Read the Arrhenius Equation Before Comparing Rate Constants
Whenever two rate constants or two parallel pathways are compared, the Arrhenius equation k = A e^(-Ea/RT) is the tool — the ratio of two k values depends on the ratio (or difference) of their activation energies at a fixed temperature.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Relating rate constants for parallel reactions with different Ea
For two parallel pathways with Ea₂ = n × Ea₁ at the same temperature, the ratio k1/k₂ comes directly from substituting both into the Arrhenius equation and dividing — the pre-exponential factors matter too unless the question states they're equal.
Zero-order vs first-order behaviour from a plotted curve
An exponentially decaying concentration-vs-time curve with two distinct linear-looking regions can represent different kinetics in each zone (e.g. zero order early, first order later) — read the shape in each region separately rather than assuming one order applies throughout the whole curve.
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- ✓k = A e^(-Ea/RT) — a ratio of two rate constants at the same T depends on the ratio of A and the difference in Ea.
- ✓A single concentration-time curve can show different kinetic orders in different regions — check each region's shape independently.
Don't assume the pre-exponential factor A is identical for two different reactions unless the question says so — it affects the k1/k₂ ratio just as much as the activation energy difference does.
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