Which is More Acidic: Ammonia or Water?

Q: Which is more acidic, Ammonia or Water?

Water (pKa 15.7) is more acidic than Ammonia (pKa 38). A lower pKa indicates a stronger acid.

Which is more acidic, Ammonia or Water?

Water (pKa 15.7) is over 22 orders of magnitude more acidic than ammonia (pKa 38). Oxygen is more electronegative than nitrogen (3.5 vs 3.0), so hydroxide (OH-) is a more stable conjugate base than amide (NH2-). Water has a pKa of 15.7, while Ammonia has a pKa of 38.

Molecule A	Ammonia (NH₃), pKa 38
Molecule B	Water (H₂O), pKa 15.7
More Acidic	Water
Governing Factor	Electronegativity
Difficulty	Beginner

Introduction

Ammonia and water are neighboring hydrides in the second period. Both can lose a proton, but one does so far more readily. Which do you think is the stronger acid?

Acidic Protons

Ammonia has three N-H bonds and water has two O-H bonds. The highlighted central atom in each molecule is where the negative charge will reside in the conjugate base.

Governing Factor: Electronegativity

The key factor is electronegativity. Oxygen (EN = 3.5) is more electronegative than nitrogen (EN = 3.0). A more electronegative atom better stabilizes the extra electron pair left behind when H+ departs.

Conjugate Base Stability

Amide (NH2-) has its negative charge on nitrogen (EN 3.0). Hydroxide (OH-) has its charge on oxygen (EN 3.5). Oxygen holds negative charge more comfortably, making hydroxide the more stable - and therefore weaker - base.

pKa Comparison

Ammonia has a pKa of ~38 and water has a pKa of 15.7. That 22-unit gap means water is roughly 10^22 times more acidic than ammonia. Electronegativity differences produce enormous effects on acidity.

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

Compare Ammonia and Water in 3D