Reaction Rate Law Order Calculator
The order of a reaction with respect to a reactant tells you how strongly its concentration controls the rate. It must be measured, not read off the balanced equation. The method of initial rates compares two experiments in which only the studied reactant changes: the rate ratio equals the concentration ratio raised to the order. This calculator takes the two concentrations and their two measured rates and returns the order, plus the rate-constant k implied if the order is treated as exact. Keep all other reactant concentrations the same between runs.
Reaction order formula
rate = k * [A]^n
rate2 / rate1 = (conc2 / conc1)^n
n = ln(rate2 / rate1) / ln(conc2 / conc1)
k = rate / [A]^n
Only the studied reactant concentration may change between the two runs. The order n follows from the logarithm of the rate ratio over the logarithm of the concentration ratio. The rate constant k is recovered from either run.
Worked example
- Doubling concentration from 0.1 to 0.2 mol/L raises rate from 0.002 to 0.008 mol/L/s.
- Rate ratio is 4; concentration ratio is 2.
- n = ln(4) / ln(2) = 1.386 / 0.693 = 2.
- The reaction is second order in this reactant.
- k = 0.002 / 0.1^2 = 0.2 L/mol/s, consistent across both runs.
Reaction order: frequently asked questions
What is reaction order?
Reaction order is the power to which a reactant concentration is raised in the rate law. For a rate law rate = k times [A] raised to n, the order with respect to A is n. It is found experimentally, not from the balanced equation, and need not be a whole number.
How does the method of initial rates work?
Run two experiments where only one reactant concentration changes. The ratio of the measured rates equals the ratio of that concentration raised to the order. Taking logarithms gives order = ln(rate2/rate1) divided by ln(conc2/conc1). This calculator applies that formula directly.
What does a result near 0, 1 or 2 mean?
An order near 0 means the rate does not depend on that reactant's concentration. An order near 1 means the rate is directly proportional to it (doubling concentration doubles rate). An order near 2 means the rate quadruples when concentration doubles. Round to the nearest sensible integer or half-integer.
Why must concentrations differ between the two runs?
If the two concentrations are equal the logarithm of their ratio is zero, and dividing by zero is undefined. The method needs a measurable change in concentration to detect how rate responds, so the two runs must use different concentrations of the studied reactant.
Official sources
- NIST Chemical Kinetics Database: measured reaction rate data.
- IUPAC Gold Book: order of reaction and rate law definitions.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.