Catalysts and measuring rate
Catalysts
- A catalyst speeds up a reaction but is not used up — it is unchanged at the end.
- It works by lowering the activation energy, so more collisions succeed.
- It does not change $\Delta H$.
- Enzymes are biological catalysts.
Practice
A catalyst speeds up a reaction by:
A catalyst lowers the activation energy so more collisions succeed; it is not used up and ΔH is unchanged.
Measuring the rate
- Change in mass: stand the flask on a balance; if gas escapes, the mass falls.
- Volume of gas: collect it in a gas syringe and read the volume.
- Precipitate: time how long a mark under the flask takes to disappear as it turns cloudy.
Practice
A gas syringe is used to measure rate by recording the:
The gas syringe collects the gas produced; its volume read at intervals tracks the rate.
The rate graph
- A graph of product against time is steepest at the start (fastest), gets less steep as reactants run out, and goes flat when the reaction finishes.
Practice
On a product-against-time graph, the line is steepest at the start and flattens when the reaction finishes.
The rate is fastest at the start (steepest) and the line goes flat once a reactant runs out.
You've got it
Key idea
- a catalyst lowers the activation energy, is not used up, and doesn't change $\Delta H$
- measure rate by mass loss, gas volume (syringe), or precipitate time
- the rate graph is steepest at the start and goes flat at the end