Problem 68 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

(a) \[ \mathrm{PCl}_{5} + 4\mathrm{H}_{2}\mathrm{O} \rightarrow 5\mathrm{HCl} + \mathrm{PO}_{4}\] (b) \[ 2\mathrm{H}_{3}\mathrm{PO}_{4} \rightarrow \mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{7} + \mathrm{H}_{2}\mathrm{O} \] (c) \[ \mathrm{P}_{4}\mathrm{O}_{10} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 4\mathrm{H}_{3}\mathrm{PO}_{4} \]

Step 1: (a) Hydrolysis of PCl5

Hydrolysis refers to the reaction of a substance with water. In this case, PCl5 reacts with water to produce hydrogen chloride (HCl) and phosphorous oxychloride (POCl3). We can write the reaction as follows: \[ \mathrm{PCl}_{5} + 4\mathrm{H}_{2}\mathrm{O} \rightarrow 5\mathrm{HCl} + \mathrm{PO}_{4}\] Now the equation is balanced, with an equal number of atoms on both sides.

Step 2: (b) Dehydration of Phosphoric Acid to form Pyrophosphoric Acid

Dehydration refers to the removal of water from a substance. In this case, two molecules of phosphoric acid (also called orthophosphoric acid, H3PO4) will lose a water molecule to form one molecule of pyrophosphoric acid (H4P2O7). We can write the reaction as follows: \[ 2\mathrm{H}_{3}\mathrm{PO}_{4} \rightarrow \mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{7} + \mathrm{H}_{2}\mathrm{O} \] Now the equation is balanced, with an equal number of atoms on both sides.

Step 3: (c) Reaction of P4O10 with water

In this reaction, P4O10 (phosphorus pentoxide) reacts with water to form phosphoric acid (H3PO4). We can write the reaction as follows: \[ \mathrm{P}_{4}\mathrm{O}_{10} + 6\mathrm{H}_{2}\mathrm{O} \rightarrow 4\mathrm{H}_{3}\mathrm{PO}_{4} \] Now the equation is balanced, with an equal number of atoms on both sides.

Key Concepts

Hydrolysis

Hydrolysis is a fascinating chemical reaction that involves breaking down a compound by adding water. It's like splitting a molecule into smaller parts. In the case of phosphorus pentachloride (\( \mathrm{PCl}_{5} \)), hydrolysis leads to two products: hydrogen chloride (\( \mathrm{HCl} \)) and phosphorous oxychloride (\( \mathrm{POCl}_3 \)). Here's how it happens:

• The water molecule breaks into its components: hydrogen and hydroxide ions.
• These interact with phosphorus pentachloride.
• The result is the liberation of hydrogen chloride gas and the formation of phosphorous oxychloride.

This reaction highlights the power of water in driving chemical transformations by splitting larger compounds into more manageable parts.

Dehydration

Dehydration in chemistry is a process where water is removed from a compound. This typically occurs through heating. One interesting example is the transformation of phosphoric acid (\( \mathrm{H_3PO_4} \)) into pyrophosphoric acid (\( \mathrm{H_4P_2O_7} \)). This is achieved by the following steps:

• Heat causes the molecules of phosphoric acid to rearrange.
• A water molecule is ejected from two phosphoric acid molecules.
• This bond-changing creates a new structure called pyrophosphoric acid.

The reaction showcases how removing specific molecules can lead to entirely new chemical structures with unique properties.

Balanced Equations

Balanced equations are crucial in chemistry as they respect the Law of Conservation of Mass. This law states that matter cannot be created or destroyed. Here's why balanced equations are important:

• They ensure the number of atoms for each element are equal on both sides of the equation.
• They confirm that no mass is lost or gained during a chemical reaction.
• They help in predicting the amounts of reactants needed and products formed.

In the reactions discussed, balancing involves arranging molecules to make sure each atom's tally is equal pre-and post-reaction. It's a vital step that ensures accuracy and clarity in chemical notation, allowing chemists to extrapolate the necessary conditions for chemical processes.

Phosphorus Compounds

Phosphorus compounds play a significant role in various chemical reactions. Phosphorus itself is inherently reactive and forms several important compounds:

• Phosphorus Pentachloride (\( \mathrm{PCl}_{5} \)): Used widely in organic chemistry as a chlorinating reagent, it reacts with water in hydrolysis reactions.
• Phosphoric Acid (\( \mathrm{H_3PO_4} \)): A versatile acid used industrially for fertilizer production, it can undergo dehydration to yield pyrophosphoric acid.
• Phosphorus Pentoxide (\( \mathrm{P_4O_{10}} \)): Known for its dehydrating capabilities, it forms phosphoric acid upon reaction with water.

These compounds are essential in numerous industrial and laboratory processes. Understanding their reactions expands our ability to harness chemical processes for practical applications.