# Suggest a method, other than spectrophotometry, for determining the reaction rate

For the hydrolysis of $\ce{\mathit{trans\hyphen}[Co(en)2Cl2]Cl}$ (and cis) we measured reaction rates via spectrophotometry. Plots of change in absorbance were used to determine the rate law and the rate for the reaction.

Suggest another method, other than spectrophotometry, for determining the reaction rate measured in this experiment.

Knowing little about measuring rates, I said one could measure the enthalpy and entropy of the reaction at a given temperature, and use those to calculate the rate constant. This is based off of the equation

$$\ln\frac{k}{T}=-\frac{\Delta H_\text{act}}{RT}+\ln\frac{k_\mathrm{B}}{h}+\frac{\Delta S_\text{act}}{R}$$

but, my professor said thermodynamics couldn’t be used to characterize kinetics. Also, reflecting on my answer, I didn’t really specify a technique.

What’s another technique that could be applied here?

Spectroscopy is often the best method, since it allows for (almost) continuous monitoring of the concentration.

If that is not wanted, ask yourself:

• What is released in the hydrolysis of the complex?

Chloride would be a good guess.

• Can I take samples and perform a wet chemical analysis to determine the concentration of new species?

Looking at the graph in your other related question, the answer is yes. The hydrolysis is slow enough.

• Which methods can I possibly use to determine the concentration $\ce{Cl-}$ in solution?

An argentometric titration, based on the low solubility of $\ce{AgCl}$ is a good idea. What about the Mohr titration, using $\ce{K2CrO4}$ as an indicator?