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Determination of the equilibrium constant for the reaction

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Introduction

Chemistry TAS Report 1. Experiment Number : 10 2. Date : 18/01/2008 3. Title : Determination of the equilibrium constant for the reaction : Fe3+(aq) + SCN-(aq) <=> FeSCN2+(aq) 4. Aims/Objective : To determine the equilibrium constant for the reaction : Fe3+(aq) + SCN-(aq) <=> FeSCN2+(aq) 5. Introduction / Theory: In this experiment, the equilibrium constant for the formation of a complex ion, FeSCN2+(aq), is determined. Complex ions, thiocyanatoiron(III) ions, are formed from iron(III) ions and thiocyanate ions in aqueous solution : Fe3+(aq) + SCN-(aq) <=> FeSCN2+(aq) (1) The equilibrium constant for this reaction is: Kc = [FeSCN2+(aq)] (2) [Fe3+(aq)][SCN-(aq)] The product complex ion is the only one of the three species which has an appreciable color (blood-red). 6. Relevant Equations/Chemical Reactions Involved : Fe3+(aq) + SCN-(aq) <=> FeSCN2+(aq) 7. Chemicals : 0.002 M KSCN(aq) 50 cm3 0.2 M Fe(NO3)3(aq) ...read more.

Middle

And the removed liquid was put into a clean and dry beaker. 6. The depth of the solution in tube 1 was then measured. 7. Steps 3 to 6 were repeated with tubes 1 and 3, 1and 4 and finally 1and 5. 10. Observations : The colour intensities of the solutions in the two tubes being observed were the same. 11. Data, Calculation and Results : * Height of solution in original tube 1 = 59 mm * Height of solution in tube 1 having the same colour intensity as tubes 2, 3, 4 and 5 are : Same colour intensity as 2 3 4 5 Height of solution in tube 1 (mm) 45 35 28 23 12. Conclusion : The equilibrium constant for the reaction : Fe3+(aq) + SCN-(aq) <=> FeSCN2+(aq) was found to be 82.1 M-1. ...read more.

Conclusion

ions: In tube 2 = 0.001 x (45 / 59) = 7.63 x 10-4 M In tube 3 = 0.001 x (35 / 59) = 5.93 x 10-4 M In tube 4 = 0.001 x (28 / 59) = 4.75 x 10-4 M In tube 5 = 0.001 x (23 / 59) = 3.90 x 10-4 M 4. [Fe3+]equil = [Fe3+]initial - [FeSCN2+]equil = 0.04 - 7.63 x 10-4 = 0.0392 M [SCN-]equil = [SCN-]initial - [FeSCN2+]equil = 0.001 -7.63 x 10-4 = 2.37 x 10-4 M 5. Room temperature = 16 oC K = [FeSCN2+]equil / ([Fe3+]equil[SCN-]equil) = 7.63 x 10-4 / [(0.0392)(2.37 x 10-4)] = 82.1 M-1 6. There are few sources of error in this experiment. (1) Determination of colour intensity in the two tubes observed might not be accurate. (2) Taking reading when measuring the depth of liquid. Error estimation - When taking initial reading, error is + 0.05 cm3. When taking final reading, error is also + 0.05 cm3. Therefore, error is + 0.1 cm3. ...read more.

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