Unit 4 · Topic 4.8 Beta

Introduction to Acid-Base Reactions

In a Brønsted-Lowry acid-base reaction, a proton (H⁺) moves from the acid to the base.

Practice 1: Models and RepresentationsPractice 6: Argumentation

Question set for this topic

Part 1 · Hook

Why this matters

Lemon juice, vinegar and stomach acid all hand out the same thing: hydrogen ions, which are bare protons. Baking soda, ammonia and soap take them in. Seeing an acid-base reaction as a single proton moving from one particle to another explains reactions that the old "acids have H, bases have OH" rule cannot.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is left when a hydrogen atom loses its electron?

  1. A proton
  2. A neutron
  3. An electron
  4. A hydride ion
Show the answer

A hydrogen atom is one proton and one electron, so H⁺ is just a proton.

  • Correct: A proton:
  • A neutron:
  • An electron:
  • A hydride ion:

2. Which part of a water molecule carries a partial negative charge and lone pairs?

  1. The oxygen atom
  2. The hydrogen atoms
  3. The O–H bonds equally
  4. Neither part
Show the answer

Oxygen is more electronegative and has two lone pairs.

  • Correct: The oxygen atom:
  • The hydrogen atoms:
  • The O–H bonds equally:
  • Neither part:

3. In H⁺(aq) + OH⁻(aq) → H₂O(l), what kind of reaction is shown?

  1. Acid-base
  2. Precipitation
  3. Combustion
  4. Decomposition
Show the answer

An H⁺ moves to OH⁻ to form water.

  • Correct: Acid-base:
  • Precipitation:
  • Combustion:
  • Decomposition:

Part 4 · See it

See it first

An HF molecule passes an H⁺ (curved arrow) to a water molecule. HF is the acid, the proton donor; water is the base, the proton acceptor. The products are F⁻, the conjugate base, and H₃O⁺, the conjugate acid. Brackets join HF with F⁻ and H₂O with H₃O⁺ as conjugate pairs, each differing by one H⁺. Equation: HF(aq) + H₂O(l) ⇌ F⁻(aq) + H₃O⁺(aq).
A proton transfer. HF gives an H⁺ to a water molecule. Each reactant and the product it becomes form a conjugate pair, differing by one H⁺. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. A Brønsted-Lowry acid gives up an H⁺ (a proton)what remains is its conjugate base, with one less H and one less + charge
  2. A Brønsted-Lowry base uses a lone pair to accept the H⁺it becomes its conjugate acid, with one more H and one more + charge
  3. Water has both O–H bonds and lone pairs on oxygenit can act as an acid or a base: it is amphoteric
  4. A bare proton bonds at once to a water moleculeacid in water gives hydronium ions, H₃O⁺

Part 6 · Key ideas

Key ideas

  • A Brønsted-Lowry acid is a proton donor; a Brønsted-Lowry base is a proton acceptor (it needs a lone pair).
  • A conjugate acid-base pair differs by exactly one H⁺: HF/F⁻, NH₄⁺/NH₃, H₂O/OH⁻, H₃O⁺/H₂O.
  • Amphoteric species such as H₂O and HCO₃⁻ can donate or accept a proton.
  • In water, H⁺ is carried as hydronium, H₃O⁺. Neutralization: H⁺ + OH⁻ → H₂O.

Part 7 · Misconception

A common mistake

The wrong idea: An acid is anything with H in its formula, and a base is anything with OH.

What actually happens: Acid and base describe what a particle does in a reaction. Methane has H but gives up no protons; ammonia has no OH but accepts a proton. Look for the H⁺ that moves.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Model

Four proton transfers

Each equation shows a proton (H⁺) moving from one particle to another.

Proton-transfer reactions
ReactionEquation
1HF(aq) + H₂O(l) ⇌ F⁻(aq) + H₃O⁺(aq)
2NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
3HCO₃⁻(aq) + OH⁻(aq) → CO₃²⁻(aq) + H₂O(l)
4CH₃COOH(aq) + NH₃(aq) → CH₃COO⁻(aq) + NH₄⁺(aq)

The double arrow (⇌) means the products also react to re-form the reactants, which you will study in Unit 7.

1. In reaction 2, which species acts as the Brønsted-Lowry acid?

  1. H₂O
  2. NH₃
  3. NH₄⁺
  4. OH⁻
Show the answer

Water gives up an H⁺ (becoming OH⁻) and ammonia accepts it (becoming NH₄⁺). The proton donor on the reactant side is water.

  • Correct: H₂O: Right: water donates the proton and becomes OH⁻.
  • NH₃: Ammonia gains an H⁺, so it is the base (proton acceptor).
  • NH₄⁺: NH₄⁺ is a product: the conjugate acid formed when ammonia accepts the proton.
  • OH⁻: OH⁻ is a product: what is left of water after it donates a proton.

2. In which reactions does water act as a base? Select all that apply.

  1. Reaction 1
  2. Reaction 2
  3. Reaction 3
  4. Reaction 4
Show the answer

In reaction 1, water accepts the proton from HF and becomes H₃O⁺, so it is a base there. In reaction 2 water donates a proton (acid). In reaction 3 water is a product, and in reaction 4 it does not appear.

  • Correct: Reaction 1: Right: water accepts H⁺ from HF and becomes hydronium.
  • Reaction 2: In reaction 2 water gives an H⁺ to ammonia, so it is the acid.
  • Reaction 3: Water is formed in reaction 3 (when OH⁻ gains a proton); it is not a reactant there.
  • Reaction 4: Water is not one of the reacting species in reaction 4.

3. In reaction 3, what is the conjugate base of HCO₃⁻?

  1. CO₃²⁻
  2. H₂CO₃
  3. OH⁻
  4. H₂O
Show the answer

Remove one H⁺ from HCO₃⁻ and you get CO₃²⁻; the charge drops by one. They differ by exactly one proton, so they are a conjugate pair.

  • Correct: CO₃²⁻: Right: HCO₃⁻ minus H⁺ is CO₃²⁻.
  • H₂CO₃: H₂CO₃ has one more H⁺, so it is the conjugate acid of HCO₃⁻.
  • OH⁻: OH⁻ is the base that takes the proton; its conjugate acid is H₂O.
  • H₂O: Water is the conjugate acid of OH⁻, not related to HCO₃⁻ by one proton.

4. HCO₃⁻ appears in reaction 3. Which statement about it is supported by its behavior?

  1. It can donate or accept a proton, so it is amphoteric
  2. It is a base, because it carries a negative charge
  3. It is an acid and not a base, as it has an H to give
  4. It is a spectator, because it keeps its carbon atom
Show the answer

In reaction 3 HCO₃⁻ donates a proton to OH⁻. It also has a site to accept one (giving H₂CO₃). A species that can do either is amphoteric, like water.

  • Correct: It can donate or accept a proton, so it is amphoteric: Right: it can donate or accept a proton.
  • It is a base, because it carries a negative charge: A negative charge does not decide; in reaction 3 HCO₃⁻ donates a proton, acting as an acid.
  • It is an acid and not a base, as it has an H to give: It does have an H to give, but it can also accept a proton on an oxygen lone pair, forming H₂CO₃.
  • It is a spectator, because it keeps its carbon atom: It changes (it loses H⁺), so it is not a spectator.

5. Which pair is a conjugate acid-base pair in reaction 4?

  1. NH₃ and NH₄⁺
  2. CH₃COOH and NH₃
  3. CH₃COO⁻ and NH₄⁺
  4. CH₃COOH and NH₄⁺
Show the answer

A conjugate pair differs by one H⁺. NH₃ gains a proton to become NH₄⁺. The other pair is CH₃COOH and CH₃COO⁻.

  • Correct: NH₃ and NH₄⁺: Right: they differ by exactly one H⁺.
  • CH₃COOH and NH₃: These are the acid and base that react with each other, not a conjugate pair.
  • CH₃COO⁻ and NH₄⁺: These are the two products: the conjugate base of one pair and the conjugate acid of the other.
  • CH₃COOH and NH₄⁺: Both are proton donors from different pairs; they do not differ by one H⁺.

Particle view

A gas dissolves in water

ClHClHClHClHHCl(g) addedOHHH+OHHH+OHHH+Cl−Cl−Cl−Cl−OHHH+In solution

Key: green Cl, chlorine atom or chloride ion (marked −); red O with white H, water-derived species; + and − mark ion charges. Water molecules that do not react are not drawn.

6. Which equation describes what the diagram shows?

  1. HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
  2. HCl(g) → H(g) + Cl(g)
  3. HCl(g) + H₂O(l) → H₂(g) + HOCl(aq)
  4. HCl(g) → HCl(aq), with no other change
Show the answer

Each HCl molecule gives its H⁺ to a water molecule. The diagram shows only H₃O⁺ and Cl⁻ afterward, with no HCl molecules left.

  • Correct: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq): Right: a proton moves from HCl to water.
  • HCl(g) → H(g) + Cl(g): The diagram shows ions, not neutral H and Cl atoms.
  • HCl(g) + H₂O(l) → H₂(g) + HOCl(aq): No hydrogen gas forms; the H ends up in hydronium.
  • HCl(g) → HCl(aq), with no other change: If HCl simply dissolved, the right box would show HCl molecules, but it shows none.

Part 9 · Summary

Summary

In a Brønsted-Lowry acid-base reaction, a proton (H⁺) moves from the acid to the base. The acid becomes its conjugate base and the base becomes its conjugate acid; each pair differs by one H⁺. Water is amphoteric, and in water a proton is carried as hydronium, H₃O⁺.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections