What is the method of initial rates?

It is an experimental technique where only one reactant concentration is changed between two experiments while all others are held constant. The ratio of the two rates gives: order = log(rate₂/rate₁) / log([A]₂/[A]₁) for the varied reactant.

What if a reactant concentration is the same in both experiments?

If the concentration of a reactant does not change between experiments, its order is indeterminate from that pair and the tool shows “?”. Additional experiment pairs are required before the overall order and rate constant can be calculated.

Can the reaction order be a non-integer?

Yes. Orders are determined experimentally and can be fractions (e.g. 0.5, 1.5). The tool rounds to two decimal places but displays the exact computed value.

How is the rate constant k calculated?

Once all orders are known, k is computed from Experiment 1 as: k = rate₁ / ([A]₁^m × [B]₁^n × ...). The result from Experiment 2 is also shown as a cross-check.

Why must the two rates be in the same units?

The method uses the ratio rate₂/rate₁, so the units cancel — but only when both rates share identical units (e.g. both in mol/L·s). Mixing units will give a wrong order.

What is the overall reaction order?

It is the sum of all individual reactant orders: m + n + p + ... This tool calculates and displays it automatically alongside the full rate law expression.

Reaction Order Calculator

Enter two experimental rate measurements to find the reaction order for each reactant and the rate constant k.

Experimental Data

Enter concentrations and rates for two experiments. If a reactant is unchanged, optionally provide its known order so the overall order and rate constant can be calculated.

Number of reactants:

All concentrations in mol/L. Rate units can be any consistent unit (mol/L·s, M/s, etc.).

Enter data and click Calculate

Summary

The reaction order calculator uses the method of initial rates to determine the reaction order for each reactant and the overall rate constant k. Enter the initial concentrations and measured rates from two experiments. For each reactant the tool solves: order = log(rate₂/rate₁) / log([A]₂/[A]₁). The overall reaction order is the sum of all individual orders, and k is back-calculated from either experiment.

How it works

Enter the initial concentration of each reactant for Experiment 1 and Experiment 2.
Enter the measured initial rate for each experiment (in the same units).
Click "Calculate" to run the method of initial rates.
The tool computes the order for each reactant by comparing the two experiments.
The overall reaction order and rate constant k are displayed with the full rate law.

Use cases

Determine reaction order from a two-experiment kinetics dataset in general chemistry.
Verify hand-calculated method-of-initial-rates results on homework problems.
Calculate the rate constant k once reaction orders are known.
Explore how changing concentrations between experiments affects the derived order.
Prepare for AP Chemistry or university physical chemistry kinetics units.
Check stoichiometry vs. experimentally derived orders.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu