Rate Constant Calculator
Use this rate constant calculator to work with common elementary-step rate laws. It can estimate the reaction rate from concentrations and k, solve backward for k, and report order-based half-life guidance where that shortcut is valid.
Enter k in units consistent with the selected total order and time basis.
Result
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Quick Answer: The core relationship is rate = k[A]^m[B]^n[C]^p. The total order is m + n + p, and the units of k change with that total order.
How to Calculate Rate Constant or Rate
- Choose the elementary step: Select uni-, bi-, or trimolecular behavior to control which reactants and order choices appear.
- Set the orders: Pick the order for each reactant. The page sums them to show total reaction order.
- Enter concentrations and k: Use consistent concentration and time units so the rate output stays meaningful.
- Review the rate and half-life: For standard zero-, first-, and second-order cases, the calculator also reports the corresponding half-life shortcut.
Rate Constant Calculator Formula
rate = k[A]^m[B]^n[C]^p
| Variable | Meaning | Unit |
|---|---|---|
| rate | Reaction rate | concentration per time |
| k | Rate constant | M^(1-n)/time where n is total order |
| [A],[B],[C] | Reactant concentrations | M |
| m,n,p | Orders with respect to each reactant | unitless |
Worked Examples
Example 1 - Unimolecular first-order step
- k: 0.20 s^-1
- [A]: 0.80 M
- Order of A: 1
Result: Rate = 0.16 M/s; Half-life = 3.47 s
For first-order behavior, half-life depends only on k and not on the current concentration.
Example 2 - Bimolecular first-order in A and B
- k: 0.50 M^-1 s^-1
- [A]: 0.30 M
- [B]: 0.40 M
- Orders: 1 and 1
Result: Rate = 0.060 M/s
Doubling either concentration would double the rate, because each reactant appears to the first power.
Example 3 - Trimolecular 2,1,1 case
- k: 0.10 M^-3 s^-1
- [A]: 0.20 M
- [B]: 0.30 M
- [C]: 0.50 M
Result: Rate = 0.0006 M/s
The squared A term makes the rate especially sensitive to changes in reactant A.
Reaction Order Reference Table
| Range | Meaning | Action |
|---|---|---|
| Total order = 0 | Rate is independent of concentration in the simplified rate law. | Zero-order half-life depends on the starting concentration and k. |
| Total order = 1 | Rate changes linearly with concentration. | First-order half-life uses t1/2 = 0.693 / k. |
| Total order >= 2 | Rate becomes more sensitive to concentration changes. | Check the units of k carefully because they change with order. |
Frequently Asked Questions
Rearrange the rate law so k equals the measured rate divided by the reactant concentrations raised to their orders.
The units of k must cancel the concentration terms in the rate law so the final rate still has units of concentration per time.
Temperature changes k strongly, and catalysts can also change it by lowering the activation-energy barrier.
The shortcut half-life formulas apply only to standard zero-, first-, and second-order cases. More complex mechanisms need a more careful treatment.
It is the sum of the individual reactant orders in the rate law and describes how strongly the overall rate responds to concentration changes.
Note: This page covers common textbook rate-law patterns and is not a substitute for a full kinetic model of a complex reaction mechanism.
References
Last reviewed: March 14, 2026