Understanding Acids and Bases: Arrhenius and Brønsted-Lowry Definitions, Study notes of Chemistry

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Three Definitions

of Acids and

Bases

• Arrhenius

• An acid is a substance that generates H 3

O

in water

• A base is a substance that generates OH - in water

• Brønsted-Lowry

• Lewis

NH

3 ( aq) + HF( aq) ⇌ NH 4

( aq) + F − ( aq) base acid H 2 O(l) + HF( aq) ⇌ H 3

O

( aq) + F − ( aq) neutral acid NH 3 ( aq) + H 2 O(l) ⇌ NH 4

( aq) + OH − ( aq) base neutral

Arrhenius Acid-

Base Reactions?

Brønsted-Lowry

Acids and Bases

NH

3 ( aq) + HF( aq) ⇌ NH 4

( aq) + F − ( aq) base acid H 2 O(l) + HF( aq) ⇌ H 3

O

( aq) + F − ( aq) base acid NH 3 ( aq) + H 2 O(l) ⇌ NH 4

( aq) + OH − ( aq) base acid

Why Two Definitions for

Acids and Bases? (1)

• Positive Aspects of Arrhenius Definitions
  • All isolated substances can be classified as acids (generate H 3 O + in water), bases (generate OH - in water), or neither.
  • Allows predictions, including (1) whether substances will react with a base or acid, (2) whether the pH of a solution of the substance will be less than 7 or greater than 7, and (3) whether a solution of the substance will be sour or bitter.
• Negative Aspects of Arrhenius Definitions
  • Does not include similar reactions (H
    • transfer reactions) as acid-base reactions.

Conjugate Acid-

Base Pairs

NH

3 ( aq ) + HF( aq ) ⇌ NH 4

( aq ) + F − ( aq ) base acid acid base

Brønsted-Lowry

Acids and Bases

H

2

PO

4 − ( aq) + HF( aq) ⇌ H 3

PO

4 ( aq) + F − ( aq) base acid acid base

• H 3

PO

4 is the conjugate acid of H 2

PO

4 − .

• H 2

PO

4 − is the conjugate base of H 3

PO

4

• H

3

PO

4 and H 2

PO

4 − are a conjugate acid- base pair.

• F − is the conjugate base of the acid HF.
• HF is the conjugate acid of the acid F − .
• HF and F − are a conjugate acid-base pair.