How this calculator works
For measured rate laws, enter up to three concentrations in mol/L and their experimentally established orders. Molecularity describes an elementary event and does not generally fix the observed rate law, so enter orders explicitly. Half-life mode applies only to the stated single-reactant integrated laws; it is not a formula for arbitrary multireactant half-lives. First-order half-life is independent of initial concentration, whereas zero- and second-order half-lives depend on it.
Formula
Rate = k product([Xi]^order i). t1/2 = [A]0/(2k) for zero order; ln(2)/k for first order; 1/(k[A]0) for second order.
Worked example
A first-order half-life of 10 s gives k = 0.0693147181 s^-1. A second-order reaction with [A]0 = 2 mol/L and half-life 10 s gives k = 0.05 L/(mol s).
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Frequently asked questions
Are reaction orders always integers?
Experimental orders can be fractional. This rate-law mode allows nonnegative orders through three.
Does bimolecular always mean second order?
For an elementary step it does, but an overall reaction mechanism can have a different observed order.
Why do k units change?
The rate has concentration/time units, so k must supply the remaining concentration power.