Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

For \(\mathrm{H}_{3} \mathrm{PO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\), the correct choice is: (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and reducing (b) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and non-reducing (c) \(\mathrm{H}_{3}^{3} \mathrm{PO}_{A}^{3}\) is tribasic and reducing (d) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is tribasic and non-reducing

Step-by-Step Solution

Answer

(a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and reducing.

Step 1: Understand the Terminology

To solve this problem, we need to know what 'dibasic', 'tribasic', 'reducing', and 'non-reducing' mean. 'Dibasic' refers to an acid that can donate two protons (hydrogen ions), while 'tribasic' refers to one that can donate three protons. A reducing agent is one that can donate electrons to another species and itself gets oxidized. A non-reducing agent cannot easily donate electrons.

Step 2: Analyze \\(\mathrm{H}_{3} \mathrm{PO}_{3}\\)

The chemical structure of \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is such that only two hydrogen atoms are directly bonded to oxygen, which means it can donate only two protons. Hence, it is dibasic. \(\mathrm{H}_{3} \mathrm{PO}_{3}\) can act as a reducing agent because it has the ability to be oxidized itself (specifically, the phosphorus atom has a lower oxidation state compared to fully oxidized phosphoric acid). Therefore, \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is reducing.

Step 3: Analyze \\(\mathrm{H}_{3} \mathrm{PO}_{4}\\)

\(\mathrm{H}_{3} \mathrm{PO}_{4}\) is phosphoric acid. In this structure, all three hydrogen atoms are bonded to oxygen. Therefore, \(\mathrm{H}_{3} \mathrm{PO}_{4}\) can donate three protons, making it tribasic. However, \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is not a reducing agent because it does not easily get oxidized.

Step 4: Determine the Correct Choice

From the analysis, we know that \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is both dibasic and reducing. On the other hand, \(\mathrm{H}_{3} \mathrm{PO}_{4}\) is tribasic and non-reducing. Thus, the correct choice is (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is dibasic and reducing.

Key Concepts

Dibasic and Tribasic Acids

In the world of chemistry, acids are often characterized by their ability to donate protons. This ability determines whether they are categorized as dibasic or tribasic acids. Let's simplify these terms:

• Dibasic Acids: These are acids that can donate up to two protons. This essentially means that their molecular structure allows them only two hydrogen ions that can be released in a reaction. Examples include sulfurous acid (\(\mathrm{H}_{2}\mathrm{SO}_{3}\) ) and phosphorous acid (\(\mathrm{H}_{3}\mathrm{PO}_{3}\)).
• Tribasic Acids: These acids can donate three protons, meaning they have three ionizable hydrogen ions. A prime example is phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)). The full dissociation results in three distinct molecules of hydrogen ions being donated.

Breaking down the structures helps us grasp why these acids act differently. In the case of \(\mathrm{H}_{3}\mathrm{PO}_{3}\), the presence of two hydrogen-oxygen bonds indicates its dibasic nature. Meanwhile, the three H-O bonds in \(\mathrm{H}_{3}\mathrm{PO}_{4}\) allow it to be tribasic.

Reducing and Non-reducing Agents

Understanding reducing and non-reducing agents is key to mastering redox reactions. The terms "reducing" and "non-reducing" refer to a substance's ability to donate electrons in a chemical reaction.

• Reducing Agents: These are chemicals that can donate electrons to another substance, and in the process, they themselves become oxidized. A classic example is \(\mathrm{H}_{3}\mathrm{PO}_{3}\), which acts as a reducing agent. This is majorly because it has a phosphorus atom in a lower oxidation state that can increase its oxidation number by losing electrons.
• Non-reducing Agents: These agents do not easily give up electrons and thus resist oxidation. An example is \(\mathrm{H}_{3}\mathrm{PO}_{4}\), which does not serve as a reducing agent because it holds its electrons tightly and doesn't change its oxidation state readily during reactions.

In summary, when an acid can donate electrons and becomes oxidized, it is labeled as reducing. Conversely, an acid that is less reactive and does not readily oxidize is termed non-reducing.

Phosphorous and Phosphoric Acid

Phosphorous acid (\(\mathrm{H}_{3}\mathrm{PO}_{3}\)) and phosphoric acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)) are significant players in chemistry, especially when talking about basicity and redox reactions.

• Phosphorous Acid (\(\mathrm{H}_{3}\mathrm{PO}_{3}\)): Despite the "tri" in its formula, phosphorous acid is dibasic. This is due to its structure where only two oxygen-bonded hydrogens can be ionized. It acts as a reducing agent because its phosphorus atom is in a lower oxidation state, allowing it to donate electrons in chemical reactions.
• Phosphoric Acid (\(\mathrm{H}_{3}\mathrm{PO}_{4}\)): This acid is tribasic, allowing it to donate three protons as it has three hydrogen atoms each bonded to an oxygen, typical for a strong acid. Unlike phosphorous acid, phosphoric acid does not act as a reducing agent as the phosphorus atom is already in a high oxidation state, limiting its ability to donate electrons.

Both acids play unique roles in chemical reactions, thanks to their distinctive structures and properties.Understanding these differences is crucial to predicting and manipulating how they behave in various reactions.