20 Chemical Equilibrium: Finding a Constant, Kc The purpose of this experimental

Question

20 Chemical Equilibrium: Finding a Constant, Kc The purpose of this experimentally determine the equilibrium constant, Kc, for the following chemical reaction ron(III) thiocyanate thiocyanoiron(I) When Fe and SCN- are combined, equilibrium is established between these two ions and the FeSCN cessary to know the concentrations of on. In order to calculate Kc for the reaction, it is ne brium leq. You will four ons at equ systems containing different concentrations of these three ions. The equilibrium concentrations of the three ions w then be experimentally determined. These values will be substituted into the equilibrium constant expression to see if Ke s indeed constant. You will use a colorimeter to determine IFescN2+] The FescN2+ on produces solutions with a leq. red color. Because the red solutions absorb blue light very well, Colorimeter users will be instructed to use the 470 nm (blue) LED. The light striking the detector is reported as absorbance percent transmittance. By comparing the absorbance of each equilibrium system, or of a standard solution, Astd, you can determine [FeSCN2 leq. The standard solution has known FeSCN2+ concentration a To prepare the standard solution, a very large concentration of Fe will be added to a small initial concentration of SCN (hereafter referred to as [SCN li. The [Fe j in the standard solution is 100 times larger [Fe3+] in the equilibrium mixtures. According to LeChatelier's principle, this high concentration forces the reaction far to the right, using up nearly 100% of the SCN ions. According to the balanced equation, for every one mole of SCN reacted, one mole of FeSCN 2+ s produced. Thus [FeSCN 1st d is assumed to be equal to SCN li Assuming [FesCN2+] and absorbance are related directly (Beer's law), the concentration of FeSCN for any of the equilibrium systems can be found by: 2+I FeSCN Astd allows you to determine the concentrations of the other two ions at Knowing the [FeSCN2+l produced, one less mole of Fe ons will be found in equilibrium. For each mole of FeSCN ions solution (see the 1:1 ratio of coefficients in the equation on the previous page). The [Fey can be determined by Fe3+l Because one mole of SCN is used up for each mole of FesCN21 ions produced, [SCN leq can be determined by SCN li-IFesCN? SCN-leq Knowing the values of IFey+leq, ISCN-lea and [FesCN2 leq you can now calculate the value equilibrium constant. 20 -1 Chemistry with Vernier

Explanation / Answer

1) Fe3+(aq) +SCN-(aq) --->FeSCN2+(aq)

iron(iii)        thiocyanate ion thiocyanoiron(III)

2) red solution (FeSCN2+(aq))

3)keq=[FeSCN2+]/[Fe3+] [SCN-]

4)Fe(NO3)3 is the source of Fe3+ (iron(III) reactant so when it is taken in excess,the entire Fe3+ gets used up .so that [FeSCN2+] =[Fe3+]=(equilibrium concentration ) approximately.So,the known concentration of the product can be approximated for the standard solutions ,and plotted against their absorbances determined spectrophotometrically.

the concentration of product in the unknown solution can then be determined from the callibration curve.

the eqm concentration is needed to calculate keq

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