Problem 62 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write a balanced equation for each of the following reactions: (a) hydrolysis of \(\mathrm{PCl}_{5},(\mathbf{b})\) dehydration of phosphoric acid (also called orthophosphoric acid) to form pyrophosphoric acid, \((\mathbf{c})\) reaction of \(\mathrm{P}_{4} \mathrm{O}_{10}\) with water.

Step-by-Step Solution

Answer

(a) Hydrolysis of PCl5: \[ PCl_{5 (s)} + 4H_{2 (l)}O \rightarrow H_{3 (aq)}PO_{4} + 5H_{(aq)}Cl \] (b) Dehydration of phosphoric acid to form pyrophosphoric acid: \[ 2H_{3 (aq)}PO_{4} \rightarrow H_{4 (aq)}P_{2}O_{7} + H_{2 (l)}O \] (c) Reaction of P4O10 with water: \[ P_{4 (s)}O_{10} + 6H_{2 (l)}O \rightarrow 4H_{3 (aq)}PO_{4} \]

Step 1: (a) Hydrolysis of PCl5

Hydrolysis of a compound involves the reaction of a substance with water, which results in the splitting of the compound into ions or simpler molecules. In the case of PCl5, it reacts with water (H2O) to form phosphoric acid (H3PO4) and hydrochloric acid (HCl) as products. The balanced equation for this reaction is: \[ PCl_{5 (s)} + 4H_{2 (l)}O \rightarrow H_{3 (aq)}PO_{4} + 5H_{(aq)}Cl \]

Step 2: (b) Dehydration of phosphoric acid to form pyrophosphoric acid

Dehydration is the process of removing water from a compound. In this case, we need to remove water from two molecules of phosphoric acid (H3PO4) to form one molecule of pyrophosphoric acid (H4P2O7). The balanced equation for this reaction is: \[ 2H_{3 (aq)}PO_{4} \rightarrow H_{4 (aq)}P_{2}O_{7} + H_{2 (l)}O \]

Step 3: (c) Reaction of P4O10 with water

The reaction of P4O10 with water involves the formation of phosphoric acid (H3PO4) as the product from tetraphosphorus decoxide. The balanced equation for this reaction is: \[ P_{4 (s)}O_{10} + 6H_{2 (l)}O \rightarrow 4H_{3 (aq)}PO_{4} \] These balanced equations show the stoichiometry of each reaction, ensuring that the number of atoms of each element on the reacting side equals the number of atoms of the same element on the product side.

Key Concepts

Hydrolysis Reaction

Hydrolysis is a fascinating chemical process where water breaks down another compound. Imagine adding water to PCl₅, a phosphorus compound. This leads to the creation of phosphoric acid (H₃PO₄) and hydrochloric acid (HCl). In simple terms, hydrolysis is nature's way of unzipping molecules to make something new.
This concept is crucial in chemistry because it explains how complex molecules transform into simpler, often more useful forms. In the balanced chemical equation for the hydrolysis of PCl₅, you can see how each component matches perfectly on both sides:

• Reactants: PCl₅ and 4H₂O
• Products: H₃PO₄ and 5HCl

Often, hydrolysis helps recycle and break down materials, proving its significance both in nature and industry.

Dehydration Reaction

Dehydration reactions involve removing water from a substance. It's like squeezing the water out of phosphoric acid to form a new molecule. Here, two molecules of phosphoric acid (H₃PO₄) undergo dehydration to form pyrophosphoric acid (H₄P₂O₇) plus a water molecule. This reaction removes water, which contrasts with hydrolysis where water is added.
Dehydration is common in forming new compounds, often with unique and desired properties. In this balanced equation:

• Reactants: 2H₃PO₄
• Products: H₄P₂O₇ and H₂O

The breakdown results not only in structural changes but also changes in behavior and function of the compounds involved. Understanding this helps in grasping how to create new materials from existing ones.

Phosphoric Acid

Phosphoric acid is a versatile molecule with the chemical formula H₃PO₄. It's produced in several chemical reactions, like the ones we've been discussing. When PCl₅ undergoes hydrolysis, phosphoric acid is one of the main products.
This acid is crucial in various industries; it's used in food and agriculture extensively. Known for being part of fertilizers, it helps in plant growth. In the world of chemistry, phosphoric acid acts as a building block for more complex molecules like pyrophosphoric acid. The balanced equations where phosphoric acid forms show how it connects different chemical processes through hydrolysis and reactions with tetraphosphorus decoxide:

• Reacted with water to form and break down compounds
• Involved in dehydration to create larger molecules

Grasping the role of phosphoric acid showcases how integral it is for synthetic processes and biological systems alike.

Stoichiometry

Stoichiometry is like the math of chemistry, ensuring that everything adds up in a chemical equation. In any reaction, it's key to have the same number of each type of atom on both sides of the equation. For example, with the hydrolysis of PCl₅ or the dehydration of phosphoric acid, stoichiometry ensures that no atoms are lost or gained during the reaction.
The balanced equations not only highlight what reacts with what but also how much of each substance you need to either start or end. Each balanced equation follows stoichiometric principles:

• The reactants' atoms exactly match the products' atoms
• Maintaining mass consistency is critical in all reactions

Understanding stoichiometry helps in predicting the amount of product formed and the reactants required in a reaction, making it invaluable for quantifying chemical processes in labs and industries.