Reaction Rate Calculator
Find a reaction rate from the rate constant, reactant concentrations, and partial orders.
Overall reaction order: 2 (m = 2).
A rate law gives the reaction rate as rate = k · [A]ᵐ · [B]ⁿ, with k the rate constant, [A] and [B] in mol/L, and m and n the partial orders. With k = 0.1, [A] = 2 mol/L and m = 2, the rate is 0.1 × 2² = 0.4 mol·L⁻¹·s⁻¹ and the overall order is 2.
What a rate law tells you
The rate law links how fast a reaction proceeds to the concentrations of its reactants. Each concentration is raised to a partial order — a number found by experiment — and multiplied by the rate constant k, which folds in temperature and the intrinsic speed of the reaction. Add the partial orders together and you get the overall order, which describes how sensitive the rate is to changing concentrations.
k is the rate constant; [A], [B] are concentrations in mol/L; m, n are the partial orders
Worked example
A reaction is second-order in A with rate constant k = 0.1 and just one reactant. The concentration of A is 2 mol/L.
- 1 Write the rate law. With a single reactant the expression is rate = k · [A]ᵐ. Here m = 2, so rate = k · [A]².
- 2 Substitute the values. Put in k = 0.1 and [A] = 2 mol/L: rate = 0.1 × 2².
- 3 Evaluate the concentration term. 2² = 4, so rate = 0.1 × 4.
- 4 Multiply for the rate. rate = 0.4 mol·L⁻¹·s⁻¹.
- 5 Add the orders. The overall order is m = 2 (there is no second reactant), so this is a second-order reaction.
How the order in a reactant changes the effect of doubling it
Doubling one reactant’s concentration multiplies the rate by 2 raised to that reactant’s partial order.
| Order in a reactant | Rate depends on | Effect of doubling that reactant |
|---|---|---|
| 0 (zero-order) | [A]⁰ = 1 | No change — rate ×1 |
| 1 (first-order) | [A]¹ | Rate ×2 |
| 2 (second-order) | [A]² | Rate ×4 |
Reading the rate law correctly
Orders come from experiment, not stoichiometry. The exponents m and n are measured — usually by seeing how the initial rate changes when you vary one concentration at a time. They are not read off the balanced equation’s coefficients, and they can be zero, fractional, or even negative.
The units of k depend on the overall order. Because the left side is always mol·L⁻¹·s⁻¹, k must carry whatever units make the equation balance: s⁻¹ for a first-order reaction, L·mol⁻¹·s⁻¹ for second-order, and mol·L⁻¹·s⁻¹ for zero-order. This calculator reports the rate assuming per-second time units.