Question

Consider the following reactions (i) \(\mathrm{CO}_{3}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCO}_{3}^{-}+\mathrm{OH}^{-}\) (ii) \(\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}\) (iii) \(\mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NH}_{4} \mathrm{OH}\) (iv) \(\mathrm{HCl}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Cl}^{-}\) Which pair of reactions proves that water is amphoteric? (1) \(i\) and ii (2) ii and iii (3) iii and iv (4) i and iii

Short answer

Option (3) iii and iv show water's amphoteric nature.

Step-by-step solution

Understand Amphoteric Substances

Amphoteric substances can act as both acids and bases. Water (H2O) is one such substance.

Analyze Reaction (i)

In reaction (i) \(\mathrm{CO}_{3}^{2-}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HCO}_{3}^{-}+\mathrm{OH}^{-}\), water donates a proton (acts as an acid according to Bronsted-Lowry theory).

Analyze Reaction (ii)

In reaction (ii) \(\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{2} \mathrm{CO}_{3}\), water combines with CO2 to form carbonic acid (acts as a base).

Analyze Reaction (iii)

In reaction (iii) \(\mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NH}_{4} \mathrm{OH}\), water donates a proton to ammonia, forming ammonium hydroxide (acts as an acid).

Analyze Reaction (iv)

In reaction (iv) \(\mathrm{HCl}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Cl}^{-}\), water acts as a base by accepting a proton from hydrochloric acid.

Identify the Pair of Reactions

From the above steps, water acts as both an acid and a base in reactions (i) and (iv). This demonstrates water's amphoteric nature clearly.

Key concepts

acid-base reactions

Acid-base reactions are fundamental in chemistry. They occur when an acid donates a proton \( \text{H}^+ \) to a base. To understand these reactions better, consider common substances. For instance, when hydrochloric acid \( \text{HCl} \) reacts with water \( \text{H}_2\text{O} \), it forms hydronium ions \( \text{H}_3\text{O}^+ \) and chloride ions \( \text{Cl}^- \). This is a classic example of an acid-base reaction.

Bronsted-Lowry theory

The Bronsted-Lowry theory provides a framework for acid-base reactions. According to this theory, an acid is a proton donor and a base is a proton acceptor. This model helps us understand the behavior of substances in different reactions. For example, in the reaction \( \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \), ammonia \( \text{NH}_3 \) acts as a base by accepting a proton from water. Water, in this case, acts as an acid by donating a proton.

amphoteric substances

Amphoteric substances can act as both acids and bases. Water is a prime example. Depending on the reaction, water can donate or accept a proton. In one reaction, water \( \text{H}_2\text{O} \) donates a proton to the carbonate ion \( \text{CO}_3^{2-} \), acting as an acid. In another reaction, water accepts a proton from hydrochloric acid \(\text{HCl} \), acting as a base. This dual behavior is what makes water amphoteric. Identifying these reactions helps in understanding the versatile nature of such substances.

water chemistry

Water chemistry is fascinating due to the unique properties of water. It is essential for life and exhibits a variety of chemical behaviors. Water can participate in acid-base reactions, as seen in the reactions with \( \text{CO}_2 \) and \( \text{NH}_3 \). Its amphoteric nature allows it to act as both an acid and a base. This makes water a versatile solvent, capable of dissolving many substances. Understanding water chemistry involves exploring its molecular structure, hydrogen bonding, and its role in facilitating chemical reactions.