Chapter 9: Unit 3. Conjugate Acid – Base Pairs

Conjugate Acid-Base Theory

Bronsted – Lowry Acids and Bases when reacting, they form conjugate Acid – Base Pairs. Let us look at the Acid – Base Reaction below:

HBr(aq) + H 2 O(l) à H 3 O + (aq) + Br–(aq)

HBr is considered as Bronsted – Lowry Acid (H + Donor)

H 2 O(l) is considered as Bronsted – Lowry Base (H + Acceptor)

H3O + is Conjugate Acid

Br– is Conjugate Base

H 3 O + is a conjugate Acid: It was a base (H 2 O) that gained H + .

Br– is a conjugate Base: It was an acid (HBr) that lost H + .

Another Example: https://www.quora.com/What–is–conjugate–acid–base–pair

A list of Acids and their Conjugate bases are listed below:

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/conjugat.php#pairs

Typical Bronsted Acids and Their Conjugate Bases

A video of You Tube illustrate the concept of Conjugate Acid – Base Pairs:

Amphoteric Compounds:

Amphoteric compounds are compounds can act as an acid or as a base depending on the reaction they are participating in.

Examples of such amphoteric compounds are given amphoteric compounds:

H 2 Ois an amphoteric compound:

H 2 O as an acid:

H 2 O(l) + NH 3 (aq) ⇔ NH 4 + (aq) + OH–(aq)

H 2 O as a base:

H 2 O(l) + HCl ⇔ H 3 O + (aq) + Cl–(aq)

HCO 3–is an amphoteric compound:

HCO 3–as an acid:

HCO 3–(aq) + NH 3 (aq) ⇔NH 4 + (aq) + CO 3 2–(aq)

HCO3–as a base:

HCO 3–(aq) + H 2 O(l) ⇔ H 2 CO 3 (aq) + OH–(aq)

H 2 PO 4–is an amphoteric compound:

H 2 PO 4–as an acid:

H 2 PO 4–(aq) + NH 3 (aq) ⇔ NH 4 + (aq) + HPO 4 2–(aq)

H 2 PO 4–as a base:

H 2 PO 4–(aq) + H 2 O(l) ⇔ H 3 PO 4 (aq) + OH–(aq)