Equilibrium Constant of a Reaction

Learning Objectives:

Upon completion of this experiment, students will have experienced:

1. To review Beer's Law and use it to determine the concentration of a complex involved in a chemical equilibrium.

2. To use experimental data to determine an equilibrium constant

3.   To explore the Le Chatelier's principle as it applies to a chemical reaction.

The product of the reaction makes a complex of very deep red color, while the Iron (III) and thiocyanate ion are almost colorless.   Because the product is colored, we can use spectrophotometry to determine the concentration of the product.  Beer's Law shows the relationship between the concentration and the absorption of light.   The relationship is a direct relationship, as the concentration increases, the absorption of light at a single wavelength also increases proportionately. 

            A α C

This relationship can be rewritten as an equation by placing a constant into the equation.   The constant, 𝛆, is known as absorptivity or molar absorptivity and b is the pathlength of the cuvette which is generally 1 cm.

                  A = 𝛆 b C

Using this relationship, the equilibrium concentrations of the product can be determined by the absorbance and the equilibrium concentration of the reactants can be determined by the change in the concentration if the initial concentrations before equilibrium are known.

There are two parts to this experiment that must be carried out in order to determine the equilibrium constant for this reaction. First, a Beer's law constant must be determined for the thiocyanate iron (III) complex. This will be done by adding a huge excess of thiocyanate ion to solutions containing iron (ill) ion to ensure that all the iron (Ill) ion has been converted to Fe(SCN)+2. The molar absorptivity, 𝛆, that has been determined from the slope of the linear plot will then be used to determine the thiocyanato iron (III) concentrations in solutions which have equilibrium mixtures of all components of the reaction.

Pre laboratory Writing Assignment

Complete the pre laboratory writing assignment in your laboratory notebook.   The prewriting must be completed before the laboratory session begins.    Use the procedure document below to explain what has to be done to collect the necessary data of the lab.   The writing should include a purpose/hypothesis of the experiment, a discussion or theory, a detailed step by step summary of the procedure explaining what data will be collected and how you will collect it.   Data Tables or space to collect the data should be included within the procedure discussion.   Look up any SDS on chemicals that you are not familiar.    

Part II:  Determination of Kc 

Use MicroLab to collect the absorbance of the B solutions to determine the concentrations used to determine the equilibrium constant for the reaction.   

Part III:  Le Chatelier’s Principle 

Use MicroLab to collect the absorbances of the C solutions to determine the concentration data and to understand Le Chatelier's Principle.