Chemical Kinetics — Rate Laws Mnemonic
Target: mnemonic for chemical kinetics rate law order of reaction
Why is this hard to memorize?
Chemical kinetics tells you HOW FAST a reaction occurs — complementing thermodynamics which tells you IF it occurs. NEET and JEE test: order of reaction (0th, 1st, 2nd), rate law expressions, integrated rate equations, half-life formulas, and Arrhenius equation (k = Ae^(-Ea/RT)). The most important distinction is that reaction ORDER is determined experimentally (not from balanced equation), and the half-life relationships: t₁/₂ for first-order = 0.693/k (independent of concentration), while zero-order t₁/₂ = [A]₀/2k.
Classic mnemonics you should know
"0th: t½ = [A]₀/2k. 1st: t½ = 0.693/k. 2nd: t½ = 1/(k[A]₀)"
Zero order: half-life depends on initial concentration. First order: half-life is CONSTANT (doesn't depend on concentration — this is the defining feature). Second order: half-life inversely depends on concentration.
"k = (2.303/t) × log([A]₀/[A]) — "Radioactive decay is always first-order""
All radioactive decay follows first-order kinetics. The integrated rate equation uses logarithm. If a graph of ln[A] vs time is linear → first order. This is tested as a graph-interpretation question.
"k = Ae^(-Ea/RT) — "Higher T or Lower Ea → faster reaction""
k increases exponentially with temperature and decreases with activation energy. Rule of thumb: for every 10°C rise, rate roughly doubles. The Arrhenius equation explains WHY temperature affects reaction rate.
The complete list
- Rate = k[A]ⁿ (rate law)
- Zero order: rate = k
- First order: rate = k[A]
- Second order: rate = k[A]²
- t½ (1st order) = 0.693/k
- Arrhenius: k = Ae^(-Ea/RT)
- Order determined experimentally
- 10°C rise ≈ doubles rate
Frequently asked questions
How do I determine the order of a reaction from experimental data?
Use the initial rates method: compare experiments where one reactant concentration changes while others are constant. If doubling [A] doubles rate → first order in A. If doubling [A] quadruples rate → second order. If rate doesn't change → zero order in A. Or plot graphs: [A] vs t linear → zero order. ln[A] vs t linear → first order. 1/[A] vs t linear → second order.
Why is first-order half-life independent of concentration?
For first-order reactions, t½ = 0.693/k. This formula has no [A] term! Physically, it means the fraction that decomposes per unit time is constant — whether you start with 100 g or 1 g, 50% will decompose in the same time. This is unique to first-order reactions and is why radioactive half-lives are constant.
What is the difference between rate and rate constant?
Rate is the speed of reaction (mol/L/s) — it changes as concentrations change during the reaction. Rate constant (k) is fixed at a given temperature — it only changes with temperature (via Arrhenius). Rate = k × [concentrations]^order. Think of k as the "speed limit" and rate as the "actual speed."
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