Determination of an Equilibrium Constant

A system is considered in a state of equilibrium when its properties do not change at time passes. Equilibrium is a state of a reaction where the concentration of all the reactants and all the products remain constant over time. Equilibrium as used in chemical systems means that all the chemical forces in a reaction are in balance and that all physical properties of the system (color, density and concentration) of all chemicals species involved remains constant. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, Keq. In this experiment the Keq for the reaction between iron (lll) ions and thiocyanate ions, SCN- is determined.

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Fe3+(aq) + SCN- (aq) FeSCN2+ (aq)
The equilibrium constant Keq, is defined by the equation;
Keq= [ FeSCN2+] / [Fe3+] [SCN-]

It is necessary to determine the molar concentration of each of the three species in solution at equilibrium to fine the value of Keq, which depends only upon temperature. In order to have a successful evaluation of this equilibrium system, it is important to conduct three tests separately. This experiment is done into two sections. First, from the series of standard solution of FeSCN2+ was prepared varying concentration of SCN- and constant concentration of H+ and Fe+ that are in stoichiometric excess. The excess of H+ ions will ensure that Fe3+ has no engage in the reaction and excess of Fe3+ ions will make the SCN- ion which is a limiting reagent. In part ll the solution of unknown [SCN-] was analyze using the same procedure as in part l.


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