# Acid-base classification: Bronsted-Lowry and Lewis

> IB Chemistry HL · Unit 3: Classification of matter
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u3-acid-base-classification-bronsted-lowry/

This module covers two core acid-base classification systems for IB Chemistry: the Bronsted-Lowry proton-transfer model, and the broader Lewis electron-pair model. You will learn to apply both and compare their scope.

**Prerequisites:** [Basic Arrhenius acid-base concepts](https://www.owlsprep.com/study/ib-chemistry-hl-u3-introduction-to-acids-and-bases/)

## Learning objectives

- Distinguish between Bronsted-Lowry and Lewis definitions of acids and bases
- Identify conjugate acid-base pairs in Bronsted-Lowry reactions
- Classify substances as acids or bases using both classification systems
- Compare the scope of Bronsted-Lowry and Lewis frameworks

## Bronsted-Lowry Acid-Base Theory

**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: $NH_3 + HCl \rightarrow NH_4^+ + Cl^-$

1. Track proton movement: HCl loses one hydrogen to become $Cl^-$, so it acts as a proton donor.
2. $NH_3$ gains one hydrogen to become $NH_4^+$, so it acts as a proton acceptor.
3. Conclusion: HCl = Bronsted-Lowry acid, $NH_3$ = Bronsted-Lowry base.

> **Exam tip:** Always check proton transfer, don't assume common acids are always the acid in every reaction.

## Conjugate Acid-Base Pairs

**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, $HCO_3^-$ and $H_2PO_4^-$.

**Worked example:** Write the conjugate base of $H_2SO_4$ and the conjugate acid of $HSO_4^-$.

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 $H_2SO_4$ = $HSO_4^-$; Conjugate acid of $HSO_4^-$ = $H_2SO_4$

**Check your understanding**

Test your understanding:

1. Which of the following are valid conjugate acid-base pairs?

   - A: $H_2O$ and $O^{2-}$
   - B: $HNO_3$ and $NO_3^-$
   - C: $NH_4^+$ and $NH_3$
   - D: $H_2SO_4$ and $SO_4^{2-}$

   *Why:* Conjugate pairs must differ by exactly one proton. A and D differ by two protons, so they are incorrect.

## Lewis Acid-Base Theory

**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.

> **mnemonic**
>
> Lewis Acid = Acceptor (both start with A), Lewis Base = Donor. This simple mnemonic prevents common mix-ups.

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 $AlCl_3$ as an acid or base according to the Lewis definition, and explain your reasoning.

1. Draw the Lewis structure of $AlCl_3$. 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 $AlCl_3$ is a Lewis acid.

**Comparing methods**

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

## Common pitfalls

- **Wrong:** Claiming conjugate pairs can differ by more than one proton
  - Why it fails: IB exam questions specifically test that conjugate acid-base pairs differ by exactly one proton
  - Correct: Always confirm the two species differ by exactly one $H^+$ when identifying conjugate pairs
- **Wrong:** Mixing up Lewis definitions: calling electron pair donors acids
  - Why it fails: Definitions are reversed from the common proton-focused Bronsted-Lowry pattern, leading to easy mix-ups
  - Correct: Use the mnemonic: Lewis Acid = Acceptor (both start with A)
- **Wrong:** Claiming Lewis theory replaces Bronsted-Lowry, and Bronsted acids are not Lewis acids
  - Why it fails: All Bronsted-Lowry acids fit the Lewis definition, because accepting a proton requires accepting an electron pair
  - Correct: Recognize Lewis theory is an extension, not a replacement, for Bronsted-Lowry
- **Wrong:** Assuming water is always a base in Bronsted-Lowry reactions
  - Why it fails: Water is amphiprotic, so its role depends on the other reactant
  - Correct: Always check proton transfer: when water reacts with a stronger base, it donates a proton and acts as an acid

## 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 |

## 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.

- [Salts and buffer solutions](https://www.owlsprep.com/study/ib-chemistry-hl-u3-salts-and-buffer-solutions/)
- [AHL: Extended periodic trends](https://www.owlsprep.com/study/ib-chemistry-hl-u3-ahl-extended-periodic-trends/)

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