Hydroiodic Acid
Hydroiodic acid is a solution of HI in water. Commercial hydroiodic acid usually contains 57% HI by mass. The solution forms an azeotrope boiling at 127 °C with 57% HI and 43% water. Hydroiodic acid is one of the strongest of all common acids due to the high stability of its corresponding conjugate base. The iodide ion is the largest of all common halides which results in the negative charge being dispersed over a larger space. By contrast, a chloride ion is significantly smaller, meaning its negative charge is more concentrated, leading to a stronger interaction between the proton and the chloride ion. This weaker H+---I– interaction in HI facilitates dissociation of the proton from the anion, and is the reason HI is the strongest acid of the hydrohalides (except for hydroastatic acid ).
- HI + H2O → H3O+ + I– Ka ≈ 1010
- HBr + H2O → H3O+ + Br– Ka ≈ 109
- HCl + H2O → H3O+ + Cl– Ka ≈ 108
Read more about this topic: Hydrogen Iodide, Properties