Question

Phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) is a triprotic acid, phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) is a diprotic acid, and hypophosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{2}\right)\) is a monoprotic acid. Explain this phenomenon.

Short answer

In conclusion, phosphoric acid (H₃PO₄) is a triprotic acid because it has three hydrogen atoms bound to oxygen atoms, which can be donated as H⁺ ions. Phosphorous acid (H₃PO₃) is a diprotic acid because it has two hydrogen atoms bound to oxygen atoms, which can be donated as H⁺ ions, and the third hydrogen atom is directly bonded to the phosphorus atom and is not acidic. Hypophosphorous acid (H₃PO₂) is a monoprotic acid because it has only one hydrogen atom bound to an oxygen atom, which can be donated as an H⁺ ion, and the other two hydrogen atoms are directly bonded to the phosphorus atom and are not acidic.

Step-by-step solution

An acid is a compound that donates hydrogen ion (H⁺) in a solution. The number of hydrogen ions that an acid can donate determines its classification: - Monoprotic acid: An acid that can donate one hydrogen ion (H⁺). - Diprotic acid: An acid that can donate two hydrogen ions (H⁺). - Triprotic acid: An acid that can donate three hydrogen ions (H⁺). #Step 2: Analyzing Phosphoric Acid (H₃PO₄)#

The structure of phosphoric acid is as follows: \(H_3PO_4: H - P(=O)(-OH)_2\) In phosphoric acid, there are three hydrogen atoms bound to oxygen atoms. These hydrogen atoms can be donated as H⁺ ions. Therefore, phosphoric acid is a triprotic acid, as it can donate three hydrogen ions. #Step 3: Analyzing Phosphorous Acid (H₃PO₃)#

The structure of phosphorous acid is as follows: \(H_3PO_3: H - P(=O)(-OH)(-H_2O)\) In phosphorous acid, there are only two hydrogen atoms bound to oxygen atoms. These hydrogen atoms can be donated as H⁺ ions. The third hydrogen atom is directly bonded to the phosphorus atom and is not acidic. Therefore, phosphorous acid is a diprotic acid, as it can donate two hydrogen ions. #Step 4: Analyzing Hypophosphorous Acid (H₃PO₂)#

The structure of hypophosphorous acid is as follows: \(H_3PO_2: H - P(-OH)(-H_2O)_2\) In hypophosphorous acid, there is only one hydrogen atom bound to an oxygen atom. This hydrogen atom can be donated as an H⁺ ion. The other two hydrogen atoms are directly bonded to the phosphorus atom and are not acidic. Therefore, hypophosphorous acid is a monoprotic acid, as it can donate one hydrogen ion. In conclusion, the phenomenon of these acids being classified as triprotic, diprotic, and monoprotic can be explained by analyzing their structures and identifying the number of hydrogen atoms that can be donated as H⁺ ions.

Key concepts

Monoprotic Acid

Monoprotic acids are the simplest form of acids that donate only one hydrogen ion (\(\text{H}^+\)) per molecule during the process of dissociation in water. An excellent example of a monoprotic acid is hypophosphorous acid (\(\text{H}_3\text{PO}_2\)). Despite its formula suggesting the presence of three hydrogens, only one is bound to an oxygen and can dissociate as \(\text{H}^+\).
The remaining two hydrogens are connected directly to phosphorus and do not ionize because hydrogen ions only dissociate effectively when attached to oxygen. Therefore, hypophosphorous acid exemplifies a monoprotic acid due to its ability to release just one proton (H⁺ ion).
This makes understanding the structure crucial in determining its acidic nature.

Diprotic Acid

Diprotic acids have the capability to donate two hydrogen ions (\(\text{H}^+\)) in two sequential steps. Phosphorous acid (\(\text{H}_3\text{PO}_3\)) is a classic example. In this molecule, two hydrogen atoms are bound to oxygen within hydroxyl groups, making them capable of dissociation.
Interestingly, the third hydrogen is directly bonded to the phosphorus atom, which means it cannot dissociate as \(\text{H}^+\). Because of this specific atomic structure, phosphorous acid can donate only two hydrogen ions, classifying it as a diprotic acid.
This dual-step ionization property influences the acid's behavior in solutions, often leading to different degrees of acidity in its two ionization phases.

Triprotic Acid

Triprotic acids are able to donate three hydrogen ions (\(\text{H}^+\)) in three steps. Phosphoric acid (\(\text{H}_3\text{PO}_4\)) serves as the exemplar of this category. It contains three hydrogen atoms each attached to an oxygen atom in hydroxyl groups, rendering them all capable of ionization.
In aqueous solutions, phosphoric acid undergoes three distinct stages of deprotonation, releasing a \(\text{H}^+\) ion in each phase. This sequential release is key to its property as a triprotic acid.
Understanding how phosphoric acid donates hydrogen ions in stages helps explain its role in biological systems and industrial applications where gradual release of protons across multiple stages is advantageous.

Hydrogen Ion Donation

Hydrogen ion donation is a fundamental concept in acid chemistry. It is the process by which an acid releases hydrogen ions (\(\text{H}^+\)) into a solution, contributing to the acidity. The number of hydrogen ions an acid can donate impacts its classification as mono-, di-, or triprotic.
When acids dissolve in water, they dissociate to varying extents based on their molecular structure. This dissociation not only determines the acid's strength but also its ability to influence pH.
The acidity of a solution is directly related to the concentration of \(\text{H}^+\) ions, making the understanding of hydrogen ion donation critical for predicting and managing pH levels in various chemical reactions.

Phosphoric Acids

Phosphoric acids refer to a group of acids that contain phosphorus and follow a systematic naming convention based on the number of ionizable hydrogen atoms they possess.
These acids include hypophosphorous (\(\text{H}_3\text{PO}_2\)), phosphorous (\(\text{H}_3\text{PO}_3\)), and phosphoric (\(\text{H}_3\text{PO}_4\)) acids, each differing in their hydrogen ion donation capacity due to unique structural characteristics.
Their applications are diverse, ranging from use in fertilizers (phosphoric acid) to reducing agents in chemistry (hypophosphorous acid). Understanding the nuances in their structure and behavior is vital for leveraging their unique properties in practical and industrial applications.