10.1.2 The Bronsted-Lowry Concept of Acids and Bases

This topic explains the Bronsted-Lowry theory — a more general theory of acids and bases that defines acids as proton donors and bases as proton acceptors.

Definition

Bronsted-Lowry Acid

An acid is a proton (H⁺) donor.

Simple explanation: A Bronsted-Lowry acid gives away a proton (H⁺).

Bronsted-Lowry Base

A base is a proton (H⁺) acceptor.

Simple explanation: A Bronsted-Lowry base accepts a proton (H⁺).

Main Idea

In 1923, J. N. Bronsted and T. M. Lowry independently proposed another theory to overcome the shortcomings of the Arrhenius theory. This theory is known as the Bronsted-Lowry theory.

• An acid is a proton donor.
• A base is a proton acceptor.

Simple explanation: The Bronsted-Lowry theory focuses on protons (H⁺), not just H⁺ and OH⁻ in water.

Core Concept

Key Definitions

Examples from the Book

Example 1: HCl and H₂O

HCl + H₂O → H₃O⁺ + Cl^-

• HCl donates a proton → acid
• H₂O accepts a proton → base
• **H₃O⁺** = hydronium ion
• **Cl⁻** = chloride ion

Example 2: NH₃ and H₂O

NH₃ + H₂O ⇌ NH₄⁺ + OH^-

• NH₃ accepts a proton → base
• H₂O donates a proton → acid
• **NH₄⁺** = ammonium ion
• **OH⁻** = hydroxide ion

Simple explanation: In the first example, HCl gives a proton; in the second, NH₃ takes a proton.

Detailed Explanation

Why the Bronsted-Lowry Theory Was Needed

• It applied only to aqueous solutions.
• It could not explain why CO₂, SO₂ are acidic (no H⁺).
• It could not explain why NH₃ is basic (no OH⁻).

The Bronsted-Lowry theory solved these problems by focusing on proton transfer.

Simple explanation: The Bronsted-Lowry theory is more general — it works for more substances and situations.

The Proton (H⁺)

A proton is a hydrogen ion (H⁺) — a hydrogen atom that has lost its electron.

• Protons are positively charged.
• In acid-base reactions, protons are transferred from one substance to another.

Simple explanation: A proton is just an H⁺ ion — a hydrogen atom without its electron.

How Bronsted-Lowry Explains Ammonia as a Base

Question from the book: Where does OH⁻ come from when ionizing bases such as ammonia?

Answer: The Arrhenius theory is not sufficient to answer this question, but the Bronsted-Lowry theory explains how ammonia acts as a base in water.

Ammonia is a gas at room temperature. When it is dissolved in water:

NH₃ + H₂O ⇌ NH₄⁺ + OH^-

• NH₃ accepts a proton from water → NH₃ is a base
• H₂O donates a proton → H₂O is an acid
• NH₄⁺ is the ammonium ion
• OH⁻ is the hydroxide ion

Simple explanation: Ammonia takes a proton from water, producing OH⁻. This is why ammonia acts as a base — even though it doesn't contain OH⁻ itself.

Conjugate Acid-Base Pairs

When an acid donates a proton, it becomes a conjugate base. When a base accepts a proton, it becomes a conjugate acid.

Example:

HCl + H₂O ⇌ H₃O⁺ + Cl^-

Example:

NH₃ + H₂O ⇌ NH₄⁺ + OH^-

Simple explanation: Every acid has a conjugate base; every base has a conjugate acid.

Amphoteric Substances

• A base (in the HCl example — accepts a proton)
• An acid (in the NH₃ example — donates a proton)

This means water behaves like an acid as well as a base. It is amphoteric in nature.

Definition: Substances that react with both acids and bases are called amphoteric substances.

Simple explanation: Amphoteric substances can act as either an acid or a base, depending on what they react with.

Example 10.1 (from the book)

Problem: Identify Bronsted-Lowry acids or bases in the following reactions.

HCl + H₂O → H₃O⁺ + Cl^-

H₂O + NH₃ → NH₄⁺ + OH^-

• An acid is a proton donor. After donating a proton, an acid forms a negative ion.
• A base is a proton acceptor. After accepting a proton from an acid, it forms a positive ion.

Solution:

• HCl is converted to Cl⁻ by donating a proton → HCl is an acid.
• H₂O accepts the proton that HCl donates and forms H₃O⁺ → water is a base.

• H₂O is converted to OH⁻ by donating a proton → H₂O is an acid.
• NH₃ accepts the proton and forms NH₄⁺ → NH₃ is a base.

Concept Assessment Exercise 10.1 (from the book)

Identify Bronsted acids and Bronsted bases in the following reactions:

1. H₂SO₄ + H₂O ⇌ HSO₄^- + H₃O⁺

• H₂SO₄ donates proton → acid
• H₂O accepts proton → base

2. CH₃COOH + H₂O ⇌ CH₃COO^- + H₃O⁺

• CH₃COOH donates proton → acid
• H₂O accepts proton → base

3. H₂S + NH₃ ⇌ NH₄⁺ + HS^-

• H₂S donates proton → acid
• NH₃ accepts proton → base

Intermediate Level

Proton Transfer in Acid-Base Reactions

In a Bronsted-Lowry acid-base reaction, a proton (H⁺) is transferred from the acid to the base.

General reaction:

HA + B ⇌ BH⁺ + A^-

• HA = acid (donates proton)
• B = base (accepts proton)
• **BH⁺** = conjugate acid
• **A⁻** = conjugate base

Simple explanation: The acid gives a proton; the base takes it.

Conjugate Acid-Base Pairs

Simple explanation: The conjugate base is what's left after an acid gives up a proton; the conjugate acid is what forms after a base takes a proton.

Why Water Is Amphoteric

• A base — when it accepts a proton (e.g., with HCl)
• An acid — when it donates a proton (e.g., with NH₃)

This is why water is amphoteric.

Comparison of Arrhenius and Bronsted-Lowry

Important Points

• - Bronsted-Lowry theory (1923): proposed by J. N. Bronsted and T. M. Lowry.
• - Acid: proton (H⁺) donor.
• - Base: proton (H⁺) acceptor.
• - Example 1: HCl + H₂O → H₃O⁺ + Cl⁻ (HCl = acid; H₂O = base)
• - Example 2: NH₃ + H₂O → NH₄⁺ + OH⁻ (NH₃ = base; H₂O = acid)
• - Conjugate acid-base pairs: after donating a proton, an acid becomes its conjugate base; after accepting a proton, a base becomes its conjugate acid.
• - Amphoteric substances: react with both acids and bases (e.g., water).
• - Overcomes limitations of Arrhenius theory: explains CO₂ (acid) and NH₃ (base).
• - Examples from exercises:
• - H₂SO₄ + H₂O ⇌ HSO₄⁻ + H₃O⁺ (H₂SO₄ = acid; H₂O = base)
• - CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺ (CH₃COOH = acid; H₂O = base)
• - H₂S + NH₃ ⇌ NH₄⁺ + HS⁻ (H₂S = acid; NH₃ = base)

Remember

• - Bronsted-Lowry acid = proton donor.
• - Bronsted-Lowry base = proton acceptor.
• - **HCl + H₂O → H₃O⁺ + Cl⁻** (HCl = acid; H₂O = base)
• - **NH₃ + H₂O → NH₄⁺ + OH⁻** (NH₃ = base; H₂O = acid)
• - Conjugate pairs: acid ↔ conjugate base; base ↔ conjugate acid.
• - Amphoteric: reacts with both acids and bases (e.g., water).
• - Explains CO₂ as acid and NH₃ as base.

Common Mistakes

• 1. Thinking Bronsted-Lowry acids must contain H⁺. They must be able to donate a proton, but not necessarily contain H⁺ as a separate ion.
• 2. Thinking Bronsted-Lowry bases must contain OH⁻. They must be able to accept a proton. NH₃ is a base but has no OH⁻.
• 3. Confusing conjugate acid and conjugate base. After donating a proton, an acid becomes its conjugate base. After accepting a proton, a base becomes its conjugate acid.
• 4. Thinking water is always a base. Water is amphoteric — it can be an acid or a base.
• 5. Confusing proton with electron. A proton is H⁺; an electron is e⁻.
• 6. Thinking the Arrhenius and Bronsted-Lowry theories are the same. They are different — Bronsted-Lowry is more general.

Quick Revision

• Bronsted-Lowry Concept (1923):
• An acid is a proton donor.
• A base is a proton acceptor.
• Examples:
• HCl + H₂O → H₃O⁺ + Cl⁻ (HCl = acid; H₂O = base)
• H₂O + NH₃ → NH₄⁺ + OH⁻ (H₂O = acid; NH₃ = base)
• Overcomes limitations of Arrhenius theory: explains why NH₃ is a base and why CO₂ is an acid.
• Water is amphoteric — it can act as both an acid and a base.
• Amphoteric substances: substances that react with both acids and bases.
• Conjugate acid-base pairs: when an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.
• Exercises:
• H₂SO₄ + H₂O ⇌ HSO₄⁻ + H₃O⁺ (H₂SO₄ = acid; H₂O = base)
• CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺ (CH₃COOH = acid; H₂O = base)
• H₂S + NH₃ ⇌ NH₄⁺ + HS⁻ (H₂S = acid; NH₃ = base)

60-Second Revision

• Bronsted-Lowry acid = proton donor.
• Bronsted-Lowry base = proton acceptor.
• **HCl + H₂O → H₃O⁺ + Cl⁻** (HCl = acid; H₂O = base)
• **NH₃ + H₂O → NH₄⁺ + OH⁻** (NH₃ = base; H₂O = acid)
• Amphoteric: water is amphoteric.
• Warning: Bronsted-Lowry acids don't have to contain H⁺ as a separate ion; they just have to donate a proton.