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Reactions and buffers

Mix an acid and a base and the pH afterward is not automatically 7 — it depends on what is left standing. And a special leftover pair, a weak acid with its conjugate base, becomes a buffer that fights pH change.

§1

Neutralization and buffers.

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When an acid and base react, they neutralize each other, but the final pH depends on what remains. Excess strong acid leaves an acidic solution; excess strong base, a basic one; an exact match may or may not be neutral.

A buffer is a weak acid together with its conjugate base (or a weak base with its conjugate acid). It resists pH change: added acid is absorbed by the conjugate base, and added base by the weak acid.

So a buffer forms, for example, when a weak acid is partially neutralized — leaving both the weak acid and its conjugate base in solution together.

UNIT 8 TOPIC 8.4 • ACID-BASE REACTIONS AND BUFFERS BUFFER ACTION A buffer holds appreciable amounts of BOTH a weak acid (HA) and its conjugate base (A⁻) that coexist in solution. BUFFER RESERVOIR weak acid HA HA HA HA HA conjugate base A⁻ A⁻ A⁻ A⁻ A⁻ Both members of the conjugate pair are present at the same time. ADD STRONG ACID (extra H⁺) A⁻ + H⁺ → HA The conjugate base soaks up the added H⁺, forming HA. ADD STRONG BASE (extra OH⁻) HA + OH⁻ → A⁻ + H₂O The weak acid soaks up the added OH⁻, forming A⁻ and water. RESULT The pH changes far less than it would in pure water — the buffer resists change. CED ANCHOR Buffers keep pH nearly constant by using an equilibrium reservoir of a conjugate acid–base pair (TRA-3): each member neutralizes one type of added strong species. A buffer can equally well be made from a weak base and its conjugate acid. AP Chemistry · Unit 8 · Acids and Bases
Fig. 8.4.1 A buffer is a weak acid paired with its conjugate base. Added acid is neutralized by the conjugate base, and added base by the weak acid. Predicting the pH after a neutralization requires tracking what remains.
§2

Finding the pH after mixing.

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Track what is left, then decide the pH.

  1. React the acid and base. Determine how many moles of each are consumed in neutralization.
  2. Identify what remains. Excess acid, excess base, or a weak acid plus its conjugate base (a buffer)?
  3. Decide the pH regime. Excess strong acid/base sets an extreme pH; a buffer sets a pH near the pKa.
  4. Do not assume neutral. A mixture is neutral only if nothing acidic or basic remains in excess.
§3

The pieces you'll meet.

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What remains sets the pH.

neutralize
Neutralization
Acid and base react, consuming each other.
remaining
What remains
Sets the final pH.
buffer
Buffer
Weak acid + conjugate base; resists pH change.
absorb acid
Absorbs acid
The conjugate base neutralizes added acid.
absorb base
Absorbs base
The weak acid neutralizes added base.
not neutral
Not always neutral
A mixed solution need not be pH 7.
§4

Worked example: partially neutralize a weak acid.

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Setup. Add some strong base to a weak acid HA, but not enough to consume it all.

Reaction. The base converts part of the HA into its conjugate base A⁻.

Result. The solution now contains both HA and A⁻ — a buffer.

Behavior. This buffer resists pH change: add more acid and A⁻ soaks it up; add more base and HA soaks it up. The pH is not simply 7; it sits near the acid's pKa.

§5

Mistakes that cost real points.

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Pitfall · 01

"After any acid-base mixing, the solution is neutral (pH 7)."

The final pH depends on what remains. Excess strong acid leaves it acidic, excess strong base leaves it basic, and even at exact equivalence a strong-weak mixture is not neutral. Only a strong-strong exact match gives pH 7.

Fix. Track the leftover species after neutralization; do not assume pH 7.

Pitfall · 02

"A buffer is just a weak acid by itself."

A buffer requires both a weak acid and its conjugate base present together. The weak acid alone can neutralize added base, but without the conjugate base it cannot absorb added acid — so it is not a buffer.

Fix. Ensure both the weak acid and its conjugate base are present for a buffer.

Pitfall · 03

"A buffer stops the pH from changing at all."

A buffer resists but does not entirely prevent pH change; adding acid or base shifts the pH slightly, and enough will overwhelm it. 'Resists' means small change, not none.

Fix. Treat a buffer as minimizing pH change, not eliminating it.

§6

Skill Check.

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