Which of the following is the strongest base? Cl- Br- NH2 – HO- HS – H2O
The Correct Answer and Explanation is:
Correct Answer: NH₂⁻
Explanation
To determine which of the given species is the strongest base, we need to examine their basicity—that is, their ability to accept a proton (H⁺). A strong base has a high tendency to accept protons, which is often associated with the conjugate base of a weak acid.
Here is the list of the species with their conjugate acids:
- Cl⁻ → HCl (strong acid)
- Br⁻ → HBr (strong acid)
- NH₂⁻ → NH₃ (weak acid)
- HO⁻ (OH⁻) → H₂O (very weak acid)
- HS⁻ → H₂S (weak acid)
- H₂O → H₃O⁺ (strong acid)
Cl⁻ and Br⁻ are conjugate bases of very strong acids (HCl and HBr), and thus they are very weak bases. They are essentially spectator ions in acid-base chemistry and do not act as bases in water.
H₂O, while a neutral molecule, can act as a very weak base, but it is far weaker than the others on this list.
OH⁻ (hydroxide) is a strong base because it is the conjugate base of water, a very weak acid. HS⁻ is a moderately strong base because H₂S is a weak acid.
However, NH₂⁻ (amide ion) is the strongest base on this list. It is the conjugate base of ammonia (NH₃), which is a weak acid. The nitrogen atom in NH₂⁻ is less electronegative than oxygen, meaning it holds onto the negative charge less tightly, making the lone pair on nitrogen more available to accept a proton. This makes NH₂⁻ highly reactive and basic in aqueous solutions.
In summary, the strength of a base is inversely related to the strength of its conjugate acid. Since ammonia (NH₃) is a much weaker acid than water or H₂S, its conjugate base (NH₂⁻) is stronger than OH⁻ or HS⁻.
Therefore, the strongest base is NH₂⁻.
