---
title: "Brønsted-Lowry Definition — AP Chem Guide & Examples"
description: "The Brønsted-Lowry definition says acids donate protons (H+) and bases accept them. It's the acid-base model AP Chem tests in Units 4 and 8, including conjugate pairs."
canonical: "https://fiveable.me/ap-chem/key-terms/bronsted-lowry-definition"
type: "key-term"
subject: "AP Chemistry"
unit: "Unit 4"
---

# Brønsted-Lowry Definition — AP Chem Guide & Examples

## Definition

The Brønsted-Lowry definition states that an acid is a proton (H+) donor and a base is a proton acceptor, so every acid-base reaction is a proton transfer that creates conjugate acid-base pairs. It is the acid-base model AP Chemistry assesses (LO 4.8.A); Lewis acid-base concepts are not on the exam.

## What It Is

The Brønsted-Lowry definition reframes acids and bases around one event, a [proton transfer](/ap-chem/key-terms/proton-transfer "fv-autolink"). An acid donates a proton (H+) and a base accepts it. That's the whole model. When HCl reacts with water, HCl hands a proton to H2O, so HCl is the acid and water is the base, producing [H3O+](/ap-chem/key-terms/h3o "fv-autolink") and Cl−.

This definition expands on the older Arrhenius definition, which only worked for species producing H+ or OH− in water. Brønsted-Lowry explains why something like NH3 is a base even though it contains no hydroxide. NH3 simply accepts a proton. It also gives you conjugate acid-base pairs, two species that differ by exactly one H+. Every Brønsted-Lowry reaction produces them, and the CED expects you to identify those pairs and compare their relative strengths when an acid or base ionizes in water. One more thing worth knowing about water itself. Its structure lets it both donate and accept [protons](/ap-chem/unit-1/atomic-structure-electron-configurations/study-guide/DiW6kVmwDRDakxKodjw5 "fv-autolink"), which is why it shows up on either side of so many acid-base reactions (and why it can autoionize).

## Why It Matters

This is the foundation of two units. In [Unit 4](/ap-chem/unit-4 "fv-autolink") (Chemical Reactions), Topic 4.8 and learning objective [AP Chem](/ap-chem "fv-autolink") 4.8.A require you to identify Brønsted-Lowry acids, bases, and conjugate acid-base pairs based on proton transfer. The CED is explicit that Lewis acid-base concepts will NOT be assessed, so Brønsted-Lowry is the model you actually need to master. The emphasis is on aqueous solutions, where water's ability to donate and accept protons makes it a participant, not just a backdrop.

Then Unit 8 (Acids and Bases) builds everything on top of it. pH and pOH calculations (AP Chem 8.1.A), water [autoionization](/ap-chem/unit-8/intro-acids-bases/study-guide/rG2ZBgD9evdBohz4a7Mj "fv-autolink") and Kw, weak acid equilibria, and buffers all assume you can spot who donated the proton and who took it. Even acid-base titrations back in Topic 4.6 are just controlled proton-transfer reactions you run to an equivalence point. If you can't track the proton, Unit 8 turns into memorization instead of logic.

## Connections

### Arrhenius Definition (Unit 4)

Arrhenius came first and is narrower. An Arrhenius acid produces H+ in water and an Arrhenius base produces OH−. Brønsted-Lowry keeps every Arrhenius acid and base but also covers cases Arrhenius can't, like NH3 acting as a base by accepting a proton with no hydroxide in sight.

### Conjugate Acid-Base Pairs (Units 4 and 8)

Conjugate pairs only exist because of the Brønsted-Lowry model. When an acid donates its proton, what's left behind is its conjugate base, and the two differ by exactly one H+. Spotting these pairs is the literal skill named in LO 4.8.A, and comparing their relative strengths powers the weak acid-base reasoning in [Unit 8](/ap-chem/unit-8 "fv-autolink").

### pH, pOH, and Water Autoionization (Unit 8)

Water autoionizing (2 H2O → H3O+ + OH−) is just one water [molecule](/ap-chem/unit-2/lewis-diagrams/study-guide/KjqTRYr5TVr2C3Be3u0J "fv-autolink") acting as a Brønsted-Lowry acid and another acting as the base. That single proton transfer gives you Kw = [H3O+][OH−] = 1.0 × 10−14 at 25°C, which is where pH = pOH = 7.0 for neutral water comes from.

### Titrations and the Equivalence Point (Unit 4)

An acid-base titration is a proton-transfer reaction you run deliberately and measure. The titrant's acid or base reacts quantitatively with the analyte, and the equivalence point is the moment the analyte's protons have all been donated or accepted. Brønsted-Lowry thinking tells you what species are actually in the flask at that point.

## On the AP Exam

Multiple-choice questions hit this term directly. Stems ask things like "which term describes a species that transfers a proton to another species in an aqueous acid-base reaction?" or "according to the Brønsted-Lowry definition, what is an acid?" The answer pattern is always the same. Acid donates the proton, base accepts it. Beyond definition recall, you'll be given a reaction equation and asked to label each species as the Brønsted-Lowry acid, base, conjugate acid, or conjugate base, which is exactly what LO 4.8.A demands. On FRQs, the model shows up implicitly any time you write a net ionic equation for an acid-base reaction, justify which species is the stronger acid, or analyze a titration. Two exam-day notes from the CED. You can write H+(aq) or H3O+(aq) and both are accepted (H3O+ is preferred), and Lewis acid-base concepts are off the table, so don't burn time on them.

## Brønsted-Lowry Definition vs Arrhenius Definition

Arrhenius defines acids and bases by what they release in water (H+ for acids, OH− for bases), so it only works in aqueous solution and can't explain bases without hydroxide. Brønsted-Lowry defines them by the proton transfer itself, donor versus acceptor. Quick test for the exam. If a base has no OH− to give (like NH3), Arrhenius fails and you need Brønsted-Lowry. Every Arrhenius acid or base is also a Brønsted-Lowry one, but not the other way around.

## Key Takeaways

- A Brønsted-Lowry acid is a proton (H+) donor and a Brønsted-Lowry base is a proton acceptor, so every acid-base reaction in this model is a proton transfer.
- When an acid donates a proton, it becomes its conjugate base, and when a base accepts a proton, it becomes its conjugate acid; the members of a conjugate pair differ by exactly one H+.
- Water can both donate and accept protons, which lets it act as an acid or a base depending on its reaction partner and explains its autoionization (Kw = 1.0 × 10−14 at 25°C).
- Brønsted-Lowry is broader than Arrhenius because it explains bases like NH3 that accept protons without containing any hydroxide.
- The AP Exam tests Brønsted-Lowry concepts (LO 4.8.A) but explicitly does NOT assess Lewis acid-base concepts.
- H3O+(aq) is the preferred symbol for the aqueous hydrogen ion, but H+(aq) is also accepted on the AP Exam.

## FAQs

### What is the Brønsted-Lowry definition of an acid and a base?

A Brønsted-Lowry acid is a species that donates a proton (H+), and a Brønsted-Lowry base is a species that accepts one. Every acid-base reaction under this model is a proton transfer, like HCl donating a proton to H2O to form H3O+ and Cl−.

### Is the Lewis definition of acids and bases on the AP Chem exam?

No. The CED states directly that Lewis acid-base concepts will not be assessed on the AP Exam. Brønsted-Lowry is the model you need for both Unit 4 and Unit 8.

### How is the Brønsted-Lowry definition different from the Arrhenius definition?

Arrhenius defines acids as producing H+ and bases as producing OH− in water, which fails for bases like NH3 that have no hydroxide. Brønsted-Lowry defines acids as proton donors and bases as proton acceptors, so it covers everything Arrhenius covers plus those extra cases.

### Can water be both a Brønsted-Lowry acid and a base?

Yes. Water's molecular structure lets it donate protons to some species and accept protons from others, so it acts as an acid with NH3 and as a base with HCl. Two water molecules even react with each other in autoionization, giving Kw = [H3O+][OH−] = 1.0 × 10−14 at 25°C.

### Should I write H+ or H3O+ on the AP Chem exam?

Both are accepted. The CED says hydronium ion and H3O+(aq) are preferred, but H+(aq) earns credit too, since the two symbols are used interchangeably for the aqueous hydrogen ion.

## Related Study Guides

- [4.6 Introduction to Titration](/ap-chem/unit-4/intro-titrations/study-guide/8XHQYjYki6GqAcrp18I2)
- [4.8 Introduction to Acid-Base Reactions](/ap-chem/unit-4/intro-acid-base-reactions/study-guide/idvZ7Ve4pFo8gFIyYfMl)
- [8.1 Introduction to Acids and Bases](/ap-chem/unit-8/intro-acids-bases/study-guide/rG2ZBgD9evdBohz4a7Mj)

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