Which is More Acidic: Hydrogen Fluoride or Hydrogen Chloride?

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

Hydrogen Chloride (pKa -7) is more acidic than Hydrogen Fluoride (pKa 3.2). A lower pKa indicates a stronger acid.

Which is more acidic, Hydrogen Fluoride or Hydrogen Chloride?

HCl is a much stronger acid than HF because Cl is larger than F. The larger Cl- anion stabilizes the negative charge over a bigger volume, making it a weaker base and HCl a stronger acid. Hydrogen Chloride has a pKa of -7, while Hydrogen Fluoride has a pKa of 3.2.

Molecule A	Hydrogen Fluoride (HF), pKa 3.2
Molecule B	Hydrogen Chloride (HCl), pKa -7
More Acidic	Hydrogen Chloride
Governing Factor	Atom Size
Difficulty	Beginner

Introduction

Both HF and HCl are hydrogen halides - a hydrogen bonded to a halogen. Which one gives up its proton more easily?

Acidic Protons

Each molecule has one acidic proton (H) bonded to a halogen. The highlighted protons are the ones that can be donated as H+.

Governing Factor: Atom Size

The key factor here is atom size. Chlorine (period 3) is larger than fluorine (period 2). When the conjugate base forms, the negative charge is spread over a larger atom in Cl- compared to F-.

Conjugate Base Stability

F- is a small, concentrated anion - it holds its charge tightly and wants the proton back (strong base, weak conjugate acid). Cl- is larger and stabilizes the charge better (weak base, strong conjugate acid).

pKa Comparison

HF has a pKa of 3.2 (weak acid), while HCl has a pKa of -7 (strong acid). The 10-unit difference reflects Cl- being far more stable than F-.

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

Compare Hydrogen Fluoride and Hydrogen Chloride in 3D