In the following reaction: NH4+ + H2O = NH3 + H3O+ A) H2O is a base and NH3 is its conjugate acid B) NH4+ is an acid and H20 is its conjugate base C) NH4+ is an acid and H3O+ is its conjugate base D) H2O is a base and H3O is its conjugate acid
The correct answer and explanation is:
Let’s analyze the reaction step-by-step:
Reaction: NH4++H2O⇌NH3+H3O+\text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{H}_3\text{O}^+
Step 1: Identify acid-base roles
- NH4+ (ammonium ion) can donate a proton (H+) to water, which means it is acting as an acid.
- H2O accepts that proton, acting as a base.
- When NH4+ loses a proton, it becomes NH3 (ammonia).
- When H2O gains a proton, it becomes H3O+ (hydronium ion).
Step 2: Definitions
- An acid is a proton donor.
- A base is a proton acceptor.
- A conjugate base is what remains after the acid has donated a proton.
- A conjugate acid is what is formed when the base accepts a proton.
Step 3: Apply to species in the reaction
- NH4+ donates a proton → NH3 is its conjugate base.
- H2O accepts a proton → H3O+ is its conjugate acid.
Step 4: Match options
A) H2O is a base and NH3 is its conjugate acid
- No, NH3 is not the conjugate acid of H2O. NH3 is formed from NH4+, not H2O.
- H2O’s conjugate acid is H3O+.
B) NH4+ is an acid and H2O is its conjugate base
- No, H2O does not come from NH4+. H2O accepts the proton; it is not a conjugate base of NH4+.
- The conjugate base of NH4+ is NH3.
C) NH4+ is an acid and H3O+ is its conjugate base
- No, H3O+ is the conjugate acid of H2O, not a conjugate base of NH4+.
D) H2O is a base and H3O+ is its conjugate acid
- Correct! H2O accepts a proton and becomes H3O+, its conjugate acid.
Correct answer: D) H2O is a base and H3O+ is its conjugate acid
Explanation (about 300 words):
In the given equilibrium reaction, ammonium ion (NH4+) reacts with water (H2O) to form ammonia (NH3) and hydronium ion (H3O+). According to the Bronsted-Lowry acid-base theory, acids are proton donors and bases are proton acceptors.
Here, NH4+ donates a proton (H+) to water. This proton transfer means NH4+ is acting as an acid. After donating the proton, NH4+ becomes NH3, which is the conjugate base because it can potentially accept a proton back.
Water (H2O), in this reaction, accepts the proton from NH4+, thus acting as a base. Upon gaining the proton, water converts to hydronium ion (H3O+), which is the conjugate acid of water. This means H3O+ has the potential to donate a proton back to revert to water.
The key to identifying conjugate acid-base pairs is understanding what species results after a proton is transferred. The acid loses a proton and becomes its conjugate base, while the base gains a proton and becomes its conjugate acid.
Option D correctly describes this process: H2O is acting as a base (accepting a proton), and H3O+ is its conjugate acid (the protonated form of H2O). This contrasts with option A, which incorrectly pairs NH3 as the conjugate acid of H2O, when in fact NH3 is the conjugate base of NH4+. Option B incorrectly labels H2O as the conjugate base of NH4+, which is not accurate because H2O was not derived from NH4+. Option C mistakes H3O+ as a conjugate base of NH4+, which it is not.
Therefore, understanding the proton transfer mechanism and applying Bronsted-Lowry definitions clarifies why option D is the correct answer.