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, (c) reaction of \(\mathrm{P}_{4} \mathrm{O}_{10}\) with water.

Short answer

(a) Hydrolysis of \(\mathrm{PCl}_{5}\): \[ \mathrm{PCl}_{5} + 4\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{PO}_{4} + 5\mathrm{HCl} \] (b) Dehydration of phosphoric acid to form pyrophosphoric acid: \[ 2\mathrm{H}_{3}\mathrm{PO}_{4} \rightarrow \mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{7} + \mathrm{H}_{2}\mathrm{O} \] (c) Reaction of \(\mathrm{P}_{4}\mathrm{O}_{10}\) with water: \[ \mathrm{P}_{4}\mathrm{O}_{10} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 4\mathrm{H}_{3}\mathrm{PO}_{4} \]

Step-by-step solution

(a) Hydrolysis of \(\mathrm{PCl}_{5}\)

Here, we are given the hydrolysis reaction of phosphorus pentachloride (\(\mathrm{PCl}_{5}\)). Upon reaction with water, phosphorus pentachloride hydrolyzes to form phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)) and hydrogen chloride (\(\mathrm{HCl}\)). The balanced equation for this reaction is: \[ \mathrm{PCl}_{5} + 4\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{PO}_{4} + 5\mathrm{HCl} \]

(b) Dehydration of phosphoric acid to form pyrophosphoric acid

In this reaction, two molecules of phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)) undergo dehydration to form one molecule of pyrophosphoric acid (\(\mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{7}\)) and a molecule of water (\(\mathrm{H}_2\mathrm{O}\)). The balanced equation for this reaction is: \[ 2\mathrm{H}_{3}\mathrm{PO}_{4} \rightarrow \mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{7} + \mathrm{H}_{2}\mathrm{O} \]

(c) Reaction of \(\mathrm{P}_{4}\mathrm{O}_{10}\) with water

When phosphorus pentoxide or diphosphorus pentoxide (\(\mathrm{P}_{4}\mathrm{O}_{10}\)) reacts with water, it forms phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)). The balanced equation for this reaction is: \[ \mathrm{P}_{4}\mathrm{O}_{10} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 4\mathrm{H}_{3}\mathrm{PO}_{4} \] Now we have written and balanced all the equations for the given reactions.

Key concepts

Understanding Hydrolysis Reactions

A hydrolysis reaction involves a chemical compound reacting with water to form two or more products. In essence, it's the process by which a water molecule is used to break the bonds in another molecule. In the context of phosphorus chemistry, the hydrolysis of phosphorus pentachloride ( PCln ) is a classic example.

When PCln is exposed to water, it undergoes a transformation where it splits into phosphoric acid ( POn ) and hydrogen chloride ( HCl ). This reaction is significant in industrial chemistry and demonstrates how water can act as a reactant to produce acids. Hydrolysis reactions are common in various biological and chemical systems, serving as a fundamental process in metabolism and other chemical pathways.

Students should remember that in hydrolysis reactions, water is not just a solvent but a reactant. The general form of a hydrolysis reaction looks something like this: A-B + HOn -> AH + BOH . The actual products will vary based on the specific reactants. In the example from the exercise, the balanced equation shows us how four water molecules ( HOn ) are needed to completely hydrolyze one molecule of phosphorus pentachloride, resulting in phosphoric acid and hydrogen chloride.

Dehydration Reactions in Chemistry

Counter to hydrolysis, a dehydration reaction involves the removal of a water molecule from a substance. It's a type of condensation reaction where two molecules combine to form one single molecule, along with the elimination of a water molecule. In biochemical systems, dehydration reactions are essential for synthesizing complex molecules from simpler ones, such as in the formation of polymers.

The dehydration of phosphoric acid to form pyrophosphoric acid, as presented in the exercise, is a perfect example of such a reaction. Dehydration reactions typically require energy input, in the form of heat or a catalyst, to proceed. The balanced chemical equation provides a clear visualization of how two molecules of phosphoric acid ( HOn ) combine, through the loss of a water molecule, to form pyrophosphoric acid ( HPn ).

When learning about dehydration reactions, it is crucial to understand that these processes are often reversible. With the presence of water and certain conditions, the reaction can reverse, meaning pyrophosphoric acid could be hydrolyzed back into phosphoric acid. This is an important concept in phosphorus chemistry, where many compounds can undergo such reversible reactions based on the environmental conditions.

Phosphorus Chemistry Fundamentals

Phosphorus chemistry is a branch of inorganic chemistry that deals with the properties and reactions of phosphorus-containing compounds. Phosphorus is a versatile element that forms a variety of compounds, from the highly reactive white phosphorus ( Pn ) to the more stable and complex phosphates used in fertilizers.

In our exercise, we encountered three key phosphorus compounds: phosphorus pentachloride ( PCln ), phosphoric acid ( HOn ), and pyrophosphoric acid ( HPn ). Each of these compounds showcases the ability of phosphorus to form strong bonds with both chlorides and oxygen, and their propensity to undergo hydrolysis and dehydration reactions. Understanding these reactions helps us appreciate the role of phosphorus in industrial applications as well as biological systems.

Another vital compound in the context of phosphorus chemistry is phosphorus pentoxide ( Pn ). It is known for its high affinity for water. This makes it a powerful drying agent or desiccant because it reacts vigorously with water to form phosphoric acid. The reaction of Pn with water in Step 3 of the original exercise illuminates the dynamic nature of phosphorus compounds and their interactions with water. This knowledge is fundamental for students who delve into advanced studies of phosphorus chemistry and its practical applications in science and technology.