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.
Summary Principles
Kinetic Factors Governing Rate
- Initial rate: Measured from the tangent (gradient) at \(t = 0\), representing the maximum rate when reactant concentrations are highest.
- Concentration / Pressure: Increases particle density \(\implies\) higher collision frequency per unit time.
- Temperature: Increases collision frequency and exponentially increases the fraction of particles with \(E \ge E_{\text{a}}\).
- Surface Area: Greater exposed reactant particles \(\implies\) higher collision frequency at phase boundaries.
Graphical Interpretation
Analysing Reaction Progress Curves
On a product vs time graph: Higher temperature, higher concentration, or powdered solid produces the steepest initial gradient (fastest initial rate). If the limiting reagent amount is held constant, all curves eventually plateau at the exact same final product mass.
AQA GCSE & IB Chemistry
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