Question

What is the anhydride for each of the following acids: (a) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (b) \(\mathrm{HClO}_{3}\) (c) \(\mathrm{HNO}_{2}\), (d) \(\mathrm{H}_{2} \mathrm{CO}_{3},\) (e) \(\mathrm{H}_{3} \mathrm{PO}_{4} ?\)

Short answer

The anhydrides for the given acids are as follows: (a) H2S2O7, (b) Cl2O6, (c) N2O3, (d) CO2 (for H2CO3), (e) H4P2O7.

Step-by-step solution

(a) Anhydride of H2SO4

To find the anhydride of H2SO4, we will first combine two molecules of H2SO4 and then remove a water molecule: \[2 \; (\mathrm{H}_{2} \mathrm{SO}_{4}) - \mathrm{H}_{2} \mathrm{O} = \mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{7}\] Thus, the anhydride of H2SO4 is H2S2O7.

(b) Anhydride of HClO3

To find the anhydride of HClO3, we will first combine two molecules of HClO3 and then remove a water molecule: \[2 \; (\mathrm{HClO}_{3}) - \mathrm{H}_{2} \mathrm{O} = \mathrm{Cl}_{2} \mathrm{O}_{6}\] Thus, the anhydride of HClO3 is Cl2O6.

(c) Anhydride of HNO2

To find the anhydride of HNO2, we will first combine two molecules of HNO2 and then remove a water molecule: \[2 \; (\mathrm{HNO}_{2}) - \mathrm{H}_{2} \mathrm{O} = \mathrm{N}_{2} \mathrm{O}_{3}\] Thus, the anhydride of HNO2 is N2O3.

(d) Anhydride of H2CO3

To find the anhydride of H2CO3, we will first combine two molecules of H2CO3 and then remove a water molecule: \[2 \; (\mathrm{H}_{2} \mathrm{CO}_{3}) - \mathrm{H}_{2} \mathrm{O} = \mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{5}\] However, H2C2O5 does not exist under normal conditions as the anhydride of carbonic acid. Carbonic acid H2CO3 instead decomposes directly into water and carbon dioxide (CO2): \[\mathrm{H}_{2} \mathrm{CO}_{3} \longrightarrow \mathrm{H}_{2} \mathrm{O} + \mathrm{CO}_{2}\] Thus, CO2 is considered the anhydride of H2CO3.

(e) Anhydride of H3PO4

To find the anhydride of H3PO4, we will first combine two molecules of H3PO4 and then remove a water molecule: \[2 \; (\mathrm{H}_{3} \mathrm{PO}_{4}) - \mathrm{H}_{2} \mathrm{O} = \mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{7}\] Thus, the anhydride of H3PO4 is H4P2O7.

Key concepts

Sulfuric Acid

Sulfuric acid, commonly represented as \( \text{H}_2\text{SO}_4 \), is a strong and industrially significant acid. It is known for its role in fertilizers, chemicals, and in petroleum refining. Understanding its anhydride is crucial in many chemical reactions and industrial processes. To find the anhydride, we pair two molecules of sulfuric acid:

• Combine two molecules of \( \text{H}_2\text{SO}_4 \).
• Remove one water molecule \( \text{H}_2\text{O} \).

This results in \( \text{H}_2\text{S}_2\text{O}_7 \), known as pyrosulfuric acid or oleum, which is an important substance in the production of various sulfonated chemicals. Its formation and composition are vital to understanding the dehydration processes and derivatives of sulfuric acid.

Chloric Acid

Chloric acid, denoted as \( \text{HClO}_3 \), is a lesser-known acid. It becomes important when studying oxidizing agents or oxyacid derivatives of chlorine. Like other acids, its anhydride tells us how it behaves in concentrated or derivative forms. To derive its anhydride:

• Start with two chloric acid molecules \( \text{HClO}_3 \).
• Remove one \( \text{H}_2\text{O} \).

This yields \( \text{Cl}_2\text{O}_6 \), a molecule relating to chlorine’s higher oxidation states. While theoretical due to its instability, knowing about \( \text{Cl}_2\text{O}_6 \) helps in understanding decomposition pathways and reaction mechanisms involving chloric acid.

Nitrous Acid

Nitrous acid \( \text{HNO}_2 \) has intriguing chemistry, especially due to its role in nitrite formation and biological processes. To comprehend its behavior in absence of water, it’s valuable to recognize its anhydride, which can be formed through the following process:

• Combine two \( \text{HNO}_2 \) molecules.
• Subtract one \( \text{H}_2\text{O} \).

You arrive at \( \text{N}_2\text{O}_3 \), also called dinitrogen trioxide. This compound partakes in forming nitrite ions and contributes to atmospheric processes. Understanding nitrous acid’s anhydride gives insight into environmental nitrogen cycles and decomposition reactions.

Carbonic Acid

Carbonic acid, or \( \text{H}_2\text{CO}_3 \), plays a pivotal role in metabolic pathways and carbon cycling. However, unlike other acids, its anhydride formation is unique. Initially combining two \( \text{H}_2\text{CO}_3 \) molecules and removing water seems to yield \( \text{H}_2\text{C}_2\text{O}_5 \). However, this compound doesn’t persist. Instead, carbonic acid decomposes to:

• \( \text{H}_2\text{O} \) (water)
• \( \text{CO}_2 \) (carbon dioxide)

Thus, \( \text{CO}_2 \) is considered its natural anhydride. This exemplifies carbon dioxide’s importance in buffering and respiratory processes.

Phosphoric Acid

Phosphoric acid \( \text{H}_3\text{PO}_4 \) is widely used in fertilizers and food processing. To grasp its behavior in compounds, one should understand its anhydride. The steps to find it include:

• Using two \( \text{H}_3\text{PO}_4 \) molecules.
• Removing one \( \text{H}_2\text{O} \).

The result is \( \text{H}_4\text{P}_2\text{O}_7 \), or pyrophosphoric acid, which features heavily in biochemistry and industrial applications where phosphate linkages play a critical role. Recognizing this conversion aids in understanding the chemistry of phosphorus-containing compounds.