chem unit 2 lect 5 ai notes

Buffer

A solution containing both members of a weak conjugate pair (HA/A⁻ or B/BH⁺) that resists pH change.

Conjugate Pair

A weak acid and its conjugate base (HA/A⁻) or a weak base and its conjugate acid (B/BH⁺).

Henderson–Hasselbalch Equation (acid form)

pH = pKa + log([A⁻]/[HA])

Henderson–Hasselbalch Equation (base form)

pOH = pKb + log([BH⁺]/[B])

Half-Equivalence Point

The point in a titration where [acid] = [conjugate base]; pH = pKa.

Buffer Region

The section of a titration curve before equivalence where both acid and conjugate base coexist.

Relationship Between Weak Acid and Strong Base Titration

Before equivalence → HA/A⁻ buffer; at half-equivalence → pH = pKa.

Relationship Between Weak Base and Strong Acid Titration

Before equivalence → B/BH⁺ buffer; at half-equivalence → pH = 14 − pKb.

Buffer Capacity

The amount of strong acid or base a buffer can neutralize before its pH changes significantly.

Limiting Factor for Buffer Capacity

The number of moles of each buffer component (acid/base); once one is used up, buffering stops.

When Adding Strong Acid to a Buffer

H⁺ reacts with and consumes the conjugate base A⁻, forming more HA.

When Adding Strong Base to a Buffer

OH⁻ reacts with and consumes HA, forming more A⁻.

Condition for Maximum Buffer Capacity

When [HA] ≈ [A⁻]; at the half-equivalence point.

Effect of Concentration on Buffer Capacity

Higher total concentrations increase capacity; dilute buffers have low capacity.

Recognizing a Buffer after Neutralization

A weak acid/base and its conjugate coexist in the same beaker after partial neutralization.

Forming a Buffer from Mixing

A weak acid + its conjugate base (like HCOOH + NaHCOO) or a weak base + its conjugate acid (NH₃ + NH₄Cl).

Buffer Formed During Titration

Stopping before equivalence in a weak acid–strong base or weak base–strong acid titration.

Buffer Breakdown Beyond Equivalence

Point where one conjugate partner is gone; only one species controls pH.

Buffer Check Question

Do I have a weak species and its conjugate in meaningful amounts? If yes → buffer.

NH₄⁺ and F⁻ Mixture

Not a buffer; they are not conjugate to each other.

HCOOH and HCOONa Mixture

A buffer; formic acid and formate are conjugate acid–base pair.

Effective Buffer Ratio Range

[A⁻]/[HA] between 0.1 and 10 gives effective buffering.

pH of Buffer with Base:Acid Ratio 2

pH = pKa + log(2) ≈ pKa + 0.30.

Condition for Buffer Formation after Neutralization

Weak acid/base and its conjugate remain after strong acid/base reaction.

Buffer Example with Ba(OH)₂ and Acetic Acid

Ba(OH)₂ neutralizes half of the weak acid → leaves HA and A⁻ → buffer formed.

Mixture of Weak Acid and Weak Base from Different Systems

Not a buffer; no conjugate relationship.

H₂SO₃ and NH₃ Reaction

All NH₃ consumed → no conjugate pair remains → not a buffer.

(CH₃)₂NH₂⁺ + HCl Mixture

Two acids, no weak base → not a buffer.

Polyprotic Acid

An acid that can donate more than one proton (e.g., H₂SO₄, H₃PO₄, H₂CO₃).

Diprotic Acid

Can donate two protons (e