๐ Acid-Base Titration Curve Types
Unit 8 | Topic 9 – Half-Equivalence Points & Curve Logic
๐ Four Types of Titration Curves
These are the key acid-base pairings you’ll see in titrations, each with unique curves and equivalence behavior:
- ๐ด Strong Acid + Strong Base
- ๐ก Weak Acid + Strong Base
- ๐ต Weak Base + Strong Acid
- ⚪ Weak Acid + Weak Base (rare!)
1️⃣ Strong Acid + Strong Base
HCl + NaOH → NaCl + H₂O
- Starts low (pH ≈ 1)
- Sharp rise near midpoint
- Equivalence point = pH 7
- Ends high (strong base excess)
2️⃣ Weak Acid + Strong Base
CH₃COOH + NaOH → CH₃COONa + H₂O
- Starts higher (pH ≈ 3)
- Buffer region develops (acid + conjugate base)
- Half-equivalence point: [HA] = [A⁻] → pKa = pH
- Equivalence point > 7 (basic solution)
3️⃣ Weak Base + Strong Acid
NH₃ + HCl → NH₄Cl
- Starts high (pH ≈ 11)
- Falls slowly (buffer zone: NH₃ + NH₄⁺)
- Half-equivalence point: [B] = [BH⁺] → pKb = pOH
- Equivalence point < 7 (acidic)
4️⃣ Weak Acid + Weak Base
CH₃COOH + NH₃ → CH₃COONH₄
- Starts around pH ≈ 3–4 or 10–11
- Very gradual pH change
- Equivalence point ≈ 7, but hard to locate
- Curve is very smooth, almost no vertical jump
❌ Not used in practice — hard to measure due to no sharp inflection!
๐ฏ Half-Equivalence Points
- Happens at the midpoint of the buffer region
- For Weak Acid: pH = pKa when [HA] = [A⁻]
- For Weak Base: pOH = pKb when [B] = [BH⁺]
- Excellent spot for calculating acid/base strength!
⚠️ Common Drawing Mistakes in Titration Curves
- ๐ Forgetting buffer regions in weak acid/base titrations
- ๐ป Placing equivalence at pH 7 even when it's wrong! (Only true for strong + strong)
- ๐ Too steep or too flat vertical rise — should reflect acid/base strength
- ๐ Misplacing half-equivalence point — it’s always before the vertical jump!
- ๐️ Flat lines drawn as verticals — be realistic! Weak titrations are smooth!
๐ก Pro Tip: Use different indicators depending on where the equivalence point lies. Don’t just use phenolphthalein for every titration!
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