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.

The lab report asks:

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?


1 Answer 1


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?


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