Reaction Order Calculator
Free Reaction order Calculator for chemical kinetics. Enter variables to compute results with formulas and detailed steps.
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The reaction order n is found by comparing rates from two experiments where only one reactant concentration changes. Taking the natural log of the rate ratio divided by the natural log of the concentration ratio gives the order. The rate constant k is then found by substituting back into the rate law.
Last reviewed: December 2025
Worked Examples
Example 1: Determining Order from Two Experiments
Example 2: First Order Reaction Identification
Background & Theory
The Reaction Order Calculator applies the following established principles and formulas. Chemistry is the science of matter's composition, structure, properties, and transformations. At the heart of quantitative chemistry lies the mole concept. One mole of any substance contains exactly 6.022×10²³ entities (Avogadro's number, Nₐ), and the molar mass of an element or compound in grams per mole is numerically equal to its atomic or molecular mass in atomic mass units. This allows chemists to convert between measurable mass and the number of reacting particles. Stoichiometry uses balanced chemical equations to relate the amounts of reactants and products. A balanced equation conserves both mass and charge. Molarity, the most common concentration unit, is defined as M = n/V, where n is moles of solute and V is volume of solution in liters, giving units of mol/L. Acidity and basicity are quantified by the pH scale, defined as pH = −log₁₀[H⁺], where [H⁺] is the molar concentration of hydrogen ions. Pure water at 25°C has pH 7.00; acids have lower values and bases higher values. Each unit change represents a tenfold change in hydrogen ion concentration. Gas behavior is described by the ideal gas law PV = nRT, where P is pressure in pascals, V is volume in cubic meters, n is moles, R = 8.314 J/(mol·K), and T is temperature in kelvin. Special cases include Boyle's Law (P₁V₁ = P₂V₂ at constant temperature) and Charles's Law (V₁/T₁ = V₂/T₂ at constant pressure). Thermochemistry quantifies heat changes in reactions through enthalpy, H. Hess's Law states that the total enthalpy change for a reaction is the sum of enthalpy changes for any sequence of steps leading to the same overall reaction, making it possible to calculate enthalpies for reactions that cannot be measured directly. Electron configuration describes the distribution of electrons in atomic orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. Periodic trends including atomic radius, ionization energy, and electronegativity arise systematically from electron configuration and nuclear charge, enabling chemists to predict and rationalize chemical behavior across the periodic table.
History
The history behind the Reaction Order Calculator traces back through the following developments. Chemistry's roots lie in alchemy, the medieval practice combining proto-scientific experimentation with mystical aims. Alchemists developed practical techniques including distillation, calcination, and the preparation of acids, building a body of empirical knowledge despite their theoretical misunderstandings. Modern chemistry is conventionally dated to Antoine Lavoisier (1743–1794), often called the father of modern chemistry. Lavoisier demonstrated the law of conservation of mass in 1789, showing that matter is neither created nor destroyed in chemical reactions. He identified oxygen's role in combustion, dismantling the phlogiston theory, and co-authored the first systematic chemical nomenclature, establishing the language still used today. John Dalton proposed the first modern atomic theory in 1803, asserting that all matter is composed of indivisible atoms, that atoms of the same element are identical in mass, and that compounds form from fixed ratios of different atoms. This provided a physical basis for Lavoisier's conservation law and Proust's law of definite proportions. Dmitri Mendeleev published his periodic table in 1869, arranging the 63 known elements by atomic mass and revealing repeating patterns of chemical behavior. He boldly left gaps for undiscovered elements and predicted their properties with remarkable accuracy, predictions confirmed by the subsequent discovery of gallium, scandium, and germanium. Ernest Rutherford's gold foil experiment in 1911 revealed the nuclear model of the atom: a tiny, dense, positively charged nucleus surrounded by electrons. Niels Bohr refined this in 1913 with a quantized model of electron orbits that explained the hydrogen emission spectrum. Quantum chemistry and molecular orbital theory, developed through the 1920s and 1930s, provided the full quantum mechanical description of chemical bonding. The latter 20th century saw the rise of computational chemistry, enabling molecular simulation at unprecedented scale. The green chemistry movement, articulated in the 12 Principles of Green Chemistry in 1998, reoriented the field toward sustainability, waste reduction, and benign chemical design, reflecting chemistry's growing awareness of its environmental responsibilities.
Frequently Asked Questions
Formula
n = ln(rate2/rate1) / ln([A]2/[A]1)
The reaction order n is found by comparing rates from two experiments where only one reactant concentration changes. Taking the natural log of the rate ratio divided by the natural log of the concentration ratio gives the order. The rate constant k is then found by substituting back into the rate law.
Frequently Asked Questions
What is reaction order in chemistry?
Reaction order describes how the rate of a chemical reaction depends on the concentration of a particular reactant. A first-order reaction rate doubles when the reactant concentration doubles, a second-order reaction rate quadruples when concentration doubles, and a zero-order reaction rate is independent of concentration. The overall reaction order is the sum of all individual orders. Reaction orders are determined experimentally using the method of initial rates or integrated rate law analysis.
Can reaction order be a fraction or negative?
Yes, reaction orders can be fractional, zero, or even negative. Fractional orders (like 0.5 or 1.5) often indicate complex reaction mechanisms with multiple elementary steps. Negative orders mean that increasing the concentration of a reactant actually slows the reaction, which can occur when a reactant inhibits a catalytic surface or competes for active sites. Zero-order reactions proceed at a constant rate regardless of concentration, commonly seen in enzyme-catalyzed reactions at saturation.
What are integrated rate laws and how do they relate to reaction order?
Integrated rate laws describe how concentration changes over time for each reaction order. For zero order: [A] = [A]0 - kt (linear in [A] vs t). For first order: ln[A] = ln[A]0 - kt (linear in ln[A] vs t). For second order: 1/[A] = 1/[A]0 + kt (linear in 1/[A] vs t). By plotting experimental data in these forms, the one that gives a straight line reveals the reaction order. The slope of the straight line gives the rate constant k.
Why might my result differ from another tool or reference?
Differences typically arise from rounding conventions, the specific version of a formula (for example, simple vs compound interest), or unit inconsistencies between inputs. Check that both tools are using the same formula variant and the same units. The References section links to the authoritative source behind the formula used here.
Is my data stored or sent to a server?
No. All calculations run entirely in your browser using JavaScript. No data you enter is ever transmitted to any server or stored anywhere. Your inputs remain completely private.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy