No, HI (hydroiodic acid) is not a weak acid; it is a strong acid.
Understanding Strong and Weak Acids
Acids donate protons (H⁺) when dissolved in water. Strong acids completely dissociate into ions in water, meaning almost all of the acid molecules break apart into H⁺ and an anion. Weak acids, on the other hand, only partially dissociate; a significant portion of the acid remains in its molecular form.
Why HI is a Strong Acid
Several sources confirm HI's classification as a strong acid:
- TutorOcean: States explicitly that HI (hydroiodic acid) is a strong acid with a pH of 3.01. https://corp.tutorocean.com/chemistry/is-hi-hydroiodic-acid-a-strong-acid-or-weak-acid/
- UCLA Chemistry Community: While discussing the weakness of HF, this source lists HI among the six main strong acids: H₂SO₄, HI, HBr, HNO₃, HCl, and HClO₄. https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=96938
- Chegg: Refers to HI as a strong acid in a problem comparing it to the weak acid HF. https://www.chegg.com/homework-help/questions-and-answers/010-m-aqueous-solutions-strong-acid-hi-weak-acid-hf-contained-separate-beakers-solutions-r-q80815829
- Stolaf.edu: Includes HI in a list of common strong acids. https://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/naming/elec.htm
The key factor contributing to HI's strength as an acid is the stability of the iodide ion (I⁻) in aqueous solution. The large size of the iodide ion effectively disperses the negative charge, making it very stable and facilitating the complete dissociation of HI. This contrasts with HF, where the small size of the fluoride ion (F⁻) leads to a strong attraction to the proton, hindering complete dissociation.
