Method of Initial Rates Helper

Determine reaction orders and rate constants from experimental initial rate data. For students, researchers, and lab technicians. Supports molar (M, mM, µM), time (s, min, h), and rate units (M/s, mol/L·min). Runs completely offline in your browser.

Educational only. Follow your lab SOPs and SDS. This tool provides estimates based on simplified models.

Experimental Data

Exp #	[A]	[B] (optional)	Initial Rate	Actions

Result Precision (decimal places)

2 4 6

Results

Enter at least two experiments with varying concentrations, then click Calculate.

Verification & Checks

Waiting for data...

Using the Method of Initial Rates

This method determines the rate law (orders and rate constant) from the initial rates of reaction measured at different starting concentrations. The reaction is assumed to be of the form: rate = k [A]^m [B]ⁿ.

Step-by-Step

Run experiments where only one reactant's concentration changes between two trials.
Measure the initial rate (slope of concentration vs. time near t=0).
Take the ratio of rates and concentrations to solve for the order:
rate2 / rate1 = ([A2]/[A1])^m
Use logarithms: m = log(rate2/rate1) / log([A2]/[A1]).
Plug orders into the rate law and solve for k using any experiment.

Example: Decomposition of NO₂

Reaction: 2 NO₂ → 2 NO + O₂

Exp	[NO₂] (M)	Initial Rate (M/s)
1	0.010	7.1e-5
2	0.020	2.8e-4
3	0.040	1.1e-3

Solution: Compare Exp2/Exp1: (2.8e-4 / 7.1e-5) ≈ 3.94. Concentration ratio = 2. Order m = log(3.94)/log(2) ≈ 2. Rate law: rate = k [NO₂]². k ≈ 0.71 M^-1s^-1.

Common Mistakes Checklist

Using concentrations after the reaction has progressed (not initial).
Not keeping temperature constant across experiments.
Assuming integer orders without statistical validation.
Forgetting to convert all concentrations and rates to consistent units.
Using only two data points without checking consistency with a third.

Predict Rate: Vary Concentration

Use the determined rate law to predict the initial rate for a new concentration.

Rate constant k

Order with respect to [A] (m)

[A] to test (M) 0.05

0.001 M 0.1 M

–

M/s

Predicted initial rate

Frequently Asked Questions

What if my reaction has more than two reactants?

Use the optional [B] column for a second reactant. For more, run experiments where only one reactant changes at a time, then repeat the analysis for each. The tool currently supports two reactants explicitly, but you can analyze them pairwise.

How accurate is the order determined this way?

The method assumes perfect data. Real experimental error, especially in measuring initial rates, affects accuracy. Always perform replicate runs and consider statistical fitting for more robust orders.

What if my order is not an integer?

Fractional or negative orders are possible for complex mechanisms. Report the value with uncertainty. The tool calculates the precise order from your inputs.

Why must temperature be constant?

The rate constant k depends strongly on temperature (Arrhenius equation). Varying temperature between experiments would change k, confounding the concentration effect.

Formulas & Assumptions

Rate law: rate = k [A]^m [B]ⁿ

Order determination (two experiments, B constant): m = log(rate₂/rate₁) / log([A]₂/[A]₁)

Rate constant: k = rate / ([A]^m[B]ⁿ) (averaged over all experiments)

Assumptions: Reaction is irreversible; rate measured at t≈0 (initial); only reactants affect rate; temperature, ionic strength, and catalyst activity constant; no side reactions.

Rounding: Orders shown to selected decimal places; k in appropriate units; intermediate logs use full precision.

Disclaimer: For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. This tool does not replace professional judgment, lab SOPs, or safety procedures. Always follow SDS guidance and institutional policies.

Sources & References: Common chemistry constants: R, N_A; Standard unit conversions and lab measurement best practices; General analytical chemistry and uncertainty concepts; Introductory chemical kinetics and rate laws; Laboratory safety basics and SDS principles.

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