Rate can be measured experimentally by tracking how a measurable quantity changes over time.
Common Methods
| Method | Measured | Example |
|---|---|---|
| Gas syringe | Volume of gas | Mg + HCl → H₂ |
| Mass loss | Mass on balance | CaCO₃ + HCl → CO₂ escapes |
| Colourimetry | Colour intensity | Disappearing cross (Na₂S₂O₃) |
| Conductivity / pH | Electrical conductivity / pH | Ionic reactions, acid-base |
Interpreting Rate Graphs
Concentration vs Time
The gradient of the tangent at any point = instantaneous rate. Steeper gradient = faster rate.
Key Ideas
- Initial rate = gradient at t = 0 (steepest point)
- Rate decreases over time as reactants are consumed
- The graph levels off when the reaction is complete (or equilibrium is reached)
Think About It
On a mass-of-product vs time graph, which reaction condition gives the steepest initial gradient but the same final mass — higher concentration or a catalyst?
Both! Higher concentration and a catalyst both increase the initial rate (steeper gradient) but give the same total product. However, increasing the amount of reactant (not just concentration) would increase the final mass too.