Reaction rates
How fast, why that fast, and what a catalyst really changes.
Scientific learning path
- 01UnderstandLive
- 02ExploreLive
- 03ExperimentLive
- 04ObserveLive
- 05ExplainLive
- 06ApplyLive
- 07TestBuilding
Concentration against time
What a catalyst changes
- Without
- With a catalyst
- Enthalpy change
Description of this simulation
First order: the rate is proportional to the concentration. Starting from 1 mol/L, the first three half-lives are 13.9, 13.9, 13.9 seconds. They are equal, which is what makes first order the special case: the half-life does not depend on how much is left. Doubling the concentration multiplies the rate by 2.
Successive half-lives
- 113.9 s
- 213.9 s
- 313.9 s
Equal, every time. This is what makes first order the special case: the half-life does not depend on how much is left, which is why radioactive decay has one number you can quote.
Double the concentration
×2
The rate is multiplied by 2.
"Double the concentration, double the rate" is true for exactly one of the three orders.
Temperature
×1.97
The rule of thumb says a rise of ten degrees roughly doubles the rate. Here it comes out at 1.97 times — from the equation, not from the rule.
What a catalyst changes
- Forward activation energy
- 50→ 25
- Reverse activation energy
- 90→ 65
- Enthalpy change
- -40unchanged
- Exothermic
- ΔH < 0
A catalyst lowers the barrier and changes nothing else. The products sit at exactly the same height, so the enthalpy change is identical and so is the position of equilibrium. It changes how fast you get there, not where you end up — and it lowers the reverse barrier by exactly as much, which is why a catalyst speeds up both directions.
What this model shows — and what it simplifies
Educational ModelConcentrations come from the integrated rate law for the chosen order, not from a curve shaped to look right, which is why a zero-order reaction stops dead when the reactant runs out instead of going negative.
Where do we see this in real life?
Food keeps in a fridge because the same reactions run several times slower ten degrees colder. A catalytic converter works because a catalyst lowers the barrier without changing what the products are.