In the reaction between hydrogen sulphate ion and water \({\rm{HSO}}_4^ - {\rm{\;}} + {\rm{\;}}{{\rm{H}}_2}0 \to {\rm{\;}} - {\rm{\gg \;}}{{\rm{H}}_3}{0^ + } + {\rm{SO}}_4^{2 - }\)
a base
Understanding the Reaction: Hydrogen Sulphate and Water
The question asks about the role of water in the reaction between the hydrogen sulphate ion \({\rm{HSO}}_4^ - \) and water \({\rm{H}}_20\). The given reaction is: \({\rm{HSO}}_4^ - {\rm{\;}} + {\rm{\;}}{{\rm{H}}_2}0 \rightarrow {{\rm{H}}_3}{0^ + } + {\rm{SO}}_4^{2 - }\) To determine the role of each substance in an acid-base reaction, we often use the Brønsted-Lowry acid-base theory.
Brønsted-Lowry Acid-Base Theory Explained
According to the Brønsted-Lowry theory:
- An acid is a species that donates a proton (\({\rm{H}}^+\)).
- A base is a species that accepts a proton (\({\rm{H}}^+\)).
In a Brønsted-Lowry acid-base reaction, a proton is transferred from the acid to the base.
Analyzing the Hydrogen Sulphate and Water Reaction
Let's examine what happens to each reactant in the given reaction:
- The hydrogen sulphate ion (\({\rm{HSO}}_4^ - \)) in the reactants changes into the sulphate ion (\({\rm{SO}}_4^{2 - }\)) in the products. To go from \({\rm{HSO}}_4^ - \) to \({\rm{SO}}_4^{2 - }\), the \({\rm{HSO}}_4^ - \) ion loses a proton (\({\rm{H}}^+\)). Therefore, \({\rm{HSO}}_4^ - \) acts as a proton donor.
- Water (\({{\rm{H}}_2}0\)) in the reactants changes into the hydronium ion (\({{\rm{H}}_3}{0^ + }\)) in the products. To go from \({{\rm{H}}_2}0\) to \({{\rm{H}}_3}{0^ + }\), the \({{\rm{H}}_2}0\) molecule gains a proton (\({\rm{H}}^+\)). Therefore, \({{\rm{H}}_2}0\) acts as a proton acceptor.
Based on the Brønsted-Lowry definitions:
- Since \({\rm{HSO}}_4^ - \) donates a proton, it acts as an acid in this reaction.
- Since \({{\rm{H}}_2}0\) accepts a proton, it acts as a base in this reaction.
Here's a summary of the proton transfer:
| Species | Change | Action | Role |
|---|---|---|---|
| \({\rm{HSO}}_4^ - \) | Loses \({\rm{H}}^+\) to form \({\rm{SO}}_4^{2 - }\) | Proton Donor | Acid |
| \({{\rm{H}}_2}0\) | Gains \({\rm{H}}^+\) to form \({{\rm{H}}_3}{0^ + }\) | Proton Acceptor | Base |
Conclusion on Water's Role
As determined by analyzing the proton transfer according to the Brønsted-Lowry theory, water (\({{\rm{H}}_2}0\)) accepts a proton from the hydrogen sulphate ion (\({\rm{HSO}}_4^ - \)) and is converted into the hydronium ion (\({{\rm{H}}_3}{0^ + }\)). Therefore, in this specific reaction, water acts as a base.
Eliminating Other Options
- An acid: Water can act as an acid (donating a proton) in reactions with stronger bases, but in this reaction, it accepts a proton.
- A salt: A salt is typically an ionic compound formed from the reaction of an acid and a base. Water (\({{\rm{H}}_2}0\)) is a covalent molecule, not a salt.
- An inert medium: An inert medium is a substance that does not participate chemically in the reaction. Water clearly participates by accepting a proton and being transformed into \({{\rm{H}}_3}{0^ + }\).
Revision Table: Key Concepts
| Concept | Definition | Relevance to Question |
|---|---|---|
| Brønsted-Lowry Acid | Proton (\({\rm{H}}^+\)) donor | \({\rm{HSO}}_4^ - \) acts as the acid. |
| Brønsted-Lowry Base | Proton (\({\rm{H}}^+\)) acceptor | \({{\rm{H}}_2}0\) acts as the base. |
| Amphoteric Substance | Can act as both an acid and a base | Water is amphoteric, but acts as a base in *this* reaction. |
Additional Information: Amphoteric Nature of Water
Water is known as an amphoteric substance because it can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base depending on the other substance it reacts with. For example:
- With a strong acid like \({\rm{HCl}}\), water acts as a base: \({\rm{HCl}} + {{\rm{H}}_2}0 \rightarrow {{\rm{H}}_3}{0^ + } + {\rm{Cl}}^-\) (Water accepts a proton).
- With a strong base like \({\rm{NH}}_3\), water acts as an acid: \({{\rm{H}}_2}0 + {\rm{NH}}_3 \rightarrow {\rm{OH}}^- + {\rm{NH}}_4^+\) (Water donates a proton).
In the reaction with the hydrogen sulphate ion (\({\rm{HSO}}_4^ - \)), which itself can act as both an acid and a base (it can donate a proton or accept one), water acts specifically as a base because \({\rm{HSO}}_4^ - \) acts as an acid and donates a proton to it.
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