; the equilibrium constant is described
by:
DETERMINATION OF AN EQUILIBRIUM CONSTANT
Kotz & Treichel, Chapter 16, pages 749-793
When two reactants are mixed, they will react to form products until a state is reached wherein the amounts of reactants and products remain constant. This is a dynamic state in which the rate of formation of the products is equal to the rate of formation of the reactants. The reactants and products are in chemical equilibrium and will remain so until the system is altered by some outside factor. The equilibrium constant, Kc, relates the concentrations of reactants and products.
For the reaction ; the equilibrium constant is described
by:
.
This is called the Law of Mass Action.
Square brackets([ ]) are used to indicate concentrations in mols/liter, that is, molarity(M).
If A, B, C and D are mixed in arbitrary amounts, they will react until their concentrations satisfy the above equation.
To demonstrate this phenomenon, we will study the equilibrium system of iron(III) and thiocyanate ion.
When solutions containing iron(III) ion and thiocyanate ion are mixed, the deep red thiocyanatoiron(III) ion is formed. The starting concentrations of iron(III) ion and thiocyanate ion SCN- decrease. By determining the concentrations of these four chemical species in several solutions, we can calculate the equilibrium constant, Kc, and note its "approximate" constancy.
The constant will change somewhat with concentration because we are not fortunate enough to be able to isolate a single uncomplicated reaction. The blood-red thiocyantoiron(III) ion reacts with more thiocyanate ion to form a dithiocyantoiron(III) ion!
Standard solutions of iron(III) nitrate and thiocyanic acid will be used to prepare the equilibrium sample solutions. Hydrogen ion will be kept approximately constant in all solutions using nitric acid. The concentration of the complex will be measured by spectrophotometry. From that information you can calculate the concentrations of all of the other species for the equilibrium expression.
SOLUTIONS USED |
Pertinent Species |
Comments |
0.00200 M Fe(NO3)3 |
0.00200 M Fe(+3) |
called "dilute" solution |
0.200 M Fe(NO3)3 |
0.200 M Fe(+3) |
called "concentrated" |
0.00200 M KSCN |
0.00200 M SCN(-1) |
thiocyanate |
0.50 M HNO3 |
used to adjust volumes |
Note: Two different solutions of iron(III) nitrate are used, in different places.
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