DETERMINATION OF THE EQUILIBRIUM CONSTANT FOR A CHEMICAL REACTION INTRODUCTION W

Question

DETERMINATION OF THE EQUILIBRIUM CONSTANT FOR A CHEMICAL REACTION INTRODUCTION When chemical substances react, the reaction typically does not go to completion. Rather, the system goes to some intermediate state in which the rates of the forward and reverse reactions are equal. In this state, the reactants and the products have concentrations which do not change with time. Such a system is said to be in chemical equilibrium. When in equilibrium at a particular temperature, a reaction mixture obeys the Law of Chemical Equilibrium, which imposes a condition on the concentrations of reactants and products. This condition is expressed in the equilibrium constant Kc for the reaction. In this experiment we will study the equilibrium properties of the reaction between iron(III) ion and thiocyanate ion Fe3* (aq) + SCN (aq) FeSCN2* (aq) When solutions containing Fe3+ ion and thiocyanate ion are mixed, they react to some extent, forming the FeSCN2+ complex ion, which has a deep red color. As a result of the reaction, the equilibrium amounts of Fe3+ and SCN will be less than they would have been if no reaction had occurred; for every mole of FeSCN2* formed, one mole of Fe3+ and one mole of SCN- will react. The equilibrium constant expression Kc for Reaction 1 is FeSCN [Fe3+[sCN-] Kc The value of Kc is constant at a given temperature. This means that mixtures containing Fe and SCN will come to equilibrium with the same value of Kc, no matter what initial amounts of Fe3 and SCN- were used. Our purpose in this experiment will be to find Kc for this reaction for several mixtures that have been made up in different ways, and to show that Kc indeed has the same value in each of the mixtures. The reaction is a particularly good one to study because Kc is of a convenient magnitude and the color of FesCN2+ ion makes for an easy analysis of the equilibrium mixture

Explanation / Answer

step 1:

concentration of Fe3+= conc. of SCN- = 2x10^-3 M

volume of Fe3+= volume of SCN- = 5 ml

therefore no. of moles of Fe2+= SCN- = 2x10^-3 * 0.005 L= 10^-5 moles.

step 2:

volume of mixture at equilibrium is sum of two volumes= 5+5=10 ml

concentration of FeSCN2+= 1.4 x10^-4 M

no. of moles= 1.4x10^-4* 0.01 = 1.4x10^-6 moles

the no. of moles of Fe2+ and SCN- is also equal to 1.4x10^-6 moles

step 3:

moles of fe2+ remaining unreacted=moles of SCN- = initial moles- moles reacted

= 10^-5 - 1.4x10^-6 moles

= 8.6x10^-6 moles

step 4:

volume of equilibrium mixture= 10 ml

concentration of Fe2+ at equilibrium=conc. of SCN-= 8.6 x 10^-6 moles

concentration of FeSCN2+ formed= 1.4x10^-6 moles

step 5:

Kc=[FeSCN2+]/[Fe2+][SCN-]

= 1.4x10^-6/(8.6x10^-6)2

   = 1.893x10^4

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