Reaction Rate Formula
Calculate the rate of a chemical reaction as the change in concentration over time.
Includes rate laws and worked examples.
The Formula
The reaction rate measures how fast a reactant is consumed or a product is formed. It is expressed as the change in concentration per unit time.
Variables
| Symbol | Meaning |
|---|---|
| Rate | Speed of the reaction (mol/(L·s) or M/s) |
| Δ[Reactant] | Change in reactant concentration (mol/L) |
| Δ[Product] | Change in product concentration (mol/L) |
| Δt | Change in time (seconds, s) |
Rate Law
The rate law relates reaction rate to the concentrations of reactants raised to their respective orders. The rate constant k and the orders m, n are determined experimentally.
| Symbol | Meaning |
|---|---|
| k | Rate constant (units depend on reaction order) |
| [A], [B] | Concentrations of reactants (mol/L) |
| m, n | Reaction orders with respect to each reactant |
Example 1
The concentration of a reactant decreases from 0.80 M to 0.60 M in 20 seconds. What is the rate?
Rate = -Δ[Reactant] / Δt
Rate = -(0.60 - 0.80) / 20
Rate = -(-0.20) / 20 = 0.20 / 20
Rate = 0.01 M/s
Example 2
For a first-order reaction with k = 0.05 s⁻¹ and [A] = 0.40 M, what is the rate?
Rate = k × [A]¹
Rate = 0.05 × 0.40
Rate = 0.02 M/s
When to Use It
Use the reaction rate formula in chemical kinetics:
- Measuring how quickly a reaction proceeds
- Comparing rates under different conditions (temperature, concentration)
- Determining the rate law and order of a reaction from experimental data
- Predicting how long a reaction will take to reach completion