Following one reaction through time
Continuous monitoring: taking repeated measurements during one reaction so that its progress can be plotted against time.
Required Practical 7 uses both an initial-rate method and a continuous monitoring method. Depending on the reaction, progress can be followed using concentration, gas volume, mass or colour.
| Approach | What is recorded | How it is analysed | Example |
|---|---|---|---|
| Continuous monitoring | Repeated readings during one run | Gradient of a tangent to a graph against time | Gas volume or colorimeter readings |
| Initial-rate clock method | Time required to reach the same fixed endpoint | Use as a quantity proportional to initial rate | Iodine clock |
Reading a time graph
- Plot the measured quantity against time and draw an appropriate best-fit curve.
- Draw a tangent at the time for which the rate is required.
- Calculate for the tangent.
The reaction is fastest where the curve is steepest. As reactants are consumed, the slope becomes less steep; when the measured quantity becomes constant, its gradient is zero.
For a reactant concentration that is decreasing, quote the rate using the magnitude of the negative tangent gradient.
For the example tangent at :
Exam Tip: If an initial rate is required from continuous data, draw the tangent at . Do not substitute an average gradient from the first few plotted points.