Required Practical 7: Measuring Reaction Rates

• Colorimetry and Absorbance: In reactions involving colored species, such as the iodination of propanone, a colorimeter is used to measure the light absorbance of the solution. According to the Beer-Lambert Law, absorbance is directly proportional to the concentration of the colored substance, allowing for non-invasive monitoring of concentration changes.

• Gas Evolution Kinetics: For reactions that produce a gaseous product, such as the reaction between a metal and an acid, the rate can be monitored by measuring the volume of gas released. This relies on the principle that the volume of gas produced is stoichiometric to the amount of reactant consumed, provided temperature and pressure remain constant.

• The Role of Tangents: Because the rate of reaction usually changes over time as reactant concentrations decrease, the instantaneous rate at any specific point is determined by the gradient of a tangent drawn to the concentration-time curve at that point.

Initial Rate via Colorimetry

• Calibration Curve: Before the main experiment, prepare a series of standard solutions of known concentration and measure their absorbance. Plotting absorbance against concentration creates a calibration curve, which is essential for converting future absorbance readings into actual concentration values.
• Maintaining Constant Volume: When varying the concentration of one reactant, the total volume of the reaction mixture must be kept constant by adding varying amounts of distilled water. This ensures that the change in rate is solely due to the change in concentration of the reactant being studied, rather than a change in the overall dilution.

Continuous Monitoring via Gas Collection

• Apparatus Setup: Connect a reaction flask to a gas syringe using a delivery tube, ensuring the entire system is gas-tight to prevent loss of product. Alternatively, gas can be collected over water in an inverted measuring cylinder if the gas has low solubility in water.
• Data Recording: Start a timer the moment the reactants are mixed and record the cumulative volume of gas produced at fixed intervals (e.g., every 20 seconds). Continue until the reaction is complete, indicated by the volume of gas reaching a constant plateau.

• Initial Rate vs. Continuous Monitoring: The initial rate method requires multiple separate experiments with different starting concentrations to determine reaction orders, whereas continuous monitoring can provide the entire kinetic profile of a single reaction mixture over time.

• Tangent Accuracy: When asked to find the rate from a graph, always draw a long tangent line that spans as much of the grid as possible. This reduces the percentage error when calculating the gradient $\frac{\Delta y}{\Delta x}$.

• Filter Selection: In colorimetry questions, remember that the chosen filter should be the complementary color to the solution. This ensures maximum absorbance and therefore the highest sensitivity to changes in concentration.

• Units Verification: Always check the units for rate. If the graph is volume vs. time, the rate is $cm^3 \, s^{-1}$; if it is concentration vs. time, it is $mol \, dm^{-3} \, s^{-1}$. Forgetting to include the time component ($s^{-1}$) is a frequent source of lost marks.

• The 'Zero Time' Error: In gas collection, there is often a slight delay between adding the reactant and sealing the flask. This can lead to an underestimate of the initial rate; using a divided flask or a small vial inside the main flask can help minimize this error.

• Temperature Fluctuations: Reaction rates are highly sensitive to temperature. If the reaction is exothermic, the heat generated can increase the rate mid-experiment, leading to an 'S-shaped' curve that does not accurately reflect the concentration-dependent kinetics.

• Ignoring the Catalyst: In reactions like the iodination of propanone, the acid catalyst concentration must remain constant across all runs. If the catalyst volume is changed without compensation, the resulting rate data will be invalid.