Question

Which from following compounds accepts proton from water molecule according to Brønsted-Lowry theory?

• A. $\text{NaOH}_{(aq)}$
• B. $\text{HCl}_{(aq)}$
• C. $\text{NH}_{3(aq)}$
• D. $\text{NH}_4\text{OH}_{(aq)}$

Hint:

Brønsted-Lowry bases always look to grab an $\text{H}^+$ directly using a lone pair. Ammonia ($\text{NH}_3$) turning into ammonium ($\text{NH}_4^+$) is the classic textbook illustration of this rule!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to find the chemical compound that acts as a Brønsted-Lowry base by directly accepting a proton ($\text{H}^+$) from a water molecule in an aqueous medium.

Step 2: Detailed Explanation:
According to the Brønsted-Lowry acid-base theory:

• An acid is a substance that acts as a proton donor.

• A base is a substance that acts as a proton acceptor.
When ammonia gas ($\text{NH}_3$) dissolves in water, it acts as a weak base. The nitrogen atom in $\text{NH}_3$ possesses a lone pair of electrons that readily accepts a proton ($\text{H}^+$) from a water molecule: $$ \text{NH}_{3(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{NH}^+_{4(aq)} + \text{OH}^-_{4(aq)} $$ In this chemical equilibrium, water acts as the Brønsted-Lowry acid (donates $\text{H}^+$) and $\text{NH}_3$ functions as the Brønsted-Lowry base because it directly captures the proton to form the ammonium ion ($\text{NH}_4^+$). In contrast, substances like NaOH and $\text{NH}_4\text{OH}$ are Arrhenius bases because they release pre-existing hydroxide ions through simple dissociation rather than a proton-acceptance step.

Step 3: Final Answer:
The compound that accepts a proton from water is $\text{NH}_{3(aq)}$, which corresponds to option (C).