Acid/Base Fundamentals

Learn Bronsted-Lowry definitions, conjugate pairs, strong vs weak, and pKa.

Hydrochloric Acid

HCl

strong acid
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Concepts

What Is an Acid/Base?

There are three ways to define acids and bases. Each builds on the last.

Definition
Acid
Base
Arrhenius
Produces H+ in water
Produces OH- in water
Bronsted-Lowry
H+ donor
H+ acceptor
Lewis
e- pair acceptor
e- pair donor

Bronsted-Lowry is the most commonly used in gen chem and orgo. This module focuses on that definition.

Strong vs Weak

Strong = dissociates completely (HCl, HBr, HI, H2SO4, HNO3, HClO3, HClO4, NaOH, KOH). Weak = dissociates partially (acetic acid, HF, NH3).

Conjugate Pairs

Every acid has a conjugate base (what remains after donating H+). Every base has a conjugate acid (what forms after accepting H+). The stronger the acid, the weaker its conjugate base.

pKa Scale

pKa measures acid strength. Lower pKa = stronger acid. Strong acids have negative pKa values. Water has pKa 15.7. Alkanes (~50) are essentially non-acidic.

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About This Molecule

Hydrochloric Acid (HCl)

Strong AcidpKa = -7

Strong acid baseline

pKa Reference

HI-10
HBr-9
HCl-7
H2SO4-3
Hydronium (H3O+)-1.7
HNO3-1.4
Phosphoric acid2.1
HF3.2

Lower pKa = stronger acid. Italic = functional group range.