Yes, HI (hydroiodic acid) is a strong acid.
Understanding Strong Acids
A strong acid is one that completely dissociates (breaks apart) into its ions (H⁺ and an anion) when dissolved in water. This means that virtually all of the HI molecules will break down into H⁺ and I⁻ ions. This high degree of dissociation results in a high concentration of H⁺ ions, which is what makes it a strong acid.
Several sources confirm this:
- TutorOcean: Explicitly states that HI (hydroiodic acid) is a strong acid with a pH of 3.01. Is HI (Hydroiodic Acid) a Strong Acid or Weak Acid? - TutorOcean
- Wikipedia: Describes hydroiodic acid as a "strong acid". Hydroiodic acid - Wikipedia
- ThoughtCo: Includes HI in its list of strong acids. What Are Strong Acids in Chemistry?
- Quora: Multiple answers on Quora confirm HI's classification as a strong acid, highlighting its complete dissociation in water. Why HI is stronger acid than HF? - Quora and Which one is the strongest acid, HI, HCL, or HBr? - Quora
- Master Organic Chemistry: Uses HI as an example of a strong acid in its discussion of ether cleavage. Acidic cleavage of ethers (SN2) – Master Organic Chemistry
The strength of HI is related to the stability of its conjugate base, the iodide ion (I⁻). The large size of the iodide ion allows for better distribution of the negative charge, making it a more stable anion. A more stable conjugate base leads to a stronger acid.
Comparison with Other Acids
While HI is a strong acid, its strength can be compared to other hydrohalic acids (HF, HCl, HBr). HI is the strongest among them due to the factors mentioned above. HF, on the other hand, is a weak acid due to the strong H-F bond and the relatively small size of the fluoride ion.
