Which is More Acidic: Hydrogen Chloride or Hydrogen Bromide?

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

Hydrogen Bromide (pKa -9) is more acidic than Hydrogen Chloride (pKa -7). A lower pKa indicates a stronger acid.

Which is more acidic, Hydrogen Chloride or Hydrogen Bromide?

HBr is stronger than HCl because Br- is larger than Cl-, spreading its negative charge over a greater volume. The H-Br bond is also weaker, making it easier to break. Hydrogen Bromide has a pKa of -9, while Hydrogen Chloride has a pKa of -7.

Molecule A	Hydrogen Chloride (HCl), pKa -7
Molecule B	Hydrogen Bromide (HBr), pKa -9
More Acidic	Hydrogen Bromide
Governing Factor	Atom Size
Difficulty	Beginner

Introduction

Two hydrogen halides from period 3 and period 4. Both are strong acids, but which is stronger?

Acidic Protons

Each has a single acidic hydrogen bonded to a halogen. Look at the relative size of the halogens.

Governing Factor: Atom Size

Atom size is the key. Bromine (period 4) is larger than chlorine (period 3). The larger Br- anion spreads its negative charge over more volume.

Conjugate Base Stability

Cl- is already a stable anion, but Br- is even more stable due to its larger size. Charge density is lower in Br-, making it a weaker base.

pKa Comparison

HCl has pKa -7 and HBr has pKa -9. Both are strong acids that fully dissociate in water, but HBr is intrinsically stronger.

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

Compare Hydrogen Chloride and Hydrogen Bromide in 3D