, the equilibrium constant is described by: 
When two reactants are mixed, they react at a given rate to form product(s). This process is called kinetics. The kinetics in a reaction relate how fast reactants are consumed to form products in the reaction. For that same reaction, at some point during the kinetics, the molar ratio of reactants to products reaches a constant value, equilibrium. Equilibrium is a dynamic state in which the rate of formation of the products is equal to the rate of formation of the reactants (also called the Law of Mass Action). Reactions in chemical equilibrium will remain so until the system is altered by some outside factor, such as removing some material from the product side. The reaction will then counter-balance that stress and restore the equilibrium concentration constant, Kc.
For the general reaction , the equilibrium constant is described by:
Recall from kinetics: Square brackets ([ ]) indicate concentration in units of molarity (M or mol/L).
For a given reaction at a given temperature, no matter how a reaction is performed (i.e. A, B, C and D are mixed in arbitrary amounts), the reaction will always reach a state of equlibrium according to its given equilibrium constant, Kc. 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(II) ion is formed. The starting concentrations of iron(III) ion and thiocyanate ion (SCN-) decrease. By determining the concentrations of these three chemical species in several solutions, we can calculate the equilibrium constant, Kc, within experimental tolerance. Note: The value of Kc will change somewhat with reactant concentration because of a complicated side reaction. The blood-red thiocyantoiron(III) ion we study also reacts with additional 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. Using that information, calculating the concentrations of all of the other species for the equilibrium expression can be accomplished.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Note: Two different solutions of iron(III) nitrate are used, in different places.
| Part I | Part II |
|
|