Is Hydroiodic Acid (HI) a Strong or Weak Acid?

Is Hydroiodic Acid (HI) a strong or weak acid?

HI is the strongest of the hydrohalic acids. Iodine is the largest halogen, forming the weakest H-X bond. The iodide ion is very stable because the negative charge is spread over a large atom.

FormulaHI
NameHydroiodic Acid
CategoryStrong acid
pKa-10
ConjugateIodide ion (I⁻)
Key ConceptStrongest hydrohalic acid

Definition

HI is a strong Bronsted-Lowry acid. It completely donates its proton to water. It is the strongest of the common hydrohalic acids.

Acidic Proton / Active Site

The hydrogen bonded to iodine is the acidic proton. The H-I bond (297 kJ/mol) is the weakest of the H-X bonds.

Conjugate Pair

HI donates H+ to form I- (iodide). Iodide is an exceptionally weak conjugate base due to its large size and high polarizability.

Strength Classification

HI has pKa = -10, making it the strongest hydrohalic acid. The trend HF < HCl < HBr < HI shows that bond strength (not electronegativity) controls acidity down a group.

See acidic protons, conjugate base overlays, and pKa labels on interactive 3D molecules.

Explore Hydroiodic Acid's Acid-Base Properties in 3D

Related Topics

Is Hydrochloric Acid Strong or Weak?

Acid-base classification of HCl

Is Hydrobromic Acid Strong or Weak?

Acid-base classification of HBr

Is Acetic Acid Strong or Weak?

Compare with weak acid

Is Ammonia Strong or Weak?

Compare with weak base

Acidity Ranking

Compare acid strength between molecule pairs

Ionic vs Covalent Bonding

Understand how electronegativity affects bond character