak acid

Nitric acid (HNO3) is a strong acid because it donates protons (H+) very easily and almost completely dissociates in water: \(HNO_{3} \rightarrow H^{+} + NO_{3}^{-}\) Phosphoric acid (H3PO4), on the other hand, is a weak acid because it does not dissociate completely in water. It ionizes in steps, similar to H3PO3, but each dissociation step is characterized by a different equilibrium constant, meaning that it does not readily lose its protons (H+): \(1^{st} \: dissociation: H_{3}PO_{4} \rightarrow H^{+} + H_{2}PO_{4}^{-}\) \(2^{nd} \: dissociation: H_{2}PO_{4}^{-}\rightarrow H^{+} + HPO_{4}^{2-}\) \(3^{rd} \: dissociation: HPO_{4}^{2-}\rightarrow H^{+} + PO_{4}^{3-}\) Strong acids like nitric acid cause a larger increase in the concentration of H+ ions in a solution, making them more acidic.

Step 3: Observation (c): Ineffectiveness of phosphate rock as a phosphate fertilizer

Phosphate rock is a naturally occurring mineral deposit composed largely of calcium phosphate (Ca3(PO4)2). It is an ineffective phosphate fertilizer because the calcium phosphate present in the rock is not readily soluble in water, making it difficult for plants to absorb the necessary nutrients. For phosphate rock to be effective, it must be converted into more soluble forms of phosphate, like superphosphate, by treatment with strong acids like sulfuric acid. This process releases soluble phosphate ions that can be absorbed by plants.

Step 4: Observation (d): Phosphorus not existing as diatomic molecules at room temperature

Phosphorus exists in several allotropes, but unlike nitrogen (N2), it does not form diatomic molecules at room temperature. Phosphorus atoms have larger atomic radii and lower electronegativity compared to nitrogen atoms, making the attractive forces between two phosphorus atoms much weaker than those between two nitrogen atoms. As a result, phosphorus forms multi-atomic molecules in its most stable form as white phosphorus (P4) or red phosphorus. Nitrogen, on the other hand, forms very stable diatomic molecules (N2) due to its strong triple covalent bond, which requires a high amount of energy to break.

Step 5: Observation (e): Basicity of Na3PO4 solutions

Na3PO4, or trisodium phosphate, is a soluble salt derived from a weak acid (phosphoric acid, H3PO4) and a strong base (sodium hydroxide, NaOH). When Na3PO4 dissolves in water, it dissociates into Na+ and PO4(3-) ions: \(Na_{3}PO_{4} \rightarrow 3Na^{+} + PO_{4}^{3-}\) The PO4(3-) ions can react with water molecules, accepting protons (H+) and generating excess hydroxide ions (OH-) in the solution: \(PO_{4}^{3-} + H_{2}O \rightarrow HPO_{4}^{2-} + OH^{-}\) Since there is an increase in OH- concentration, the solution becomes quite basic.

Key Concepts

Diprotic Acids

Diprotic acids, such as phosphorous acid (H_3PO_3), have two hydrogen atoms that can dissociate and release protons (H\(^+\)). This characteristic affects how these acids behave in solution. Initially, the acid dissociates to form H_2PO_3^- and releases one proton. The second step involves the dissociation of H_2PO_3^- into HPO_3^{2-}, releasing another proton.
This stepwise release is important because each step is associated with a specific equilibrium constant, making the second dissociation event less favorable than the first. The third hydrogen in H_3PO_3 is not involved in the acid behavior because it is bonded directly to phosphorous (P-H bond), making it much harder to ionize. Understanding this concept helps us predict how diprotic acids react in chemical environments.

Strong and Weak Acids

Acids are categorized into strong and weak based on their ability to dissociate in water. Strong acids like nitric acid (HNO_3) dissociate completely, releasing a greater concentration of hydrogen ions (H\(^+\)) into the solution. This results in a lower pH, indicating a high acidity level.
In contrast, weak acids such as phosphoric acid (H_3PO_4) do not fully dissociate. They release their protons in stages, each with varying equilibrium constants. This incomplete dissociation means fewer free hydrogen ions are present in the solution, leading to higher pH compared to strong acids. The stepwise dissociation of weak acids plays a crucial role in buffering capacities which is essential in biological and environmental systems.

Phosphate Fertilizers

Phosphate rock, largely composed of calcium phosphate ( Ca_3(PO_4)_2 ), is a mineral with limited use as a direct fertilizer due to its low solubility in water. Plants require phosphate ions to grow, and insoluble compounds cannot meet this need because they do not dissolve readily in the soil's moisture.
To make the phosphorus in phosphate rock accessible to plants, it’s frequently treated with acids like sulfuric acid to produce superphosphate. This conversion turns the mineral into more soluble forms, improving the uptake by plant roots. Understanding this process is key to managing soil health and agricultural productivity.

Solutions and pH

The pH of a solution is an indicator of its acidity or basicity. It is directly influenced by the presence of hydrogen (H\(^+\)) and hydroxide (OH\(^-\)) ions. For example, when sodium phosphate (Na_3PO_4) dissolves in water, it dissociates to form sodium and phosphate ions. The phosphate ion (PO_4^{3-}) reacts with water to accept protons, producing hydroxide ions, which increases the solution's pH level, making it basic.
Understanding pH is crucial in many fields such as biology, chemistry, and environmental science. Controlling pH is essential in processes like fermentation, biochemical reactions, and maintaining stable aquatic environments.