Read the Order Before Picking a Rate Equation
Every rate-law question starts with the same fork: is this zero, first or second order? The shape of a concentration-vs-time plot, or the units of the rate constant, tell you the order before you touch an equation.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Reading order from a concentration-time graph
A straight-line ln[A] vs t plot signals first order; a straight-line [A] vs t plot (linear decay) signals zero order; a straight-line 1/[A] vs t plot signals second order — match the graph shape to the order before selecting an integrated rate law.
Units of k reveal the order directly
Rate constant units change with order: first order k has units of (time)⁻¹; zero order k has units of concentration/time; second order k has units of concentration⁻¹ time⁻¹ — if a question gives you the units of k, you can read off the order without any other data.
First-order time and half-life calculations
radioactive decay problems (comparing two isotopes' activities over time) use exactly this same first-order framework.
OlympIQ exam lens
- ✓ln[A] vs t linear → first order; [A] vs t linear → zero order; 1/[A] vs t linear → second order.
- ✓Units of k alone identify the reaction order.
- ✓First-order half-life is constant and independent of starting concentration — use this to compare two decaying species directly.
Don't assume a reaction is first order by default — check the units of k or the linear graph shape given in the question before applying the first-order integrated rate law.
Ready to try the pattern?
100% coverage across the 9 linked questions.
Practise 9 linked questions