Acid-base classification: Bronsted-Lowry and Lewis

IB Chemistry HL· 45 min read

1. Bronsted-Lowry Acid-Base Theory★★☆☆☆⏱ 15 min

📘 Definition

Bronsted-Lowry Acids and Bases

A Bronsted-Lowry acid is a proton ( $H^{+}$ ) donor, and a Bronsted-Lowry base is a proton ( $H^{+}$ ) acceptor. This model expands on the earlier Arrhenius framework to include reactions outside aqueous solution.

Example:

When HCl dissolves in water, HCl donates a proton to water, so HCl is the acid and water is the base.

Unlike the Arrhenius model, Bronsted-Lowry theory correctly identifies bases that do not produce hydroxide ions directly, such as ammonia, and describes acid-base reactions in non-aqueous solvents.

📐 Worked Example

Identify the Bronsted-Lowry acid and base in the reaction: $N H_{3} + H C l \to N H_{4}^{+} + C l^{-}$

1
Track proton movement: HCl loses one hydrogen to become $C l^{-}$ , so it acts as a proton donor.
2
$N H_{3}$ gains one hydrogen to become $N H_{4}^{+}$ , so it acts as a proton acceptor.
3
Conclusion: HCl = Bronsted-Lowry acid, $N H_{3}$ = Bronsted-Lowry base.

Exam tip:

Always check proton transfer, don't assume common acids are always the acid in every reaction.

2. Conjugate Acid-Base Pairs★★☆☆☆⏱ 15 min

📘 Definition

Conjugate Acid-Base Pair

Two species that differ by exactly one proton, formed by proton transfer in a Bronsted-Lowry reaction. The conjugate base is what remains after an acid donates a proton, and the conjugate acid forms after a base accepts a proton.

A substance that can act as both a Bronsted-Lowry acid and base is called amphiprotic. Common examples include water, $H C O_{3}^{-}$ and $H_{2} P O_{4}^{-}$ .

📐 Worked Example

Write the conjugate base of $H_{2} S O_{4}$ and the conjugate acid of $H S O_{4}^{-}$ .

1
To form a conjugate base, remove one proton ( $H^{+}$ ) from the acid.
2
$H_2SO_4 - H^+ = HSO_4^-$
3
To form a conjugate acid, add one proton ( $H^{+}$ ) to the base.
4
$HSO_4^- + H^+ = H_2SO_4$
5
Final answer: Conjugate base of $H_{2} S O_{4}$ = $H S O_{4}^{-}$ ; Conjugate acid of $H S O_{4}^{-}$ = $H_{2} S O_{4}$

✓ Quick check

Test your understanding:

Which of the following are valid conjugate acid-base pairs?
- A: $H_{2} O$ and $O^{2 -}$
- B: $H N O_{3}$ and $N O_{3}^{-}$
- C: $N H_{4}^{+}$ and $N H_{3}$
- D: $H_{2} S O_{4}$ and $S O_{4}^{2 -}$
Reveal answer
[ "B", "C" ] —
Conjugate pairs must differ by exactly one proton. A and D differ by two protons, so they are incorrect.

3. Lewis Acid-Base Theory★★★☆☆⏱ 20 min

📘 Definition

Lewis Acids and Bases

A Lewis acid is an electron pair acceptor, and a Lewis base is an electron pair donor. This classification focuses on electron pair movement rather than proton transfer.

The Lewis model is broader than Bronsted-Lowry: all Bronsted-Lowry acids and bases are also Lewis acids and bases, but many Lewis reactions do not involve proton transfer. This is especially useful for describing coordinate covalent bond formation in coordination complexes.

📐 Worked Example

Classify $A l C l_{3}$ as an acid or base according to the Lewis definition, and explain your reasoning.

1
Draw the Lewis structure of $A l C l_{3}$ . Aluminum has 3 valence electrons, all bonded to chlorine, giving it only 6 valence electrons and an empty outer orbital.
2
Aluminum can accept an electron pair from another species to complete its octet.
3
By definition, Lewis acids accept electron pairs, so $A l C l_{3}$ is a Lewis acid.

Methods compared

Comparison of the two classification frameworks:

Bronsted-Lowry

Focuses on proton transfer

+ Pros: Simple to apply for common aqueous reactions

− Cons: Limited to reactions involving protons

Lewis

Focuses on electron pair transfer

+ Pros: Broad scope, includes non-proton reactions like coordination complex formation

− Cons: Less intuitive for common aqueous acid-base reactions

4. Common Pitfalls

Wrong move:

Claiming conjugate pairs can differ by more than one proton

Why:

IB exam questions specifically test that conjugate acid-base pairs differ by exactly one proton

Correct move:

Always confirm the two species differ by exactly one $H^{+}$ when identifying conjugate pairs

Wrong move:

Mixing up Lewis definitions: calling electron pair donors acids

Why:

Definitions are reversed from the common proton-focused Bronsted-Lowry pattern, leading to easy mix-ups

Correct move:

Use the mnemonic: Lewis Acid = Acceptor (both start with A)

Wrong move:

Claiming Lewis theory replaces Bronsted-Lowry, and Bronsted acids are not Lewis acids

Why:

All Bronsted-Lowry acids fit the Lewis definition, because accepting a proton requires accepting an electron pair

Correct move:

Recognize Lewis theory is an extension, not a replacement, for Bronsted-Lowry

Wrong move:

Assuming water is always a base in Bronsted-Lowry reactions

Why:

Water is amphiprotic, so its role depends on the other reactant

Correct move:

Always check proton transfer: when water reacts with a stronger base, it donates a proton and acts as an acid

5. Quick Reference Cheatsheet

Classification	Acid Definition	Base Definition	Key Note
Bronsted-Lowry	Proton ( $H^{+}$ ) donor	Proton ( $H^{+}$ ) acceptor	Only applies to proton transfer reactions
Lewis	Electron pair acceptor	Electron pair donor	Broader scope, includes non-proton reactions
Conjugate Pair	N/A	N/A	Must differ by exactly one proton

6. Frequently Asked

Can a substance be an acid by one definition and a base by another?

Yes, this is common. For example, water is amphiprotic (can be acid or base) under Bronsted-Lowry, and always acts as a Lewis base when it forms bonds with metal cations.

Do I need to remember both definitions for the IB exam?

Yes, IB exam questions regularly ask to classify substances using both definitions, and often test your ability to compare their scope.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Identify Bronsted-Lowry acid-base pairs
2023 · 2
Compare Lewis and Bronsted-Lowry definitions

Going deeper

syllabusIB Chemistry HL Topic 8: Acids and Bases

What's Next

Mastering these two acid-base classification systems is the foundation for all subsequent acids and bases topics in IB Chemistry HL. Next, you will apply these definitions to understand the pH scale, acid strength, and acid-base equilibria. Bronsted-Lowry theory is core to acid-base titrations and buffer calculations, while Lewis theory is critical for understanding coordination compound chemistry later in the course. Building a solid understanding of classification will simplify all more complex topics that follow.