10.1.1 Arrhenius Concept of Acids and Bases

This topic explains the Arrhenius theory — the first successful theory of acids and bases, proposed by Svante Arrhenius in 1887.

Definition

Arrhenius Acid

An acid is a substance that ionizes in water to produce H⁺ ions.

Simple explanation: An Arrhenius acid releases hydrogen ions (H⁺) when dissolved in water.

Arrhenius Base

A base is a substance that ionizes in water to produce OH⁻ ions.

Simple explanation: An Arrhenius base releases hydroxide ions (OH⁻) when dissolved in water.

Main Idea

In 1887, a Swedish chemist Svante Arrhenius proposed the first successful theory of acids and bases.

• An acid is a substance that ionizes in water to produce H⁺ ions.
• A base is a substance that ionizes in water to produce OH⁻ ions.

Simple explanation: Arrhenius said acids produce H⁺ in water, and bases produce OH⁻ in water.

Core Concept

Key Points of the Arrhenius Theory

Examples from the Book

HCl_(g) H₂O→ H⁺_(aq) + Cl^-_(aq)

NaOH_(s) H₂O→ Na⁺_(aq) + OH^-_(aq)

More examples:

HNO_3(l) H₂O→ H⁺_(aq) + NO^-_3(aq)

H₂SO_4(l) H₂O→ 2H⁺_(aq) + SO^2-_4(aq)

KOH_(s) H₂O→ K⁺_(aq) + OH^-_(aq)

NH₄OH_(aq) H₂O→ NH⁺_4(aq) + OH^-_(aq)

Simple explanation: These equations show acids producing H⁺ and bases producing OH⁻ in water.

Detailed Explanation

What Is Ionization?

Ionization is the process by which a substance separates into ions when dissolved in water.

• Acids ionize to produce H⁺ ions.
• Bases ionize to produce OH⁻ ions.

Simple explanation: When acids and bases dissolve in water, they break apart into charged particles called ions.

Examples of Arrhenius Acids

Hydrochloric acid (HCl):

HCl_(g) H₂O→ H⁺_(aq) + Cl^-_(aq)

Nitric acid (HNO₃):

HNO_3(l) H₂O→ H⁺_(aq) + NO^-_3(aq)

Sulphuric acid (H₂SO₄):

H₂SO_4(l) H₂O→ 2H⁺_(aq) + SO^2-_4(aq)

Simple explanation: All these acids produce H⁺ ions in water.

Examples of Arrhenius Bases

Sodium hydroxide (NaOH):

NaOH_(s) H₂O→ Na⁺_(aq) + OH^-_(aq)

Potassium hydroxide (KOH):

KOH_(s) H₂O→ K⁺_(aq) + OH^-_(aq)

Ammonium hydroxide (NH₄OH):

NH₄OH_(aq) H₂O→ NH⁺_4(aq) + OH^-_(aq)

Simple explanation: All these bases produce OH⁻ ions in water.

Limitations of the Arrhenius Theory

The Arrhenius theory has its limitations:

1. It applies only to aqueous solutions. It does not explain acid-base behaviour in non-aqueous solvents.

• There is no H in CO₂ — yet CO₂ is acidic.
• CO₂ reacts with water to form carbonic acid (H₂CO₃), which produces H⁺.

• There is no OH in NH₃ — yet NH₃ is basic.
• NH₃ accepts a proton from water to form NH₄⁺ and OH⁻.

Simple explanation: The Arrhenius theory couldn't explain all acids and bases — especially those without H⁺ or OH⁻ directly.

Why the Arrhenius Theory Was Important

• The first successful theory of acids and bases.
• Able to explain the behaviour of many common acids and bases.
• The foundation for later theories (Bronsted-Lowry, Lewis).

Simple explanation: Arrhenius was the first to explain acids and bases in terms of ions — a huge step forward.

Intermediate Level

Ionization vs Dissociation

• Ionization: When a molecular substance (like HCl) forms ions in water.
• Dissociation: When an ionic compound (like NaOH) separates into ions in water.

Simple explanation: Both processes produce ions, but ionization starts with molecules, while dissociation starts with ionic compounds.

Strong vs Weak Arrhenius Acids

Strong acids ionize completely in water: - HCl, HNO₃, H₂SO₄

Weak acids ionize partially in water: - CH₃COOH (acetic acid)

Note: This is covered in more detail in Section 10.2.

Strong vs Weak Arrhenius Bases

Strong bases ionize completely in water: - NaOH, KOH, Ca(OH)₂

Weak bases ionize partially in water: - NH₄OH, Al(OH)₃

Limitations in Detail

Simple explanation: The Arrhenius theory works for common acids and bases but fails for some important cases.

How Bronsted-Lowry Solved These Problems

• Acid = proton donor
• Base = proton acceptor

This explained why CO₂ is acidic (it donates a proton) and why NH₃ is basic (it accepts a proton).

Important Points

• - Arrhenius theory (1887): proposed by Svante Arrhenius.
• - Arrhenius acid: substance that ionizes in water to produce H⁺ ions.
• - Arrhenius base: substance that ionizes in water to produce OH⁻ ions.
• - Examples of acids: HCl, HNO₃, H₂SO₄.
• - Examples of bases: NaOH, KOH, NH₄OH.
• - Limitations:
• - Applies only to aqueous solutions.
• - Cannot explain why CO₂, SO₂ are acidic (no H).
• - Cannot explain why NH₃ is basic (no OH).
• - Importance: first successful theory; foundation for later theories.

Remember

• - Arrhenius acid → H⁺ in water.
• - Arrhenius base → OH⁻ in water.
• - Examples:
• - HCl → H⁺ + Cl⁻
• - NaOH → Na⁺ + OH⁻
• - Limitations:
• - Only aqueous solutions.
• - Cannot explain CO₂ (acid) or NH₃ (base).
• - Bronsted-Lowry solved these limitations.

Common Mistakes

• 1. Thinking all acids contain H⁺ and all bases contain OH⁻. CO₂ is acidic but has no H⁺; NH₃ is basic but has no OH⁻.
• 2. Thinking the Arrhenius theory explains everything. It only applies to aqueous solutions.
• 3. Confusing ionization and dissociation. Ionization starts with molecules; dissociation starts with ionic compounds.
• 4. Thinking all acids are strong. Some acids (like acetic acid) are weak.
• 5. Thinking all bases are strong. Some bases (like NH₄OH) are weak.
• 6. Forgetting the limitations of the Arrhenius theory. This led to the development of the Bronsted-Lowry theory.

Quick Revision

• Arrhenius Concept (1887):
• An acid is a substance that ionizes in water to produce H⁺ ions.
• A base is a substance that ionizes in water to produce OH⁻ ions.
• Examples:
• HCl(g) → H⁺(aq) + Cl⁻(aq)
• NaOH(s) → Na⁺(aq) + OH⁻(aq)
• HNO₃(l) → H⁺(aq) + NO₃⁻(aq)
• H₂SO₄(l) → 2H⁺(aq) + SO₄²⁻(aq)
• KOH(s) → K⁺(aq) + OH⁻(aq)
• NH₄OH(aq) → NH₄⁺(aq) + OH⁻(aq)
• Limitations:
• Applies only to aqueous solutions.
• Does not explain why CO₂, SO₂ are acidic (no H).
• Does not explain why NH₃ is basic (no OH).
• Importance: first successful theory of acids and bases; foundation for later theories.

60-Second Revision

• Arrhenius acid → H⁺ in water.
• Arrhenius base → OH⁻ in water.
• Examples: HCl, NaOH.
• Limitations: only aqueous; cannot explain CO₂ (acid) or NH₃ (base).
• Warning: the Arrhenius theory does not explain all acids and bases — Bronsted-Lowry is more general.