Rate Constant Units Mnemonics-Easy to memorize | Kinetics Guide
Mnemonic: “1-2-3, Power Down!”
Remember the exponent on M decreases by 1 as the order increases:
Order 0 → M⁺¹ | Order 1 → M⁰ | Order 2 → M⁻¹ | Order 3 → M⁻²
(Always multiplied by t⁻¹)
M = mole dm-3
Units & Explanation:
| Order | Rate Law | Units of Rate Constant (k) | Mnemonic Phrase |
|---|---|---|---|
| 0 | rate = k | M t⁻¹ (e.g., mol L⁻¹ s⁻¹) | “Zero? M¹ per time!“ (Concentration appears once) |
| 1 | rate = k[A] | t⁻¹ (e.g., s⁻¹) | “First? Time inverse!“ (No concentration) |
| 2 | rate = k[A]² | M⁻¹ t⁻¹ (e.g., L mol⁻¹ s⁻¹) | “Second? Per-M per time!“ (Concentration in denominator once) |
| 3 | rate = k[A]³ | M⁻² t⁻¹ (e.g., L² mol⁻² s⁻¹) | “Third? Per-M² per time!“ (Concentration in denominator twice) |
Key Memory Aids:
- Exponent Rule:
Exponent on M = 1 – Order
*(e.g., Order 2 → 1–2 = –1 → M⁻¹)* - Time: Always t⁻¹ (per second/minute).
- Pattern: Units progress as M⁺¹ → M⁰ → M⁻¹ → M⁻² for orders 0→3.
✅ Quick Recap:
- 0 = M⁺¹/t | 1 = 1/t | 2 = 1/(M·t) | 3 = 1/(M²·t)
- Remember: “Higher order, lower exponent (1–order)!”
