Equilibrium Constant
Topics: Chemical equilibrium, Laboratory glassware / Pages: 11 (2698 words) / Published: Apr 2nd, 2015

Chemical reactions don’t typically go to completion. Instead, the system goes to an intermediate state where the rate of the forward reaction and the rate of the reverse reaction equal each other. At this point, the concentrations do not change with time. These reactions are said to be in equilibrium. Equilibrium is depended on a particular temperature, and the concentrations of reactants and products have to follow a rule demonstrated by the equilibrium constant Kc.
The equilibrium concentrations that will be studied is the reaction between iron (III) ion and thiocyanate ion:

The mixture of Fe3+ and SCN- react to form a compound that produces a dark brown/red color. Since this is the case, it means the complex absorbs the most light at a wavelength of around 450-480nm. Using the balanced equation, it can be seen that for every one mole of FeSCN2+ formed, one mole of Fe3+ and one mole of SCN- will react. Therefore, the equilibrium constant expression for the reaction is:

Kc is constant at any given temperature. Then it can be deduced that mixtures with any different initial amounts of Fe3+ and SCN- will always come to equilibrium with the same value of Kc. This lab will use several mixtures of varying amounts of Fe3+ and constant amount of SCN- using serial dilutions to show that Kc is indeed the same value for each mixture.
Using Le Chatelier’s principle, the amoung to FeSCN2+ formed can be found. As more of the same reactant is added, more of the product will be formed. It can go to a point where so much of the reactant is added that all of the other reactant will be turned into product. In the experiment, limiting amounts of Fe3+ will be used and large amounts of SCN- will be mixed to produce known amounts of FeSCN2+, because the compound formed will be essentially equal to the starting amount of the limiting reactant.

Experimental Procedure
Before beginning, acquire six test tubes, 2.0x10-4M KSCN (aq), 0.20M Fe(NO3)3, a 5ml volumetric

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