Reaction Rate Law Calculator with Rate Constant

A reaction rate law calculator with a rate constant calculates the reaction rate from a rate-law equation and the concentrations of the reactants. Enter the rate constant, reactant concentrations, and reaction orders; the result is the predicted rate, with units determined by the inputs and the overall order.

Formula

For the rate law rate = k[A]^m[B]ⁿ, the calculator multiplies the rate constant, k, by the concentration of A raised to its order, m, and the concentration of B raised to its order, n. The rate is the change in concentration per unit time, commonly expressed in mol/(L·s), also written M/s.

• k is the rate constant. Its units depend on the overall reaction order and on the time and concentration units used.
• [A] and [B] are reactant concentrations, entered here in mol/L (M).
• m and n are the experimentally determined reaction orders for A and B. They have no units.
• The overall order is m + n. For this calculator’s units, k is entered in (mol/L)^1−(m+n) s⁻¹.

The calculator accepts nonnegative concentrations, rate constants, and orders. The default values give a first-order dependence on each reactant. For other reaction orders, check that the units of your rate constant match the selected orders and the rate unit shown.

Worked Example

Using the defaults, k = 1 (mol/L)⁻¹s⁻¹, [A] = 0.5 mol/L, m = 1, [B] = 0.25 mol/L, and n = 1:

1. Write the rate law: rate = k[A]^m[B]ⁿ.
2. Substitute the values: rate = 1 × (0.5)¹ × (0.25)¹.
3. Multiply: rate = 0.125 mol/(L·s).

Rounded to four decimal places, the predicted reaction rate is 0.1250 mol/(L·s).

What a Reaction Rate Law Describes

A rate law expresses how a measured reaction rate depends on reactant concentrations under specified conditions. It is a quantitative model: changing one concentration changes the predicted rate according to that reactant’s order. For example, in a rate law with first order in A, doubling [A] while keeping everything else fixed doubles the rate. If the order in A is two, doubling [A] multiplies the rate by four.

Reaction orders are generally established from experiments, not read directly from the balanced chemical equation. The equation’s coefficients give stoichiometric proportions for the overall reaction, but they do not normally determine the exponents in an empirical rate law. An elementary reaction step is a special case where its molecularity can connect to a rate expression; do not assume that applies to an overall reaction.

How to Choose Inputs and Units

Use concentrations and a rate constant measured or reported in compatible units, and enter the orders for the exact rate law you intend to evaluate. If your measured rate is reported per minute, convert the time basis consistently; a k value in seconds cannot be combined directly with a concentration-rate result per minute.

• For a rate in mol/(L·s), use concentrations in mol/L and k units that reduce to that rate unit.
• For a rate in mol/(L·min), use a rate constant based on minutes, or convert the final result and k consistently.
• If a reactant is absent from the rate law, it has no concentration factor in that expression. Do not add one merely because it appears in the balanced equation.
• For a zero-order reactant, its concentration is raised to zero and does not affect the calculated rate, provided the expression applies under those conditions.

The calculator’s concentration unit is metric (mol/L). Some chemistry sources also use mol/m³; 1 mol/L equals 1,000 mol/m³. Convert both concentrations and k as required rather than mixing these conventions. US customary volume units are uncommon in standard kinetic rate laws; if a source gives concentration per cubic foot or gallon, convert it to a consistent concentration unit before calculating.

Reading the Result and Its Limits

The output is an instantaneous rate predicted by the entered rate law at the entered concentrations. It is not automatically the time required to finish a reaction, the total amount of product, or the average rate over a long interval. Those quantities require additional information, such as a suitable integrated rate law, reaction stoichiometry, and the way concentrations change over time.

The result also depends on conditions under which the rate law and k apply. Temperature, solvent, catalysts, pressure for gas-phase systems, and reaction mechanism can affect kinetics. A rate constant is not a universal constant for a reaction; it is tied to specified conditions and a particular rate-law form.

This calculator handles a power-law expression with two concentration factors. More complicated laws may include products, inhibitors, saturation terms, reverse reactions, or concentration-independent terms. Use the relevant model rather than forcing such behavior into this simple equation. A numerically precise answer does not make uncertain or approximate input measurements more accurate.

Common Mistakes and Checks

• Using stoichiometric coefficients as orders: this is generally unjustified unless the reaction is an elementary step or experimental evidence supports it.
• Using incompatible units: k’s units change with the overall order. For an overall first-order law, k has units s⁻¹; for an overall second-order law, it has units L/(mol·s).
• Confusing rate with concentration: the answer has concentration-per-time units, not mol/L alone.
• Rounding too early: keep extra digits through intermediate calculations, then round the final rate to match the precision of the inputs.
• Applying a rate law outside its conditions: a law measured at one temperature or concentration range may not accurately predict behavior elsewhere.

A useful check is dimensional analysis: multiply the units of k by each concentration unit raised to its order. The result should simplify to the units of the rate being reported. If it does not, verify the rate-law order, concentration units, and time basis.

Frequently Asked Questions

How do I calculate reaction rate from a rate constant?
Substitute the rate constant and reactant concentrations into the experimentally established rate law, raising each concentration to its reaction order, then multiply the terms.

Does the rate constant change when concentration changes?
For a fixed reaction and conditions, concentration changes alter the rate through the concentration terms, not by changing k. The value of k can change when conditions such as temperature change.

Can reaction orders be negative or fractional?
Yes. Orders can be zero, fractional, or negative in experimentally observed rate laws. This calculator restricts orders to nonnegative values, so it cannot represent those cases; evaluate them with a tool or expression that allows the applicable exponents.

Can I get the time for a reaction to finish from this calculator?
No. It gives a rate at one set of concentrations. Finding concentration or time as a reaction proceeds generally requires an integrated rate law and appropriate initial conditions; a reaction may also approach equilibrium rather than finish completely.

Why does k have different units for different reactions?
The units of k must balance the concentration powers so that the rate law produces concentration per time. Consequently, its units depend on the overall reaction order and on the chosen concentration and time units.

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in k | Rate constant k | (mol/L)^(1-n) s^-1 | 1 | 0
in a | Concentration of A | mol/L | 0.5 | 0
in m | Order with respect to A | - | 1 | 0
in b | Concentration of B | mol/L | 0.25 | 0
in n | Order with respect to B | - | 1 | 0
out rate | Reaction rate | mol/(L s) | k * a^m * b^n | 4