20 Chemical Equilibrium: Finding a Constant, Kc The purpose of this lab is to ex
ID: 498285 • Letter: 2
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20 Chemical Equilibrium: Finding a Constant, Kc The purpose of this lab is to experimentally determine the equilibrium constant, Kc. for the following chemical reaction: Fe3+(aq) SCN-(aq) FeSCN2 (aq) iron (I) thiocyanate thiocyanoiron(II) When Fe and SCN- are combined, equilibrium is established between these two ions and the FeSCN2+ ion. In order to calculate Kc for the reaction, it is necessary to know the concentrations of at equilibrium: [FeSCN2 leg, [SCN-leg, and [Fes jeg. You will prepare four equilibrium systems containing different concentrations ofthese three ions. The equilibrium o the three ions will then be experimentally determined. These values will be substituted into the equilibrium constant expression Ke constant. You will use a to determine [FeSCN? leg. The FeSCN ion produces solutions with a the red solutions absorb blue light very well, users be instructed to use the 470 nm LED. The light striking the detector is reported as absorbance of a standard comparing the ofeach equilibrium standard solution has solution, you can determine [FeSCN The a known FeSCN2+ concentration To prepare the standard solution, a very large concentration of Fe3+ will be added to a smal concentration ofSCN (hereafter referred to as [Fe lin the standard solution is 100 times larger than [Fe3+] in the equilibrium mixtures According to LeChatelier's principle, this high concentration forces the reaction far to the right, using up nearly 100% SCN is produced to the balanced equation, for every one mole of SCN reacted, one mole of [FeSCN 1std is to be equal to li Assuming absorbance are related directly (Beer's law), the concentration of FeSCN2+ for any of the equilibrium systems can be found by x [FesCN2+lstd FeSCN2+ eq Astd Knowing the 2+]eg allows you to determine the concentrations of the other two be found in FeSCN2 produced, one less mole of Fe3+ ions will [Fes For each mole of ions previous page). the (see the 1:1 ratio ofcoefficients in the equation on the determined by Fe3+] one of SCN is used up for each mole of FescN2 ions produced, scN-leq can be determined by: ISCN leq Knowing the values of Feu leq, IscN-leq, and IFescN2+leq, you can now calculate the value of Kc, the equilibrium constant 20-1Explanation / Answer
(1)
Yes, the equilibrium constant Kc for all the test samples should ideally be the same, because the value of Kc doesn't depend on the initial concentrations of reactants or the products, but it is dependent on the temperature of the reaction.
Since T remains constant, so Kc should also come same for all the values.
(2)
The reason for the observed differences in the Kc values for all the test samples could be caused during the transferring of solutions. This can result in some reagent being added in slight excess or shortage than the theoretically reported values for the experiment.
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