ChemistryChapter 9: Acids – Bases Chemistry
9.1 Conjugate Acid-Base Pairs
Conjugate Acid:
Explanation level
This topic explains conjugate acid-base pairs. It describes how acids and bases are related by the transfer of a proton and how to identify conjugate pairs in acid-base reactions.
Definition
Conjugate Acid: A species formed as a result of the acceptance of a proton by a base.
Conjugate Base: The species that remains after donating a proton from an acid.
Main Idea
In an acid-base reaction, the acid forms a base (conjugate) and the base, having accepted a proton, forms a conjugate acid. The conjugate acid-base pairs are species on opposite sides of an equation that differ by a proton. The weaker acids have stronger conjugate bases and stronger acids have weaker conjugate bases.
Core Concept
- Conjugate acid-base pair: Two species that differ by one proton (H⁺).
- Acid → Conjugate base: Acid donates a proton.
- Base → Conjugate acid: Base accepts a proton.
- Strong acid → Weak conjugate base.
- Strong base → Weak conjugate acid.
Detailed Explanation
What is a Conjugate Acid-Base Pair?
- A conjugate acid is a species formed as a result of the acceptance of a proton by a base.
- A conjugate base is the species that remains after donating a proton from an acid.
- In an acid-base reaction, the acid forms a base (conjugate) and the base, having accepted a proton, forms a conjugate acid.
- The conjugate acid-base pairs are species on opposite sides of an equation that differ by a proton.
- The weaker acids have stronger conjugate bases and stronger acids have weaker conjugate bases.
Do You Know?
A strong acid produces a relatively weak conjugate base. Likewise a strong base produces a relatively weak conjugate acid.
General Acid-Base Reaction
A1 + B2 → B1 + A2
- A1 = Brønsted acid
- B2 = Brønsted base
- B1 = Conjugate base
- A2 = Conjugate acid
Examples
1. Ionization of HCl in Water:
HCl + H2O ⇌ H3O+ + Cl-
- Acid: HCl
- Base: H₂O
- Conjugate acid: H₃O⁺
- Conjugate base: Cl⁻
- HCl / Cl⁻
- H₂O / H₃O⁺
2. Ionization of NH₄OH in Water:
H3N: + H2O ⇌ NH4+ + OH-
- Base: NH₃
- Acid: H₂O
- Conjugate acid: NH₄⁺
- Conjugate base: OH⁻
- NH₃ / NH₄⁺
- H₂O / OH⁻
3. Ionization of H₂O:
H2O + H2O ⇌ H3O+ + OH-
- Base: H₂O (one molecule)
- Acid: H₂O (another molecule)
- Conjugate acid: H₃O⁺
- Conjugate base: OH⁻
- H₂O / OH⁻
- H₂O / H₃O⁺
Amphoteric Nature of Water
- Water is amphoteric in nature, i.e., it is acidic as well as basic in nature.
- It can act as an acid (donate a proton) or as a base (accept a proton).
Intermediate Level
Conjugate Acid-Base Pairs
| Species | Role | Conjugate |
|---|---|---|
| HCl | Acid | Cl⁻ (conjugate base) |
| H₂O | Base | H₃O⁺ (conjugate acid) |
| NH₃ | Base | NH₄⁺ (conjugate acid) |
| H₂O | Acid | OH⁻ (conjugate base) |
Examples
1. HCl in water: HCl + H2O ⇌ H3O+ + Cl- - Pairs: HCl/Cl⁻, H₂O/H₃O⁺
2. NH₃ in water: NH3 + H2O ⇌ NH4+ + OH- - Pairs: NH₃/NH₄⁺, H₂O/OH⁻
3. Water ionization: H2O + H2O ⇌ H3O+ + OH- - Pairs: H₂O/OH⁻, H₂O/H₃O⁺
Amphoteric Nature
- Water is amphoteric (acidic and basic).
Key Points
- Conjugate pairs differ by one H⁺.
- Strong acid → weak conjugate base.
- Strong base → weak conjugate acid.
Important Points
- 1. Conjugate acid: Formed when base accepts H⁺.
- 2. Conjugate base: Remains after acid donates H⁺.
- 3. Conjugate pairs differ by one H⁺.
- 4. Strong acid → weak conjugate base.
- 5. Strong base → weak conjugate acid.
- 6. Water is amphoteric.
- 7. Examples: HCl/H₂O, NH₃/H₂O, H₂O/H₂O.
Remember
- - Conjugate acid: Base + H⁺
- - Conjugate base: Acid − H⁺
- - Pairs differ by one H⁺
- - Strong acid → weak conjugate base
- - Strong base → weak conjugate acid
- - Water is amphoteric
Common Mistakes
- 1. Forgetting that conjugate pairs differ by one H⁺: They differ by exactly one proton.
- 2. Mixing up conjugate acid and base: Acid → conjugate base; Base → conjugate acid.
- 3. Thinking strong acids have strong conjugate bases: Strong acids have weak conjugate bases.
- 4. Forgetting water is amphoteric: It can act as both acid and base.
Quick Revision
- Conjugate acid: Formed when base accepts H⁺.
- Conjugate base: Remains after acid donates H⁺.
- Conjugate pairs differ by one H⁺.
- Strong acid → weak conjugate base.
- Strong base → weak conjugate acid.
- Water is amphoteric.
- Examples: HCl/H₂O, NH₃/H₂O, H₂O/H₂O.
60-Second Revision
- Conjugate acid: Base + H⁺
- Conjugate base: Acid − H⁺
- Pairs differ by one H⁺
- Strong acid → weak conjugate base
- Strong base → weak conjugate acid
- Water is amphoteric
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- Chapter: 9 (Acids-Bases Chemistry)
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Sticky Notes · Quick Revision
Key Point 1
- Conjugate acid: Base + H⁺
Key Point 2
- Conjugate base: Acid − H⁺
Key Point 3
- Pairs differ by one H⁺
Key Point 4
- Strong acid → weak conjugate base
Key Point 5
- Strong base → weak conjugate acid
Key Point 6
- Water is amphoteric
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