Which is More Acidic: Hydrogen Bromide or Hydrogen Iodide?

Q: Which is more acidic, Hydrogen Bromide or Hydrogen Iodide?

Hydrogen Iodide (pKa -10) is more acidic than Hydrogen Bromide (pKa -9). A lower pKa indicates a stronger acid.

Which is more acidic, Hydrogen Bromide or Hydrogen Iodide?

HI is the strongest hydrohalic acid because I- is the largest halide ion. The large size of iodide provides maximum charge stabilization. Hydrogen Iodide has a pKa of -10, while Hydrogen Bromide has a pKa of -9.

Molecule A	Hydrogen Bromide (HBr), pKa -9
Molecule B	Hydrogen Iodide (HI), pKa -10
More Acidic	Hydrogen Iodide
Governing Factor	Atom Size
Difficulty	Beginner

Introduction

The final pair in the hydrogen halide series. HBr and HI are both strong acids - which one is stronger?

Acidic Protons

Each has one acidic proton. Notice how much larger iodine is compared to bromine.

Governing Factor: Atom Size

Atom size continues to be the dominant factor. Iodine (period 5) is even larger than bromine (period 4).

Conjugate Base Stability

I- is the largest common halide ion. Its negative charge is spread over the greatest volume, giving it the lowest charge density and making it the weakest base of the halides.

pKa Comparison

HBr has pKa -9 and HI has pKa -10. HI is the strongest hydrohalic acid.

Interactive side-by-side 3D viewer with acidic proton highlights, conjugate base overlays, and pKa labels.

Compare Hydrogen Bromide and Hydrogen Iodide in 3D