Chemical Reaction Rate Law Calculator
Understanding Rate Law Calculations
In chemical kinetics, the rate law is an equation that links the reaction rate with the concentrations of its reactants. Unlike the stoichiometric equation, the rate law is determined experimentally. This calculator helps you determine the speed of a reaction based on the rate constant and molar concentrations.
Where:
- Rate: The speed of the reaction (usually in M/s).
- k: The rate constant, which depends on temperature.
- [A] and [B]: Molar concentrations of reactants.
- m and n: The partial reaction orders (typically 0, 1, or 2).
Step-by-Step Example Calculation
Let's look at a realistic scenario for a second-order reaction. Suppose you have the following data:
- Rate constant (k): 0.045 M⁻¹s⁻¹
- Concentration of Reactant A: 0.50 M
- Concentration of Reactant B: 0.20 M
- Reaction order for A: 1 (First order)
- Reaction order for B: 1 (First order)
The Calculation:
Rate = 0.045 × (0.50)¹ × (0.20)¹
Rate = 0.045 × 0.50 × 0.20 = 0.0045 M/s
How to Determine Reaction Orders
Reaction orders (m and n) cannot be reliably predicted just by looking at the balanced chemical equation. Chemists use the Method of Initial Rates. By running the reaction multiple times while keeping one concentration constant and varying the other, they observe how the rate changes:
- Zero Order: Doubling the concentration has no effect on the rate.
- First Order: Doubling the concentration doubles the rate.
- Second Order: Doubling the concentration quadruples (2²) the rate.
Frequently Asked Questions
What are the units of the rate constant (k)?
The units for k change depending on the overall order of the reaction (m + n). For a first-order reaction, the unit is s⁻¹. For second-order, it is M⁻¹s⁻¹. For third-order, it is M⁻²s⁻¹.
Can reaction orders be fractions?
Yes, while most introductory chemistry examples use integers (0, 1, 2), complex reaction mechanisms can result in fractional orders like 0.5 or 1.5.