How do I identify the conjugate acid–base pairs in a reaction?

Look for two species, one on each side, that differ by exactly one H⁺: the one with an extra H and a charge one higher is the conjugate acid. In HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻, H₂CO₃/HCO₃⁻ is one pair (HCO₃⁻ accepts a proton, so it is the base) and H₂O/OH⁻ is the other (water donates the proton). Every Brønsted reaction has the form acid₁ + base₂ ⇌ base₁ + acid₂.

How do pKa values predict the direction of an acid–base reaction?

Protons move from the stronger acid to the stronger base, so the equilibrium favours the side with the weaker acid. K = Ka(reactant acid)/Ka(product acid) = 10^(pKa(product acid) − pKa(reactant acid)). Acetic acid (pKa 4.75) reacting with bicarbonate to form carbonic acid (pKa 6.35) has K ≈ 40, favouring products, while NH₃ + H₂O has K = 10^(9.25 − 14) = 1.8×10⁻⁵, which is simply Kb of ammonia.

Which species are amphiprotic, and are their solutions acidic or basic?

Amphiprotic species can both donate and accept a proton: water, HCO₃⁻, HSO₃⁻, H₂PO₄⁻, HPO₄²⁻, HS⁻ and HC₂O₄⁻ are common examples. Compare their acid constant Ka with their base constant Kb = Kw/Ka(conjugate acid). HCO₃⁻, HPO₄²⁻ and HS⁻ have the larger Kb and give basic solutions; HSO₃⁻, H₂PO₄⁻ and HC₂O₄⁻ have the larger Ka and give acidic ones.

What is the levelling effect?

Any acid stronger than H₃O⁺ transfers its proton completely to water, so in water HCl, HBr, HI, HNO₃ and HClO₄ all appear equally strong — they are levelled to H₃O⁺. A less basic solvent such as glacial acetic acid separates them: HClO₄ > HI > HBr > HCl > H₂SO₄ > HNO₃. Likewise bases stronger than OH⁻ (NH₂⁻, H⁻, O²⁻) are levelled to OH⁻ in water.

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🤝Conjugate Acid-Base Pair Finder - Brønsted Acids & Bases, Equilibrium Direction, Lewis & HSAB

In the Brønsted–Lowry theory an acid donates a proton and a base accepts one; an acid and its conjugate base differ by exactly one H⁺ (one hydrogen atom and one unit of charge). This tool compares the element counts and charges of every species in an equation to find the proton transfers automatically. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, water is the acid and OH⁻ its conjugate base, while NH₃ is the base and NH₄⁺ its conjugate acid; the two pairs are colour-coded and spectator ions such as Na⁺ or Cl⁻ are recognised. If you enter only reactants, the tool looks up each species as an acid and as a base in a 25 °C pKa table, picks the most favourable proton transfer, writes the products and judges the equilibrium with K = 10^(pKa of product acid − pKa of reactant acid): for CH₃COOH + HCO₃⁻, K = 10^(6.35 − 4.75) ≈ 40, so acetate and carbonic acid are favoured. The conjugate mode gives the conjugate acid and base of any species with Ka, Kb, pKa and pKb (pKa + pKb = 14) and decides whether amphiprotic species such as HCO₃⁻, H₂PO₄⁻, HPO₄²⁻, HS⁻, HSO₃⁻ and water behave mainly as acids or bases — HCO₃⁻ has Kb = 2.2×10⁻⁸ > Ka = 4.7×10⁻¹¹, which is why sodium bicarbonate solution is basic. Further tabs rank acids by pKa with a levelling-effect note, classify Lewis acids (BF₃, AlCl₃, Fe³⁺, CO₂) and Lewis bases (NH₃, OH⁻, CN⁻, CO), show Pearson's hard–soft acid–base table and compare the Arrhenius, Brønsted–Lowry and Lewis definitions.

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How to use Conjugate acid–base pairs

1In Conjugate pairs, type a full acid–base reaction (e.g. NH3 + H2O ⇌ NH4+ + OH-) or just the reactants (e.g. CH3COOH + HCO3-), or tap an example
2Read the role of each species, the colour-coded conjugate pairs, their pKa values, K and which side is favoured
3Switch tabs to get conjugates of a single species, rank a list of acids, look up Lewis acid/base and HSAB class, or compare the three theories

FAQ

How do I identify the conjugate acid–base pairs in a reaction?: Look for two species, one on each side, that differ by exactly one H⁺: the one with an extra H and a charge one higher is the conjugate acid. In HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻, H₂CO₃/HCO₃⁻ is one pair (HCO₃⁻ accepts a proton, so it is the base) and H₂O/OH⁻ is the other (water donates the proton). Every Brønsted reaction has the form acid₁ + base₂ ⇌ base₁ + acid₂.
How do pKa values predict the direction of an acid–base reaction?: Protons move from the stronger acid to the stronger base, so the equilibrium favours the side with the weaker acid. K = Ka(reactant acid)/Ka(product acid) = 10^(pKa(product acid) − pKa(reactant acid)). Acetic acid (pKa 4.75) reacting with bicarbonate to form carbonic acid (pKa 6.35) has K ≈ 40, favouring products, while NH₃ + H₂O has K = 10^(9.25 − 14) = 1.8×10⁻⁵, which is simply Kb of ammonia.
Which species are amphiprotic, and are their solutions acidic or basic?: Amphiprotic species can both donate and accept a proton: water, HCO₃⁻, HSO₃⁻, H₂PO₄⁻, HPO₄²⁻, HS⁻ and HC₂O₄⁻ are common examples. Compare their acid constant Ka with their base constant Kb = Kw/Ka(conjugate acid). HCO₃⁻, HPO₄²⁻ and HS⁻ have the larger Kb and give basic solutions; HSO₃⁻, H₂PO₄⁻ and HC₂O₄⁻ have the larger Ka and give acidic ones.
What is the levelling effect?: Any acid stronger than H₃O⁺ transfers its proton completely to water, so in water HCl, HBr, HI, HNO₃ and HClO₄ all appear equally strong — they are levelled to H₃O⁺. A less basic solvent such as glacial acetic acid separates them: HClO₄ > HI > HBr > HCl > H₂SO₄ > HNO₃. Likewise bases stronger than OH⁻ (NH₂⁻, H⁻, O²⁻) are levelled to OH⁻ in water.

Features

Automatic Brønsted acid, base, conjugate acid and conjugate base labelling with colour-coded pairs
Spectator ions detected; school shorthand such as HAc, Ac⁻ and NH₃·H₂O accepted
Product prediction from reactants only, with K from pKa values and equilibrium direction
Conjugate acid and base of any species with Ka, Kb, pKa and pKb
Amphiprotic check comparing Ka with Kb to tell whether the solution is acidic or basic
Acid strength ranking table with Ka and conjugate-base pKb, plus levelling effect
Lewis acid/base classifier and Pearson HSAB hard/borderline/soft table
Side-by-side comparison of Arrhenius, Brønsted–Lowry and Lewis theories

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