What reaction rate tells you
The rate of a reaction is how quickly reactant concentration is consumed, usually written in moles per liter per second (M/s). A rate law connects that speed to concentration through a rate constant k: the larger k is, or the more concentrated the reactant, the faster the reaction goes. Knowing the instantaneous rate helps you predict how much reactant is used in a short interval and compare conditions such as different temperatures, which change k.
This calculator handles a single reactant A whose rate law is rate = k[A]^n, where n is the reaction order you choose: zero, first, or second. Use it when the order and rate constant are known from experiment or from your course material and you want the rate at a given concentration. It does not fit a rate law to data; for that you need several trials with varied concentrations.
The formula and its variables
rate = k x [A]^n
- k is the rate constant. Its units depend on the order: M/s for zero order, s-1 for first order, and M-1 s-1 for second order. Enter k in units that give a rate in M/s.
- [A] is the reactant concentration in mol/L.
- n is the order in A: 0, 1 or 2. Doubling [A] leaves a zero-order rate unchanged, doubles a first-order rate, and quadruples a second-order rate.
Worked example
A first-order reaction has k = 0.05 s-1 and [A] = 0.20 M. The rate is 0.05 x 0.20 = 0.0100 M/s. If the same reaction were second order with k = 0.5 M-1 s-1 at the same concentration, the rate would be 0.5 x 0.20^2 = 0.5 x 0.04 = 0.0200 M/s. The calculator shows 0.01000 M/s and 0.02000 M/s for these two cases.
Common mistakes and how to interpret the result
- Assuming order from the balanced equation. The order is found by experiment and usually does not equal the stoichiometric coefficient.
- Using k with the wrong units. A first-order k in s-1 will not give a sensible M/s answer if you select second order.
- Treating the result as constant. The rate is instantaneous; as A is consumed, [A] falls and the rate drops (except for zero order).
- Comparing rates at different temperatures with one k. k changes with temperature, so use the value measured at the temperature of interest.