Spectophotometric Determination of the Equilibrium Constant of a Reaction

Z.C. G. Hacherodepartment of mining, metallurgical and materials engineering, college of engineeringuniversity of the philippines, diliman, quezon city, philippinesdate performed: March 3 and 5, 2015instructors name: Maro Pea

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ABSTRACT

A spectrophotometer is a device that measures the amount of light that can pass through a given substance. The quantitative measurement of light absorption as a function of wavelength can establish both the identity and the concentration of a substance in a solution through the application of Beer-Lamberts law. Standard and unknown solutions containing varying amounts of FeCl3, KSCN and HCl wereprepared and tested for absorptivity through the use of a UV-Vis spectrophotometer. After obtaining the equilibrium values of the concentrations of [Fe3+],[SCN-],and[Fe(SCN2+] the,Keq was calculated, as well as the percent difference of the experimental value from the literature value. It was obtained that the experimentally determinedKEq (331.10) differed by 68% from the known Keq (890) of the reaction. It can be concluded that the experiment was a failure due to experimental inaccuracy. Itwas then recommended that this experiment to be done in a fairly consistent temperature using more test reaction systems obtained from reagents with greater purity.

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CONCLUSION AND RECOMMENDATION

The experimentally determinedKEq (331.10) differed by 68% from the known Keq (890) of the reaction. The experiments final data deviated from the literature value. Many factors could have contributed to the error, like experimental mistakes such as the reagents nothaving a high enough purity, thetemperature where the experiment was done was not consistent, or that the apparatus, especially the cuvette, was nothandled properly and efficiently.

This experiment can be improved by using reagents which are of greater purity for the preparation of the stock solutions It is highly recommended to take good care when handling the cuvette, as fingerprints in it can drastically change the data. Lastly, it is

recommended that the test solutions be within the testing room for at least 30 minutes so that the solutions would have the same temperature, andthe Keq of the solutions would not have drastic differences such as in the experiment.

As an overall conclusion, the experiment was not successful because of the huge deviation of the equilibrium constant from the theoretical range of equilibrium constants. It failed due to different factors that can affect a certain procedure or a certain part in the experiment. By carefully performing the experiment for accuracy and precision, by carefully preventing errors by applying different techniques, and by using uncontaminated reagents, then the experiment could have been successful.REFERENCES

[1] Petrucci, R., Herring, F., Madura, J., & Bissonnette, C. General Chemistry: Principles and Modern Applications, 10th ed.; Pearson: Canada, 2011.

[2] Chang, R. General Chemistry: Essential Concepts, 6th ed.; McGraw-Hill: New York, 2011.

[3]Spectophotometry; http://employees.oneonta.edu/kotzjc/LAB/Spec_intro.pdf (accessed March 9, 2015)