Subjects and lessons to explore.

Law

Chemistry: Atoms First 2e · Acid-Base Equilibria

Brønsted-Lowry Acids and Bases

8 min read
7 key termsSourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 8 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Check yourself
  7. Study tools
  8. Sources & references

In 30 seconds

The Brønsted-Lowry theory identifies acids and bases by the single event at the heart of every acid-base reaction: the transfer of a proton, H+. An acid is any species that donates a proton; a base is any species that accepts one. This improves on the older Arrhenius definition, which required acids to release H+ and bases to release OH− in water: under Brønsted-Lowry rules, ammonia (NH3), which contains no hydroxide at all, is a perfectly good base because it accepts a proton from water:

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Every proton transfer creates a conjugate pair: the acid that gives up the proton becomes its conjugate base, and the base that takes it becomes its conjugate acid. The two members of a pair differ by exactly one proton. Because a bare proton cannot survive on its own in water, it is always carried by a water molecule as the hydronium ion, H3O+ (so H+(aq) is simply shorthand for H3O+(aq)).

Why this matters

Proton transfer is one of the most common chemical events in biology, medicine, and the environment:

  • Human physiology: The carbon dioxide we exhale dissolves in blood as carbonic acid, H2CO3, which donates protons to form bicarbonate, HCO3−. This acid-base pair is the body's main pH regulator, with the lungs and kidneys managing the balance.
  • Drug behavior: Many medicines are weak acids or bases. Whether a drug molecule carries a proton at body pH decides whether it can cross cell membranes — which is why aspirin (a weak acid) is absorbed differently in the acidic stomach than in the small intestine.
  • Environmental chemistry: Acid rain, limestone weathering, and ocean acidification are all proton-transfer problems; identify the donor and acceptor and you can reason about these systems without memorizing reactions.

The college version

Core Concepts

Acids donate protons; bases accept them

An acid is a proton donor and a base a proton acceptor — nothing more. The proton is a hydrogen nucleus, a tiny, highly charged particle that attaches to any electron-rich site; in water that site is a lone pair on an oxygen atom, producing hydronium. Consider nitrous acid dissolving in water:

HNO2(aq) + H2O(l) ⇌ H3O+(aq) + NO2-(aq)

Nitrous acid hands a proton to water, so HNO2 is the acid and H2O the base — water itself, not hydroxide. Bases need only accept H+: ammonia, carbonate (CO32−), and phosphate are all bases whose formulas contain no hydroxide.

Conjugate pairs differ by one proton

A consists of two species that differ by exactly one H+. Remove a proton from an acid and you get its conjugate base; add a proton to a base and you get its conjugate acid.

  • HNO2 / NO2− — nitrite is the conjugate base of nitrous acid.
  • NH4+ / NH3 — ammonium is the conjugate acid of ammonia.
  • H2O / OH− — hydroxide is the conjugate base of water.

Every acid-base reaction contains exactly two conjugate pairs: the acid on the left becomes its conjugate base on the right, and the base becomes its conjugate acid. To find the pairs, scan the species present and match those that differ by one proton.

Amphiprotic species

Some species can play either role. substances can donate a proton (acting as an acid) or accept one (acting as a base), depending on their partner. Water, bicarbonate (HCO3−), dihydrogen phosphate (H2PO4−), and hydrogen sulfate (HSO4−) are the most important examples:

  • With a strong base, bicarbonate acts as an acid: HCO3− + OH− ⇌ CO32− + H2O.
  • With a strong acid, bicarbonate acts as a base: HCO3− + H3O+ ⇌ H2CO3 + H2O.

This dual personality is exactly what makes bicarbonate the buffer that holds blood pH near 7.4.

Strong versus weak: complete versus partial transfer

Strong acids and bases transfer protons essentially completely in water; weak ones transfer only part of their protons, leaving an equilibrium. The common strong acids are HCl, HBr, HI, HNO3, HClO4, and the first proton of H2SO4. The common strong bases are the group 1 metal hydroxides (NaOH, KOH) and the heavier group 2 hydroxides such as Ba(OH)2. Almost everything else — acetic acid, carbonic acid, ammonia — is weak and described by an equilibrium constant Ka or Kb, the subject of the relative-strengths topic.

Water molecules constantly exchange protons with one another:

2H2O(l) ⇌ H3O+(aq) + OH-(aq)

At 25 °C the product of the two ion concentrations is the ion-product constant of water:

Kw = [H3O+][OH−] = 1.0 × 10-14

This relationship couples the two ion concentrations in every aqueous solution — know one, know the other — and it is the foundation of the pH and pOH scale developed next.

How It Works / Step-by-Step Process

Worked example 1: labeling the players

Problem. In HNO2(aq) + H2O(l) ⇌ H3O+(aq) + NO2-(aq), name the acid, base, conjugate acid, and conjugate base.

Solution.

  1. Find the two species that differ by one proton: HNO2/NO2− form one pair; H2O/H3O+ form the other.
  2. Follow the proton: HNO2 loses one, so it is the acid, leaving NO2− as its conjugate base. H2O gains one, so it is the base, becoming H3O+, its conjugate acid.
  3. Answer: acid HNO2, base H2O, conjugate acid H3O+, conjugate base NO2−.

Worked example 2: predicting products from the pairs

Problem. Hydrogen cyanide (HCN) reacts with hydroxide ion in water. Predict the products and identify the two conjugate pairs.

Solution.

  1. Assign roles: HCN has a proton to give, so it is the acid; OH− can accept one, so it is the base.
  2. Move the proton: HCN becomes CN−; OH− becomes H2O.
  3. Balanced reaction:

HCN(aq) + OH-(aq) ⇌ CN-(aq) + H2O(l)

  1. Pairs: HCN/CN− and H2O/OH−. Each product is the conjugate partner of a reactant — a built-in check that no atoms were created or destroyed.

Worked example 3: using Kw with dimensional analysis

Problem. At 25 °C, [H3O+] = 5.0 × 10-9 M. Find [OH−] and state whether the solution is acidic or basic.

Solution.

  1. Write the relationship first: Kw = [H3O+][OH−] = 1.0 × 10-14.
  2. Rearrange to isolate the unknown:

[OH−] = Kw[H3O+] = 1.0 × 10-145.0 × 10-9 = 2.0 × 10-6 M

Dimensional analysis: Kw carries units of M², so (M²)/(M) = M — the answer comes out in molarity.

  1. Since [OH−] = 2.0 × 10-6 M exceeds [H3O+] = 5.0 × 10-9 M, the solution is basic.

Common Confusions

Do Not ConfuseWithDifference
Base that contains OH−Base that accepts a protonNH3 and CO32− are bases with no hydroxide; basicity means accepting H+, not containing OH−.
Conjugate pair membersAny two species in the equationPair members differ by exactly one proton; HNO2 and H2O in worked example 1 are NOT a pair.
H+ floating alone in waterH3O+A bare proton cannot survive in water; H+(aq) is shorthand for hydronium.
Strong acidConcentrated acidStrength is about completeness of ionization, not amount present; a dilute strong acid is still strong.
AmphiproticAmphotericAmphiprotic specifically means able to donate or accept a proton; it describes a species' proton behavior.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine molecules playing tag with one hydrogen "tag." The molecule that passes the tag is the acid; the one that catches it is the base. Once the tag moves, the passer is missing it (its conjugate base) and the catcher is holding it (its conjugate acid). Some molecules, like water, play either role depending on who they meet.

Key takeaways

  • Brønsted-Lowry: acid = proton donor; base = proton acceptor. No OH− is required for basicity.
  • In water, H+(aq) is shorthand for hydronium ion, H3O+(aq).
  • A conjugate pair differs by exactly one proton; every acid-base reaction contains two pairs.
  • Amphiprotic species (H2O, HCO3−, H2PO4−, HSO4−) can donate or accept a proton.
  • Strong acids and bases ionize completely; weak ones establish equilibria quantified by Ka / Kb.
  • Water autoionizes: Kw = [H3O+][OH−] = 1.0 × 10-14 at 25 °C.
  • The stronger an acid, the weaker its conjugate base.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

  1. State the Brønsted-Lowry definitions of acid and base.

    Show answer

    An acid donates a proton; a base accepts a proton.

  2. In HCN(aq) + H2O(l) ⇌ H3O+(aq) + CN-(aq), label all four species by role.

    Show answer

    Acid HCN, base H2O, conjugate acid H3O+, conjugate base CN−.

  3. Why does a "proton" in water actually exist as H3O+?

    Show answer

    A bare proton is a tiny, highly charged nucleus that instantly attaches to a water molecule's lone pair, forming hydronium.

  4. Name two amphiprotic species and write a reaction showing one of them acting as an acid.

    Show answer

    HCO3− and H2PO4− are examples. Acting as an acid: HCO3− + OH− ⇌ CO32− + H2O.

  5. At 25 °C, [OH−] = 1.0 × 10-12 M. What is [H3O+]? Is the solution acidic or basic?

    Show answer

    [H3O+] = Kw/[OH−] = 1.0 × 10-14/1.0 × 10-12 = 1.0 × 10-2 M. Since [H3O+] > [OH−], the solution is acidic.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Brønsted-Lowry acid
Any species that donates a proton (H+)
Brønsted-Lowry base
Any species that accepts a proton
conjugate acid-base pair
Two species differing by exactly one proton
hydronium ion (ceH3O+)
The form a proton takes when it attaches to water
amphiprotic
Able to act as either an acid or a base
ion-product constant of water (Kw)
[H3O+][OH−] = 1.0 × 10-14 at 25 °C
conjugate acid–base pair
Two species differing by one proton, e.g., HC2H3O2 and C2H3O2-

Sources & references

  1. openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.