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General Chemistry I · Aqueous Reactions

Acid–Base Reactions

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Acids and bases are described by two complementary models. Arrhenius defines an acid as a substance that produces H⁺ in water and a base as one that produces OH⁻. Brønsted–Lowry generalizes this: an acid is a proton (H⁺) donor and a base is a proton acceptor, and every acid–base reaction transfers a proton from an acid to a base, generating a conjugate base and a conjugate acid. When an acid and base react in water, the net result of a neutralization is the combination of H⁺ and OH⁻ to form water, plus a salt.

Why this matters

Acid–base chemistry explains digestion (stomach HCl), antacids (bases neutralizing acid), blood pH buffering, and soil treatment. Neutralization is used to titrate unknown concentrations and to treat acid spills. The Brønsted–Lowry view generalizes acid–base behavior beyond water, laying the foundation for buffer and pH calculations in later units.

The college version

Key Ideas

Two definitions

  • Arrhenius acid: produces H⁺ in water (HCl → H⁺ + Cl⁻).
  • Arrhenius base: produces OH⁻ in water (NaOH → Na⁺ + OH⁻).
  • Brønsted–Lowry acid: proton donor. Brønsted–Lowry base: proton acceptor.

Conjugate acid–base pairs

  • When an acid donates H⁺, the species left behind is its conjugate base (HCl → Cl⁻).
  • When a base accepts H⁺, the species formed is its conjugate acid (NH₃ → NH₄⁺).
  • A conjugate pair differs by exactly one H⁺.

Neutralization

  • General form: acid + base → salt + water.
  • HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
  • Net ionic for strong acid + strong base: H⁺(aq) + OH⁻(aq) → H₂O(l).

Equations and Variables

  • Arrhenius acid: HCl(aq) → H⁺(aq) + Cl⁻(aq)
  • Arrhenius base: NaOH(aq) → Na⁺(aq) + OH⁻(aq)
  • Neutralization: acid + base → salt + H₂O
  • Net ionic (strong acid + strong base): H⁺(aq) + OH⁻(aq) → H₂O(l)

How It Works (Problem-Solving Method)

  1. Identify the acid and base in the reaction.
  2. Track the proton to find the conjugate base (acid minus H⁺) and conjugate acid (base plus H⁺).
  3. For neutralization, predict products: salt (cation of base + anion of acid) and water.
  4. Balance the equation.
  5. Write the net ionic equation by canceling spectator ions (strong acids/bases split into ions).

Worked Example

Write the neutralization of hydrochloric acid and sodium hydroxide, then give the net ionic equation.

  • Molecular: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
  • Complete ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l).
  • Spectators: Na⁺ and Cl⁻. Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l).

Conjugate-pair identification for NH₃ + H₂O ⇌ NH₄⁺ + OH⁻:

  • NH₃ (base) accepts H⁺ → NH₄⁺ (conjugate acid).
  • H₂O (acid) donates H⁺ → OH⁻ (conjugate base).
  • Pairs: NH₃/NH₄⁺ and H₂O/OH⁻.

Common Confusions

  • "Arrhenius and Brønsted–Lowry are competing, contradictory theories." — They are compatible; Brønsted–Lowry generalizes Arrhenius to bases that don't contain OH⁻ (like NH₃).
  • "An acid 'contains' H⁺ it releases." — More precisely, the acid donates a proton to water; H⁺ is not a free-floating particle in solution.
  • "A salt is always table salt (NaCl)." — "Salt" in chemistry means any ionic product of neutralization (KNO₃, CaSO₄, etc.).
  • "The conjugate base is a different compound added to the reaction." — It is simply the acid after losing H⁺ (HCl → Cl⁻), not a separate reagent.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Think of an acid as a kid eager to give away a hot potato (a proton, H⁺), and a base as a kid eager to catch it. When the acid tosses the proton to the base, the acid is left holding less (that's the conjugate base) and the base is now holding the proton (the conjugate acid). Neutralization is the special case where the proton lands on an OH⁻ to make plain water — hot potato handed off, everyone calm. The catch: the "hot potato" is really a hydrogen ion with no electrons, and it never floats alone in water — it rides on a water molecule as H₃O⁺, which the simplified equations usually leave out.

Key takeaways

  • Arrhenius: acid → H⁺, base → OH⁻ in water.
  • Brønsted–Lowry: acid = proton donor, base = proton acceptor.
  • Conjugate pairs differ by one H⁺.
  • Neutralization: acid + base → salt + water.
  • Strong acid + strong base net ionic: H⁺ + OH⁻ → H₂O.
  • Strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄.
  • Arrhenius: acids give H⁺, bases give OH⁻.
  • Brønsted–Lowry: acids donate protons, bases accept protons.
  • Conjugate acid–base pairs differ by one H⁺.
  • Neutralization → salt + water; net ionic H⁺ + OH⁻ → H₂O.

Keep learning

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Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • State the Arrhenius and Brønsted–Lowry definitions of acids and bases.
  • Identify conjugate acid–base pairs.
  • Write and balance neutralization reactions.
  • Predict the products of an acid–base reaction and write its net ionic equation.

Sources & references

  1. Chemistry LibreTexts, "4.3: Acid-Base Reactions."
  2. OpenStax, "14.1 Brønsted-Lowry Acids and Bases." *Chemistry 2e*.
  3. Chemistry LibreTexts, "7.8: Acid–Base and Gas Evolution Reactions."

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

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