Question

What do you understand by the term acid anhydride? What are the anhydrides of (a) \(\mathrm{HNO}_{2}\) (b) \(\mathrm{HNO}_{3}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) \((d) H_{3} P O_{3}\) (e) HOCl and \((f) H I O_{3} ?\)

Short answer

An acid anhydride forms an acid with water. Anhydrides are: (a) \( \mathrm{N}_2\mathrm{O}_3 \), (b) \( \mathrm{N}_2\mathrm{O}_5 \), (c) \( \mathrm{P}_2\mathrm{O}_5 \), (d) \( \mathrm{P}_2\mathrm{O}_3 \), (e) \( \mathrm{Cl}_2\mathrm{O} \), (f) \( \mathrm{I}_2\mathrm{O}_5 \).

Step-by-step solution

Defining Acid Anhydride

An acid anhydride is a non-metal oxide that reacts with water to form an acid. It is essentially a compound that can be considered as derived from an acid by the removal of water (dehydration). For example, if you remove a water molecule from a carboxylic acid, the resulting compound is an acid anhydride.

Determine the Anhydride of (a) \( \mathrm{HNO}_{2} \)

To find the anhydride of \( \mathrm{HNO}_{2} \), we need to remove water from it. \( \mathrm{HNO}_{2} \) is nitrous acid. Its anhydride can be represented as \( \mathrm{N}_2\mathrm{O}_3 \). This compound is called dinitrogen trioxide.

Determine the Anhydride of (b) \( \mathrm{HNO}_{3} \)

For \( \mathrm{HNO}_{3} \), nitric acid, the anhydride is formed by its dehydration. The anhydride of \( \mathrm{HNO}_{3} \) is \( \mathrm{N}_2\mathrm{O}_5 \), which is known as dinitrogen pentoxide.

Determine the Anhydride of (c) \( \mathrm{H}_3 \mathrm{PO}_4 \)

Phosphoric acid, \( \mathrm{H}_3\mathrm{PO}_4 \), when dehydrated forms its anhydride \( \mathrm{P}_2\mathrm{O}_5 \), which is also called diphosphorus pentoxide.

Determine the Anhydride of (d) \( \mathrm{H}_3\mathrm{PO}_3 \)

For phosphorous acid, \( \mathrm{H}_3\mathrm{PO}_3 \), the anhydride formed is \( \mathrm{P}_2\mathrm{O}_3 \), known as diphosphorus trioxide.

Determine the Anhydride of (e) \( \mathrm{HOCl} \)

Hypochlorous acid, \( \mathrm{HOCl} \), does not have a typical anhydride that forms like the others listed. However, the closest related compound typically identified is \( \mathrm{Cl}_2\mathrm{O} \) or dichlorine monoxide.

Determine the Anhydride of (f) \( \mathrm{HIO}_{3} \)

Iodic acid, \( \mathrm{HIO}_{3} \), forms its anhydride by removal of water to give \( \mathrm{I}_2\mathrm{O}_5 \), which is called iodine pentoxide.

Key concepts

Non-Metal Oxides

Non-metal oxides are fascinating compounds that play a critical role in our understanding of acids and anhydrides. These oxides are formed when non-metals, like nitrogen or phosphorus, react with oxygen. They are not to be confused with metal oxides, which generally form basic solutions. Instead, non-metal oxides often react with water to create acidic solutions. This unique reaction is a key reason these compounds are so central to the study of chemistry, particularly for students learning about chemical reactions.

A non-metal oxide can be seen as a precursor to acids. For example:

• Combining nitrogen oxide with water forms nitric acid or nitrous acid.
• Phosphorus pentoxide reacts with water to form phosphoric acid.

These reactions exemplify the transformative nature of non-metal oxides and why understanding them is crucial in chemistry education.

Dehydration

Dehydration in chemistry is all about the removal of water molecules. When we think about dehydration, we're not just talking about losing water. It's about transforming chemical compounds by extracting water.

In the world of acid anhydrides, dehydration plays a pivotal role. Removing water from an acid can transform it into its corresponding anhydride.

• Take \( ext{HNO}_3, \) for example. When dehydrated, it yields \( ext{N}_2 ext{O}_5, \)which is nitric acid's anhydride.
• Similarly, phosphoric acid (\( ext{H}_3 ext{PO}_4 \)) loses water and forms diphosphorus pentoxide.

These transformations show the importance of viewing dehydration not just as water removal, but as a process that promotes entirely new chemical compounds.

Carboxylic Acid

Carboxylic acids are organic acids characterized by the presence of a carboxyl group \( ext{(-COOH)}. \) Examples include acetic acid and citric acid. These acids become especially interesting because they can form anhydrides through dehydration.

Anhydrides formed from carboxylic acids are important both in nature and the laboratory.

• For instance, acetic anhydride, formed from acetic acid, is used in the synthesis of aspirin.
• Understanding how carboxylic acids behave when they lose water helps students appreciate the organic synthesis process.

Hence, carboxylic acids don't just play a vital role in biological systems, but also in many industrial applications.

Anhydride Formation

Anhydride formation is a fascinating process where acids transform into anhydrides, often involving dehydration. By removing water from an acid, you essentially change its structure and reactivity characteristics.

• For nitrous acid (\( ext{HNO}_2\)), removing a water molecule results in dinitrogen trioxide (\( ext{N}_2 ext{O}_3\)).
• Phosphoric acid (\( ext{H}_3 ext{PO}_4\)) becomes diphosphorus pentoxide through a similar process.

Students often find this process engaging because it illustrates how mere water removal can lead to new chemicals with entirely different properties.

When students understand this, they grasp the broader concept of chemical transformations and the intricate dance of atoms that defines chemistry.

Chemistry Education

Chemistry education offers students an opportunity to delve into the world of atoms and molecules. A crucial part of this education is understanding terms like acid anhydride, which opens doors to advanced chemistry topics.

Educators use concepts like non-metal oxides and anhydride formation to teach about reactions and transformations. Here's how these concepts fit into the broader learning goals:

• They help explain how substances change under various conditions.
• They highlight the significance of chemical reactions in both natural and industrial contexts.
• They provide the foundational knowledge needed for more advanced studies in organic and inorganic chemistry.

By breaking down complex processes into understandable chunks, chemistry education equips students with tools to explore the world scientifically.