Which of the following is a Bronsted-Lowry base but NOT an Arrhenius base? NaOH NH3 KOH HC
The Correct Answer and Explanation is:
The correct answer is NH₃ (ammonia).
Explanation:
To understand why ammonia (NH₃) is a Brønsted-Lowry base but not an Arrhenius base, we need to consider the definitions of both theories:
- Arrhenius Definition:
- An Arrhenius base is a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH⁻).
- For example, NaOH and KOH are both Arrhenius bases because they dissociate in water to form OH⁻ ions.
- Brønsted-Lowry Definition:
- A Brønsted-Lowry base is a substance that accepts protons (H⁺).
- Ammonia (NH₃) fits this definition because it can accept a proton to form ammonium ion (NH₄⁺). This is shown by the reaction: NH3+H+⟶NH4+\mathrm{NH}_3 + \mathrm{H}^+ \longrightarrow \mathrm{NH}_4^+NH3+H+⟶NH4+
- Even though ammonia doesn’t produce OH⁻ ions directly in water, it still behaves as a base by accepting a proton (H⁺), which makes it a Brønsted-Lowry base.
Why is NH₃ not an Arrhenius base?
- NH₃ does not directly increase the concentration of OH⁻ ions in solution. Instead, it reacts with water molecules to produce OH⁻ ions through a different mechanism, as shown in the following equation: NH3+H2O⇌NH4++OH−\mathrm{NH}_3 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{NH}_4^+ + \mathrm{OH}^-NH3+H2O⇌NH4++OH−
- Therefore, while ammonia leads to the formation of OH⁻ ions in solution, it doesn’t do so by directly releasing them like NaOH or KOH. Thus, it doesn’t fit the stricter Arrhenius definition.
Conclusion:
Ammonia is a Brønsted-Lowry base because it accepts protons, but it is not an Arrhenius base because it does not directly increase the concentration of OH⁻ ions in water.
