RP7: Initial Rates and Continuous Monitoring

The two required approaches answer related questions using different observations. In a clock investigation, record the time to a defined small reaction extent. In continuous monitoring, record a quantity such as gas volume, mass or absorbance repeatedly during one reaction. Choose a signal linked to a reactant or product by known chemistry; not every reaction generates a gas or a useful colour change.

For a concentration investigation keep other reactant concentrations, total volume, temperature, catalyst and mixing procedure fixed. Replace a reduced reactant volume with solvent so changing one solution does not inadvertently change every concentration. Calculate the concentration after mixing rather than quoting the stock-bottle label.

An iodine clock includes a reagent that removes iodine until a small fixed amount has been generated; the subsequent colour change defines the endpoint. With the same threshold amount in every run, rate is proportional to 1/t while the reaction is still near its initial composition. Keep the clock reagent and total volume fixed. A changed threshold would invalidate a simple time comparison.

Repeat each condition and inspect scatter. An objective colour sensor reduces subjective endpoint judgment; consistent rapid mixing reduces the delay before timing. The relative-rate units are s⁻¹. A concentration production rate requires the threshold amount and volume, and determining k requires the rate law. Follow the approved clock reagents and waste arrangements.

In a gas-producing reaction, a gas syringe records volume against elapsed time. Check for leaks, a freely moving plunger and sufficient capacity. Start the reaction without losing its earliest gas where the approved apparatus permits. If a bung is fitted after mixing, early loss can distort the initial rate. A sealed reaction must retain an open path to the measuring syringe and must not be allowed to overpressurise.

Draw a tangent to a smooth curve at the time of interest. Its gradient is a rate in volume/time; a chord between two points is an average over that interval. A plateau shows little further measured gas production, which can reflect exhaustion of a limiting reactant, but a leak or stuck syringe must be excluded. Compare final yields separately from initial gradients.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.