2
2
2.0000e-1
2
2
2.0000e-1
The Reaction Order Calculator determines the order of a chemical reaction with respect to a reactant using the method of initial rates. Reaction order tells you how sensitively the rate depends on the concentration of each reactant and is crucial for writing the correct rate law. Unlike stoichiometric coefficients, reaction orders must be determined experimentally — they cannot be predicted from the balanced equation alone (except for elementary reactions). This calculator takes two experimental measurements of concentration and rate, computes the order using logarithmic analysis, and also extracts the rate constant. It is an essential tool for students learning kinetics, researchers characterizing new reactions, and engineers developing rate models for reactor design.
The method of initial rates compares two experiments where only the concentration of the species of interest changes while all other conditions remain constant. Starting from the rate law:
$$\text{Rate} = k[A]^n$$
Taking the ratio of two experiments:
$$\frac{\text{Rate}_2}{\text{Rate}_1} = \left(\frac{[A]_2}{[A]_1}\right)^n$$
Taking the logarithm of both sides:
$$n = \frac{\ln(\text{Rate}_2/\text{Rate}_1)}{\ln([A]_2/[A]_1)}$$
Once n is determined, the rate constant is calculated from either experiment:
$$k = \frac{\text{Rate}_1}{[A]_1^n}$$
The calculator provides both the exact computed order and the nearest integer value, since reaction orders are most commonly integers (0, 1, 2, 3) or simple fractions (0.5, 1.5).
Order 0: Rate is independent of [A] — common in enzyme-saturated or surface-saturated conditions. Order 1: Rate is directly proportional to [A] — doubling [A] doubles the rate. Order 2: Rate depends on the square of [A] — doubling [A] quadruples the rate. Order 3: Rare, but tripling indicates a three-body collision or complex mechanism. Fractional orders (e.g., 0.5, 1.5) suggest a complex mechanism with multiple steps. If the computed order is not near an integer or simple fraction, check for experimental errors.
Inputs
Results
Rate ratio = 0.008/0.002 = 4. Concentration ratio = 0.2/0.1 = 2. Order n = ln(4)/ln(2) = 2. This is second-order: doubling [A] quadruples the rate. k = 0.002/(0.1)² = 0.2 L/(mol·s).
Inputs
Results
Rate ratio = 0.003/0.001 = 3. Concentration ratio = 0.15/0.05 = 3. Order n = ln(3)/ln(3) = 1. This is first-order: tripling [A] triples the rate. k = 0.001/0.05 = 0.02 s⁻¹.
Reaction order is the exponent to which the concentration of a reactant is raised in the rate law. It indicates how the rate changes when that reactant's concentration changes. The overall reaction order is the sum of individual orders for all reactants.
Only for elementary reactions (single-step processes), where the order equals the molecularity. For complex multi-step reactions, the order must be determined experimentally because the rate law reflects the mechanism, not the overall stoichiometry.
Use the method of initial rates while keeping all other reactant concentrations constant. Determine the order with respect to each reactant separately, then combine them into the full rate law.
Experimental errors in rate and concentration measurements cause deviations from exact integers. If the order is close to an integer (e.g., 1.95 or 2.08), it is likely an integer order. Values like 1.5 or 0.5 may indicate genuine fractional orders from complex mechanisms.
The rate does not depend on that reactant's concentration. This occurs when the reactant is in large excess, when the reaction is catalyzed by a surface that is fully covered, or when an enzyme is saturated with substrate.
Yes, negative orders are possible and indicate that increasing the concentration of that species actually decreases the reaction rate. This occurs with inhibitors in catalytic reactions or in complex mechanisms where excess reactant blocks the active pathway.
A technique where the initial rate of a reaction is measured at different starting concentrations. By comparing rates when only one concentration varies, you can isolate the order with respect to each reactant. It is the most common experimental method for determining reaction orders.
A minimum of two experiments (varying one concentration) determines the order for one reactant. For a reaction with two reactants, at least three experiments are needed: two varying [A] at constant [B], and one varying [B]. More experiments improve accuracy.
When one reactant is in vast excess, its concentration barely changes and can be treated as constant. The reaction appears to follow a lower overall order. For example, a second-order reaction A + B → products becomes pseudo-first-order if [B] >> [A].
Use multiple data pairs and average the results. Plot log(rate) vs log([A]) — the slope gives the order, and linear regression provides an error estimate. Using more than two data points significantly improves the reliability of the determined order.
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