# AHL: Advanced Lewis acid-base concepts

> IB Chemistry HL · IB Chemistry HL (2025 syllabus)
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u3-ahl-advanced-lewis-acid-base/

This sub-topic extends core Lewis acid-base theory, covering adduct formation, hard-soft acid-base (HSAB) classification, and applications to coordinate chemistry regularly assessed in IB HL Chemistry.

**Prerequisites:** [Core Lewis acid-base theory](https://www.owlsprep.com/study/ib-chemistry-hl-u3-core-lewis-acid-base/); [Coordinate covalent bonding](https://www.owlsprep.com/study/ib-chemistry-hl-u2-coordinate-bonding/)

## Learning objectives

- Distinguish Lewis acid-base definitions from Brønsted-Lowry models
- Identify adduct formation and coordinate bonding in Lewis reactions
- Classify Lewis acids and bases using the hard-soft acid-base (HSAB) framework
- Predict reaction outcomes and adduct stability using HSAB theory

## Extended Lewis Model: Definitions and Adduct Formation

**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: $BF_3$, Lewis base: $NH_3$

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: $NH_3 + BF_3 \rightarrow NH_3BF_3$

1. Analyze the valence electrons of each reactant: Ammonia ($NH_3$) has one lone pair of electrons on the central nitrogen atom.
2. Boron trifluoride ($BF_3$) 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.

## Hard-Soft Acid-Base (HSAB) Theory

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.

**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: $Ag^+$ (acid), $I^-$ (base)

> **mnemonic**
>
> Hard is small and tight, soft is big and bendable; hard likes hard, soft likes soft.

**Worked example:** Using HSAB theory, predict whether $AgF$ or $AgI$ is more thermodynamically stable.

1. Classify all species: $Ag^+$ 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 $Ag^+$ matches with soft $I^-$, so $AgI$ 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.

## Applications to IB Chemistry HL

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

**Exam command terms**

- **Classify** — Assign each species as acid/base and hard/soft, with a 1-sentence justification based on electron structure or polarizability *(Justify AlCl3 as Lewis acid by noting empty orbital on Al.)*

- **Predict** — State the outcome (product, stability) and explicitly reference the Lewis or HSAB rule to gain full marks.

**Worked example:** Identify the Lewis acid in the formation of the tetraamminecopper(II) complex ion: $Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}$

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 $Cu^{2+}$, so $Cu^{2+}$ is the Lewis acid, and $NH_3$ is the Lewis base (ligand).

## Common pitfalls

- **Wrong:** Claiming all Lewis acids are also Brønsted-Lowry acids
  - Why it fails: Many common Lewis acids (e.g. $BF_3$, $AlCl_3$, transition metal cations) do not contain hydrogen to donate as protons, so they cannot be Brønsted-Lowry acids.
  - Correct: Remember: All Brønsted-Lowry acids are Lewis, but not all Lewis acids are Brønsted-Lowry.
- **Wrong:** Classifying a species with a lone pair as a Lewis acid
  - Why it fails: Lewis acids accept electron pairs, so they need an empty orbital, not a free lone pair.
  - Correct: Memorize: Lone pair = donor = Lewis base.
- **Wrong:** Matching hard acids to soft bases in HSAB predictions
  - Why it fails: The core HSAB rule is that like polarity matches produce more stable bonds.
  - Correct: Recall the mnemonic: hard likes hard, soft likes soft.
- **Wrong:** Treating coordinate covalent bonds as chemically different from other covalent bonds after formation
  - Why it fails: Only the origin of the shared electron pair differs; the bond itself has identical bond energy and length to any other covalent bond.
  - Correct: Do not label coordinate bonds separately in final Lewis structures unless explicitly asked to do so.

## Cheatsheet

| Species Type | Key Property | Common Examples |
| --- | --- | --- |
| Lewis Acid | Accepts electron pair, empty orbital | $H^+, BF_3, AlCl_3, M^{n+}$ (metal ions) |
| Lewis Base | Donates electron pair, has lone pair | $NH_3, OH^-, H_2O, CN^-$ |
| Hard Acid | Low polarizability, high charge density | $H^+, Li^+, Al^{3+}, BF_3$ |
| Soft Acid | High polarizability, low charge density | $Ag^+, Hg^{2+}, Cu^+, Pt^{2+}$ |
| Hard Base | Low polarizability, high electronegativity | $OH^-, F^-, NH_3, H_2O$ |
| Soft Base | High polarizability, low electronegativity | $I^-, CN^-, CO, S^{2-}$ |
| HSAB Rule | Hard prefers hard, soft prefers soft | Predicts relative adduct stability |

## 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.

- [AHL: Acid-base titration curves](https://www.owlsprep.com/study/ib-chemistry-hl-u3-ahl-acid-base-titration-curves/)
- [R1: What drives chemical reactions?](https://www.owlsprep.com/study/ib-chemistry-hl-u4-overview/)
- [Enthalpy change and calorimetry](https://www.owlsprep.com/study/ib-chemistry-hl-u4-enthalpy-change-and-calorimetry/)

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