How do I determine the Kc of the reaction between Fe3+(aq) + SCN-(aq)-->Fe(SCN)2+(aq) at different temperatures using spectophotometry? [closed]








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Beer's law states that A=elc where A is the molar absorptivity, e is the molar absorption coefficient, l is the pathlength of light and c is the concentration. However, I am not sure how to obtain values for this reaction.


Should I use different concentrations of $\ce{Fe(SCN)++}$ and plot a graph to calculate it or is there a better way? At different temperatures will e change? Thank you.




The Beer Lambert law is $I\_{trans} =I\_0 \exp(-\epsilon [C]l)$ where $I\_{trans}$ is the transmitted and $I\_0$ the incident light intensity. Usually this is used as $I\_{trans}=I\_010^{(-\epsilon[C]l)}$ in which case $\epsilon$ is the decadic extinction coefficient. The term $\epsilon[C]l$ is called the optical density. The extinction coefficient depends on wavelength and is obtained by plotting optical density vs. concentration then the slope gives the extinction coefficient at each wavelength used. 


Generally the extinction coefficient does not change greatly with temperature. Any changes are due to vibrational level's populations changing as the temperature changes and this is hard to predict exactly; usually bands broaden a little and become less intense but some vibrational bands may increase and others decrease so it would be best simply to measure at each temperature.


As far as you reaction goes you need to know the extinction coefficient from a separate experiment with known concentrations before trying to get the equilibrium constant.


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