This is an equilibrium lab question Even though you used different amounts of Na

Question

This is an equilibrium lab question

Even though you used different amounts of NaSCN for each test mixture the values of Kc you measured for each should be similar. Explain why you obtained a similar value of Kc eventhough each mixture had a different [SCN-]init.

Hint: consider the [Fe2+]eq and [FeSCN 2+]eq in each mixture.

Solution # Vol SCN- [Fe3+] initial [SCN-] initial [FeSCN2+]eq [Fe3+]eq [SCN-]eq Kc Average 2 1 0.001 0.0005 2.23E-05 9.78E-04 4.78E-04 4.77E+01 40.42403501 3 2 0.001 0.0010 3.13E-05 9.69E-04 9.69E-04 3.33E+01 4 3 0.001 0.0015 5.22E-05 9.48E-04 1.45E-03 3.81E+01 5 4 0.001 0.0020 7.58E-05 9.24E-04 1.92E-03 4.26E+01

Explanation / Answer

Answer: Equilibrium constant (Kc) for this reaction

Fe+3 +SCN- ? [FeSCN]+2

Kc = [FeSCN]+2/[Fe+3 ][SCN]

The value of Kc remains constant for the given concentrations because of the reason that intiail concentration of Fe+3 is very high compared to SCN- , and most of it unreacted and during intially. But when the concentration of SCN- is increasing it starts reacting with unreacted Fe+3 ,so the overall equilibrium constant remains constant thorugh the the reaction with different concentrations of SCN-.

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