Which of the following are weak acids?
A. HI, HNO3, HBr
B. HF
C. HI, HF
D. HF, HBr
E. None of the above
The correct Answer and Explanation is:
The correct answer is B. HF.
Explanation:
To understand why HF (hydrofluoric acid) is a weak acid and the others listed are not, let’s first explore the concept of weak and strong acids.
Strong vs. Weak Acids:
- Strong acids completely ionize in water, meaning they dissociate fully into their constituent ions. This full dissociation leads to a high concentration of hydrogen ions (H⁺), resulting in a very low pH.
- Weak acids, on the other hand, only partially ionize in water. A significant amount of the acid remains in its molecular form, and only a small fraction dissociates to release H⁺ ions.
Strong Acids:
There are a few commonly recognized strong acids, including:
- HI (hydroiodic acid)
- HNO₃ (nitric acid)
- HBr (hydrobromic acid)
- HCl (hydrochloric acid)
- HClO₄ (perchloric acid)
- H₂SO₄ (sulfuric acid)
These acids dissociate almost entirely in water. For example:
- HI: HI → H⁺ + I⁻
- HBr: HBr → H⁺ + Br⁻
- HNO₃: HNO₃ → H⁺ + NO₃⁻
As you can see, HI, HNO₃, and HBr are strong acids, so any answer choice containing them (A, C, D) can be ruled out for “weak acids.”
Weak Acid – HF (Hydrofluoric Acid):
Hydrofluoric acid (HF) is classified as a weak acid. This is because when dissolved in water, it does not fully dissociate into ions:
- HF ⇌ H⁺ + F⁻
The equilibrium arrows indicate that the reaction is reversible, meaning not all of the HF molecules ionize to form hydrogen ions and fluoride ions. This limited dissociation is what characterizes HF as a weak acid. Despite being weak in terms of ionization, HF is still very corrosive and dangerous due to the high reactivity of the fluoride ion (F⁻).
Why is HF Weak?
The bond between hydrogen and fluorine in HF is stronger than the bonds between hydrogen and other halides (like HI or HBr). This stronger bond makes it harder for HF to fully ionize, which explains its weak acid behavior. The electronegativity of fluorine also causes the hydrogen-fluorine bond to be more polarized, but the overall effect still leads to partial dissociation in water.
Conclusion:
- Answer B (HF) is the correct choice because it is the only weak acid in the list.
- HI, HNO₃, and HBr are strong acids, which fully dissociate in water.
- Weak acids like HF only partially dissociate, making them significantly different from strong acids in terms of chemical behavior.
In this context, “none of the above” (choice E) would be incorrect because option B provides the correct identification of a weak acid.