Rate Constant (k) Units Calculator
Enter the sum of the exponents in the rate law.
Seconds (s)
Minutes (min)
Hours (h)
Days
Years (yr)
Resulting Units for k:
How to Calculate Units of Rate Constant
In chemical kinetics, the rate constant (k) is a proportionality constant that relates the molar concentration of reactants to the rate of a chemical reaction. Unlike the reaction rate itself, which always has units of concentration divided by time (usually M/s), the units of the rate constant change depending on the overall reaction order.
The General Formula
To find the units for any order n, you can use the following universal derivation:
Units of k = M(1 – n) · t-1
Where:
- M = Molarity (mol/L)
- n = Overall order of the reaction
- t = Time (seconds, minutes, etc.)
Common Reaction Orders and Their Units
| Order (n) | Rate Law Example | Units of k |
|---|---|---|
| 0 (Zero Order) | Rate = k | M · s-1 or mol/(L·s) |
| 1 (First Order) | Rate = k[A] | s-1 |
| 2 (Second Order) | Rate = k[A]2 | M-1 · s-1 or L/(mol·s) |
| 3 (Third Order) | Rate = k[A]2[B] | M-2 · s-1 or L2/(mol2·s) |
Practical Example Calculation
Suppose you have a reaction that is 1.5 order overall. What are the units of k if time is measured in minutes?
- Identify n = 1.5 and t = min.
- Apply the formula: M(1 – 1.5) · min-1.
- Simplify: M-0.5 · min-1.
- Expanded: (mol/L)-0.5 · min-1 = L0.5 · mol-0.5 · min-1.