Understanding Ka, Kb, pKa & pH — and what really makes an acid strong
A bright, no-clutter walkthrough of equilibrium constants, the pH scale, and the four real reasons one acid or base beats another — built for quick, accurate revision. Includes calculators, video lectures, and full reference tables.
The strength of an acid or base is determined by the extent to which it ionises in aqueous solution. A strong acid donates protons completely (or nearly completely) to water, producing high concentrations of hydronium ions (\(H_3O^+\)). A weak acid ionises only partially, establishing an equilibrium between the molecular form and ions. The same principle applies to bases: strong bases dissociate fully, while weak bases reach an equilibrium.
Here, HA is the acid, \(A^-\) its conjugate base, and \(H_3O^+\) the conjugate acid of water. The equilibrium constant for this reaction is the acid‑ionisation constant (\(K_a\)):
Similarly, for a base \(B\) reacting with water:
A larger \(K_a\) (or \(K_b\)) indicates a stronger acid (or base). Often, we use the p\(K_a\) = –log \(K_a\) scale: the smaller the p\(K_a\), the stronger the acid.
The pH scale: counting H⁺ the easy way
pH is just a compact way to write hydrogen-ion concentration. Every 1-unit drop in pH means a 10× jump in [H⁺] — that’s why the scale feels so dramatic.
Lower pH = more H⁺
As [H⁺] rises, pH falls — the relationship is inverse and logarithmic, not straight-line.
Common reference points
- Gastric acid — pH ≈ 1.5–2
- Pure water — pH = 7 (neutral)
- Seawater — pH ≈ 8.1
- Household ammonia — pH ≈ 11
Ka and Kb: measuring how far a reaction goes
Weak acids and bases don’t fully ionize — they settle into equilibrium. Ka and Kb simply tell you where that equilibrium sits.
Bigger Ka → equilibrium sits further right → more ionization → stronger acid.
Bigger Kb → more OH⁻ produced → stronger base.
pKa: the strength number that flips direction
pKa = −log(Ka). Because of the minus sign, the relationship inverts: a smaller pKa means a stronger acid.
How pH, pOH and Kw stay locked together
Water’s self-ionization fixes a constant relationship — push one value up, the other must come down.
What actually decides acid/base strength?
Ka and Kb are the measurement. These four structural factors are the cause — the reasons one molecule ionizes more than another.
Electronegativity of the atom holding H
A more electronegative atom pulls electron density away from H, polarizing the bond and releasing H⁺ more easily.
Atomic size & bond strength
Down a group, atoms get bigger, the H–X bond gets longer and weaker, and it breaks more easily — this outweighs electronegativity.
Resonance stability of the conjugate base
If the negative charge left behind can spread over several atoms, the conjugate base is more stable — so the acid ionizes more readily.
Inductive (electron-withdrawing) effect
Nearby electronegative groups pull electron density through the bonds, stabilizing the conjugate base and boosting acidity.
🔁 Predicting the Direction of Acid‑Base Reactions
A fundamental rule governs any acid‑base equilibrium: the reaction always favours the side with the weaker acid and weaker base. The proton (\(H^+\)) will always bind to the stronger base. Consequently, the equilibrium lies toward the formation of the weaker acid‑base pair.
- Hydrochloric acid (HCl, strong) reacts completely with water: HCl + H₂O → H₃O⁺ + Cl⁻ (equilibrium far right).
- Acetic acid (CH₃COOH, weak) establishes an equilibrium: CH₃COOH + H₂O ⇌ H₃O⁺ + CH₃COO⁻ (left‑favoured).
- Ammonia (NH₃, weak base) with water: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (equilibrium left, because OH⁻ is a strong base and NH₄⁺ is a stronger acid than water).
📊 Common Strong and Weak Acids (with Ka values)
| Acid | Formula | \(K_a\) (at 25°C) | p\(K_a\) | Strength |
|---|---|---|---|---|
| Hydrochloric | HCl | ~10⁶ | -6 | Strong |
| Sulfuric (first H) | H₂SO₄ | very large | -3 | Strong |
| Acetic | CH₃COOH | 1.8 × 10⁻⁵ | 4.74 | Weak |
| Carbonic (first H) | H₂CO₃ | 4.3 × 10⁻⁷ | 6.37 | Weak |
| Ammonium ion | NH₄⁺ | 5.6 × 10⁻¹⁰ | 9.25 | Very weak |
🧮 Interactive: pH & pOH Calculator
🔄 Ka – Kb Converter
This video explains the concepts of Ka, pKa, and factors affecting acid strength with examples.
Quick revision sheet
Download Complete Notes Below
Proudly Powered By
Leave a Comment