Rate Law Order Calculator
Find the reaction order in a reactant from two experiments — the initial concentration and initial rate for each.
How it works
order n = ln(rate₂ ÷ rate₁) ÷ ln([A]₂ ÷ [A]₁)
For a rate law rate = k[A]ⁿ, the order n is the exponent that tells you how sensitively the rate responds to concentration. Comparing two experiments where only [A] changes, the ratio of rates equals the ratio of concentrations raised to n. Taking logs of both sides isolates n as the log of the rate ratio divided by the log of the concentration ratio; the answer normally rounds to a small whole number or a simple fraction.
Worked example
Doubling [A] from 0.1 M to 0.2 M raises the rate from 0.02 M/s to 0.08 M/s — a fourfold increase. n = ln(0.08 ÷ 0.02) ÷ ln(0.2 ÷ 0.1) = ln 4 ÷ ln 2 = 2, so the reaction is second order in A.
Sources: Rate equation — Wikipedia
Frequently asked questions
Why must the two concentrations differ?
The method reads the order from how the rate changes as concentration changes. If both concentrations were equal, ln(1) = 0 would appear in the denominator and the order would be undefined.
What does a zero order mean?
If the rate does not change when you change [A], the log of the rate ratio is zero and the order is zero — the rate is independent of that reactant’s concentration.
Should I use initial rates?
Yes. Use initial rates and initial concentrations so that other reactants have not yet changed appreciably, isolating the effect of the single reactant you are varying.
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This calculator is for educational purposes. Double-check important results before acting on them.