AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-8)
Unit 4 · Topic 4.8
4.8 Introduction to Acid-Base Reactions
In the Brønsted-Lowry model, an acid donates a proton (H⁺) and a base accepts one. An acid that gives up its proton becomes its conjugate base, and a base that gains one becomes its conjugate acid. Water can act as either an acid or a base.
Key terms
- Brønsted-Lowry acid
- Brønsted-Lowry base
- conjugate acid-base pair
- proton transfer
- amphoteric
Acids donate, bases accept
A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor. A 'proton' here means an H⁺ ion: a hydrogen atom that has lost its electron, which is just a bare proton.
Because a proton has to go somewhere, an acid only acts as an acid when a base is there to accept the proton. Every acid-base reaction has both.
Conjugate acid-base pairs
When an acid donates a proton, what's left is its conjugate base. When a base accepts a proton, it becomes its conjugate acid. The two members of a conjugate pair differ by exactly one H⁺.
In HF + H₂O ⇌ F⁻ + H₃O⁺, HF is the acid and F⁻ is its conjugate base. H₂O is the base and H₃O⁺ (the hydronium ion) is its conjugate acid.
In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, it's the other way around for water. Water donates a proton, so it's the acid, and OH⁻ is its conjugate base. NH₃ accepts the proton, so it's the base, and NH₄⁺ is its conjugate acid.
| Acid | Conjugate base |
|---|---|
| HCl | Cl⁻ |
| H₂SO₄ | HSO₄⁻ |
| HSO₄⁻ | SO₄²⁻ |
| H₃O⁺ | H₂O |
| H₂O | OH⁻ |
| NH₄⁺ | NH₃ |
Water's special role
In water solutions, water itself often takes part. Its structure, with polar O–H bonds and lone pairs on oxygen, lets it accept a proton (becoming H₃O⁺) or donate one (becoming OH⁻). A substance that can act as either an acid or a base is called amphoteric. Other amphoteric species include HCO₃⁻ and HPO₄²⁻: each has a proton it can give away and a lone pair that can accept one.
Water reacts with itself slightly: H₂O + H₂O ⇌ H₃O⁺ + OH⁻. You'll use that equilibrium in Unit 8.
Comparing strengths
When an acid ionizes in water, you can compare the strengths of the conjugate pairs. A strong acid like HCl gives up its proton almost completely, which means its conjugate base, Cl⁻, has almost no tendency to take the proton back. Cl⁻ is a negligibly weak base.
A weak acid like HF gives up its proton only partly, so its conjugate base, F⁻, has a real tendency to accept a proton. In general, the stronger the acid, the weaker its conjugate base. Unit 8 makes this quantitative.
You won't be tested on Lewis acids and bases (electron-pair acceptors and donors). AP Chemistry focuses on proton transfer in water.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Labeling acids, bases and conjugates
Identify the acid, base, conjugate acid and conjugate base in CH₃COOH + NH₃ ⇌ CH₃COO⁻ + NH₄⁺.
Show the solutionHide the solution
- Step 1: CH₃COOH loses a proton to become CH₃COO⁻, so CH₃COOH is the acid and CH₃COO⁻ is its conjugate base.
- Step 2: NH₃ gains a proton to become NH₄⁺, so NH₃ is the base and NH₄⁺ is its conjugate acid.
- Step 3: Check: each pair differs by exactly one H⁺.
Answer: Acid CH₃COOH, base NH₃, conjugate base CH₃COO⁻, conjugate acid NH₄⁺
- Example 2
An amphoteric ion
Write equations showing HCO₃⁻ acting (a) as an acid with water and (b) as a base with water.
Show the solutionHide the solution
- Step 1: (a) As an acid, HCO₃⁻ donates a proton to water: HCO₃⁻ + H₂O ⇌ CO₃²⁻ + H₃O⁺.
- Step 2: (b) As a base, HCO₃⁻ accepts a proton from water: HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻.
- Step 3: In (a) water is the base; in (b) water is the acid. Both HCO₃⁻ and water are amphoteric.
Answer: (a) HCO₃⁻ + H₂O ⇌ CO₃²⁻ + H₃O⁺; (b) HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻
- Example 3
One proton at a time (classic trap)
What is the conjugate base of H₂SO₄? A student answers SO₄²⁻.
Show the solutionHide the solution
- Step 1: A conjugate base has exactly one fewer H⁺ than its acid.
- Step 2: Removing one H⁺ from H₂SO₄ gives HSO₄⁻.
- Step 3: SO₄²⁻ is the conjugate base of HSO₄⁻, not of H₂SO₄.
Answer: HSO₄⁻
Common mistakes
- Removing more than one H⁺ when writing a conjugate base, or forgetting to change the charge.
- Assuming water is always the base. With NH₃ or other bases, water is the acid.
- Pairing species that don't differ by one H⁺, such as calling H₃O⁺ the conjugate acid of OH⁻.
- Thinking a strong acid has a strong conjugate base. It's the opposite.
On the exam
- Expect multiple-choice questions that ask you to identify a conjugate pair or the species acting as a base. Draw an arrow showing where the proton goes.
- In free response, use the terms 'proton donor' and 'proton acceptor' when you justify a label.
Connected topics
Videos
Check yourself
4 questions on 4.8 Introduction to Acid-Base Reactions. Pick an answer to see if you got it, and why.
For the reaction HCO₃⁻(aq) + H₂O(l) ⇌ H₂CO₃(aq) + OH⁻(aq), which species acts as the Brønsted-Lowry acid in the forward reaction, and what is its conjugate base?
Which of the following species can act as either a Brønsted-Lowry acid or a Brønsted-Lowry base in water?
In the reaction H₂PO₄⁻(aq) + NH₃(aq) ⇌ HPO₄²⁻(aq) + NH₄⁺(aq), which two species act as Brønsted-Lowry bases?
In which of the following reactions does water act as a Brønsted-Lowry acid?
0 of 4 answered