Problem 214 - A Complete Resource Book in Chemistry for JEE Main

Question

Three reactions involving \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)are given below:[2010] (i) \(\mathrm{H}_{3} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) (ii) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{HPO}_{4}^{2-}+\mathrm{H}_{3} \mathrm{O}^{+}\) (iii) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}+\mathrm{OH}^{-} \longrightarrow \mathrm{H}_{3} \mathrm{PO}_{4}+\mathrm{O}^{2}\) In which of the above does \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)act as an acid? (a) (ii) only (b) (i) and (ii) (c) (iii) only (d) (i) only

Step-by-Step Solution

Answer

In reaction (ii) only, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as an acid. Therefore, the answer is (a) (ii) only.

Step 1: Define Acid and Base

In Bronsted-Lowry theory, an acid is a substance that donates a proton (H^+ ion), and a base is a substance that accepts a proton. We'll use this definition to identify whether \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as an acid in each reaction.

Step 2: Analyze Reaction (i)

In reaction (i), \(\mathrm{H}_{3} \mathrm{PO}_{4}\) donates a proton to \(\mathrm{H}_{2}\mathrm{O}\), forming \(\mathrm{H}_{3}\mathrm{O}^{+}\) and \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\). Here, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) is not donating a proton, so it does not act as an acid.

Step 3: Analyze Reaction (ii)

In reaction (ii), \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) donates a proton to \(\mathrm{H}_{2}\mathrm{O}\), which becomes \(\mathrm{HPO}_{4}^{2-}\) and \(\mathrm{H}_{3}\mathrm{O}^{+}\). Here, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as an acid by donating a proton.

Step 4: Analyze Reaction (iii)

In reaction (iii), \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) accepts a hydroxide ion (\(\mathrm{OH}^{-}\)) and forms \(\mathrm{H}_{3} \mathrm{PO}_{4}\). Here, it accepts a proton by bonding with \(\mathrm{OH}^{-}\), so it does not act as an acid but rather acts similarly to a base.

Step 5: Conclusion

Based on the analysis, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as an acid in reaction (ii) by donating a proton. In reactions (i) and (iii), it does not act as an acid.

Key Concepts

Bronsted-Lowry theory

The Bronsted-Lowry theory is a fundamental concept in understanding acid-base reactions. According to this theory, an acid is any substance that can donate a proton (\(\mathrm{H}^{+}\) ion), while a base is a substance that can accept a proton. This definition helps us understand and predict the behavior of different substances in chemical reactions.

For example, in the reactions involving \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), this ion can act as either a donor or an acceptor of protons, depending on the situation. Knowing the roles of acids and bases in reactions is crucial for analyzing and predicting reaction outcomes, allowing scientists and students to understand how different species interact at the molecular level.

Proton donors

In the Bronsted-Lowry theory, substances that donate protons are called acids. In the context of the exercise, \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as a proton donor in some reactions. When \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) donates a proton to another molecule, it fulfills its role as an acid, according to the Bronsted-Lowry definition.

In one of the given reactions, specifically reaction (ii), \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) donates a proton to water, forming \(\mathrm{HPO}_{4}^{2-}\) and \(\mathrm{H}_{3}\mathrm{O}^{+}\). Here:

• \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) is the acid because it donates a proton.
• Water (\(\mathrm{H}_{2}\mathrm{O}\)) acts as the base since it accepts the proton.

Understanding proton donors helps us determine which substances are acting as acids in a reaction, providing a clear view of the acidic participants in various chemical scenarios.

Chemical reactions analysis

Analyzing chemical reactions to determine the role of different substances involves carefully evaluating which participants act as acids and which act as bases. In the exercise's step-by-step solution, each reaction is scrutinized to identify if \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) acts as an acid.

For example:

• In Reaction (i), \(\mathrm{H}_{3}\mathrm{PO}_{4}\) donates a proton to \(\mathrm{H}_{2}\mathrm{O}\), and \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) does not donate or accept protons, indicating it doesn't act as an acid.
• In Reaction (ii), \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) donates a proton to water, clearly acting as an acid.
• In Reaction (iii), \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) accepts a hydroxide ion, resembling the behavior of a base rather than an acid.

Through detailed analysis, one can determine the specific roles of substances in a reaction, enabling a comprehensive understanding of the acid-base interactions. This kind of analysis not only characterizes the behavior of chemicals in specific reactions but also aids in predicting outcomes in various other chemical processes.