Buffers, pH curves and indicators (HL)
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All 14 Flashcards — Buffers, pH curves and indicators (HL)
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Question
What is a buffer solution?
Answer
A solution that **resists pH change** when a small amount of acid or alkali is added.
Question
What two components make an acidic buffer?
Answer
A **weak acid + its conjugate base** (e.g. ethanoic acid + sodium ethanoate), both present in appreciable amounts.
Question
What two components make a basic buffer?
Answer
A **weak base + its conjugate acid** (e.g. ammonia + ammonium chloride).
Question
How does a buffer mop up added H⁺?
Answer
The **conjugate base** reacts with it: A⁻ + H⁺ → HA (so the ratio [A⁻]/[HA] barely changes).
Question
How does a buffer mop up added OH⁻?
Answer
The **weak acid** reacts with it: HA + OH⁻ → A⁻ + H_{2}O.
Question
Henderson–Hasselbalch equation?
Answer
$\text{pH} = \text{p}K_a + \log_{10}\dfrac{[A^-]}{[HA]}$ — buffer pH from pK_{a} and the base/acid ratio.
Question
When does a buffer have pH = pK_{a}?
Answer
When **[A⁻] = [HA]** (equal amounts), so the log term is zero — the **half-equivalence point**.
Question
Equivalence-point pH of a strong acid–strong base titration?
Answer
**Exactly 7** — the salt formed is neutral.
Question
Equivalence-point pH of a weak acid–strong base titration?
Answer
**Greater than 7** — the salt of a weak acid hydrolyses to a basic solution.
Question
Equivalence-point pH of a strong acid–weak base titration?
Answer
**Less than 7** — the salt of a weak base hydrolyses to an acidic solution.
Question
What is an acid–base indicator, chemically?
Answer
A **weak acid**, HIn ⇌ H⁺ + In⁻, whose acid form and conjugate base are **different colours**.
Question
How do you choose a suitable indicator?
Answer
Pick one whose **colour-change range (≈ pK_{a}(In)) lies inside the vertical jump** at the equivalence point.
Question
Best indicator for weak acid–strong base (equivalence > 7)?
Answer
**Phenolphthalein** (range 8.3–10.0) — it changes within the basic vertical jump.
Question
Best indicator for strong acid–weak base (equivalence < 7)?
Answer
**Methyl orange** (range 3.1–4.4) — it changes within the acidic vertical jump.
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Full study notes for Buffers, pH curves and indicators (HL)
Topic 6.1 hub
Proton transfer reactions
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