AHL: Advanced Lewis acid-base concepts

IB Chemistry HL· 3.8 AHL Advanced Lewis acid-base concepts· 15 min read

1. Extended Lewis Model: Definitions and Adduct Formation★★☆☆☆⏱ 5 min

📘 Definition

Lewis Acid and Lewis Base

A Lewis acid accepts an electron pair from a donor species, and requires an empty valence orbital to do so. A Lewis base donates an electron pair to an acceptor species, and requires at least one non-bonding lone pair to do so.

Example:

Lewis acid: $B F_{3}$ , Lewis base: $N H_{3}$

Unlike Brønsted-Lowry theory, which is limited to reactions involving proton transfer, the Lewis model describes acid-base behavior across a much wider range of chemical systems, including reactions that do not contain hydrogen at all.

📐 Worked Example

Identify the Lewis acid and Lewis base in the reaction: $N H_{3} + B F_{3} \to N H_{3} B F_{3}$

1
Analyze the valence electrons of each reactant: Ammonia ( $N H_{3}$ ) has one lone pair of electrons on the central nitrogen atom.
2
Boron trifluoride ( $B F_{3}$ ) only has 6 valence electrons around the central boron atom, leaving an empty p-orbital available to accept an electron pair.
3
Classify each species: Ammonia donates the electron pair, so it is the Lewis base. Boron trifluoride accepts the electron pair, so it is the Lewis acid. The product is a Lewis adduct.

Exam tip:

The presence of a lone pair is the most reliable indicator of a Lewis base, regardless of proton accepting ability.

2. Hard-Soft Acid-Base (HSAB) Theory★★★☆☆⏱ 5 min

HSAB theory classifies Lewis acids and bases by their polarizability and charge density to predict the relative stability of adducts. This is a common topic for both multiple choice and extended response questions.

📘 Definition

Hard and Soft Classification

Hard species have small ionic/atomic radius, high charge density, and low polarizability. Soft species have larger radius, low charge density, and high polarizability.

Example:

Hard: $H^{+}$ (acid), $F^{-}$ (base); Soft: $A g^{+}$ (acid), $I^{-}$ (base)

📐 Worked Example

Using HSAB theory, predict whether $A g F$ or $A g I$ is more thermodynamically stable.

1
Classify all species: $A g^{+}$ is a large cation with a +1 charge, so it is a soft Lewis acid. $F^{-}$ is small, highly charged, and a hard base; $I^{-}$ is large, polarizable, and a soft base.
2
Apply the core HSAB rule: hard acids preferentially bind hard bases, and soft acids preferentially bind soft bases.
3
Soft $A g^{+}$ matches with soft $I^{-}$ , so $A g I$ is the more stable adduct, which matches its observed low solubility in water.

Exam tip:

You only need to remember general classification rules, not the category of every rare ion; the question will give context for classification if required.

3. Applications to IB Chemistry HL★★★★☆⏱ 5 min

Advanced Lewis concepts are applied across multiple HL topics, including transition metal complex formation, organic reaction mechanisms, and solubility predictions.

📐 Worked Example

Identify the Lewis acid in the formation of the tetraamminecopper(II) complex ion: $C u^{2 +} + 4 N H_{3} \to [C u (N H_{3})_{4}]^{2 +}$

1
The copper(II) cation has empty 3d orbitals that can accept electron pairs from donor ligands.
2
Each ammonia molecule has a lone pair on nitrogen that it donates to form a coordinate bond to the metal center.
3
The electron pair acceptor is $C u^{2 +}$ , so $C u^{2 +}$ is the Lewis acid, and $N H_{3}$ is the Lewis base (ligand).

4. Common Pitfalls

Wrong move:

Claiming all Lewis acids are also Brønsted-Lowry acids

Why:

Many common Lewis acids (e.g. $B F_{3}$ , $A l C l_{3}$ , transition metal cations) do not contain hydrogen to donate as protons, so they cannot be Brønsted-Lowry acids.

Correct move:

Remember: All Brønsted-Lowry acids are Lewis, but not all Lewis acids are Brønsted-Lowry.

Wrong move:

Classifying a species with a lone pair as a Lewis acid

Why:

Lewis acids accept electron pairs, so they need an empty orbital, not a free lone pair.

Correct move:

Memorize: Lone pair = donor = Lewis base.

Wrong move:

Matching hard acids to soft bases in HSAB predictions

Why:

The core HSAB rule is that like polarity matches produce more stable bonds.

Correct move:

Recall the mnemonic: hard likes hard, soft likes soft.

Wrong move:

Treating coordinate covalent bonds as chemically different from other covalent bonds after formation

Why:

Only the origin of the shared electron pair differs; the bond itself has identical bond energy and length to any other covalent bond.

Correct move:

Do not label coordinate bonds separately in final Lewis structures unless explicitly asked to do so.

5. Quick Reference Cheatsheet

Species Type	Key Property	Common Examples
Lewis Acid	Accepts electron pair, empty orbital	$H^{+}, B F_{3}, A l C l_{3}, M^{n +}$ (metal ions)
Lewis Base	Donates electron pair, has lone pair	$N H_{3}, O H^{-}, H_{2} O, C N^{-}$
Hard Acid	Low polarizability, high charge density	$H^{+}, L i^{+}, A l^{3 +}, B F_{3}$
Soft Acid	High polarizability, low charge density	$A g^{+}, H g^{2 +}, C u^{+}, P t^{2 +}$
Hard Base	Low polarizability, high electronegativity	$O H^{-}, F^{-}, N H_{3}, H_{2} O$
Soft Base	High polarizability, low electronegativity	$I^{-}, C N^{-}, C O, S^{2 -}$
HSAB Rule	Hard prefers hard, soft prefers soft	Predicts relative adduct stability

6. Frequently Asked

What is the key difference between Lewis and Brønsted-Lowry acid-base definitions?

Brønsted-Lowry theory only describes proton transfer reactions, while Lewis theory describes all electron pair donation/acceptance reactions. All Brønsted-Lowry acids/bases are Lewis acids/bases, but not all Lewis species are Brønsted-Lowry.

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.

2025 · 2
HSAB stability prediction question
2023 · 1
Lewis acid classification multiple choice

Going deeper

Official syllabusIB Chemistry HL 2025 SyllabusFull reference for 3.8 AHL content

What's Next

Advanced Lewis acid-base concepts are the foundation for understanding transition metal complex chemistry, a major assessed topic in IB Chemistry HL. This framework also underpins all organic reaction mechanisms you will study in AHL organic chemistry, including acid catalysis, nucleophilic attack, and electrophilic substitution. Mastery of Lewis theory and HSAB will help you quickly solve reaction prediction and stability questions, saving valuable time in both Paper 1 and Paper 2 exams.