Determination of Kc for Fescn2
Chemical reactions that produce products at the same rate at which they produce reactants is said to be at equilibrium and are expressed using double arrows. However while an equation may be at equilibrium the concentrations of the products and reactants are not necessarily equal (Tro, 2011). In order to quantify the relationship of the products to the reactants the equilibrium constant is used. In this experiment, the reactants are ferric ion and thiocyanate ion which form the product . The equation for this reversible reaction and the is:
By calculating one can determine to which side of the equilibrium the reaction lies. If is greater than 1 the forward reaction is favored, there are higher concentration of products and the equilibrium point lies to the right. However if is less than 1, the contrary is true and the equilibrium lies towards the left, the reverse reaction is favored, and there are higher concentrations reactants (Tro, 2011).
In this experiment, when ferric ion and thiocyanate ion were mixed to create the resulting solution has a deep red color. The light absorbance of the solution can then be measured with a spectrophotometer. Then as the concentration of the thiocyanate ion was varied, one can use the absorbance measurements to calculate the equilibrium constant using Beers Law.
Beers Law states if the concentration and absorbance of a standard solution is known, then a sample of the solution with an unknown concentration can be determined using its absorbance. The purpose of this experiment was to be able to determine the equilibrium constant as the concentration of the thiocyanate ion was changed, (thus the concentration of the product would be different from that of the standard solution) using the absorbance of each equilibrium solution.
This experiment began by preparing the standard solution. This was done by initially placing 30mL of 0.0020M into a 100mL graduated cylinder. Then 18.0mL of 0.200M was measured into a 25mL graduated cylinder; then transferred to a large test tube. Finally 2.0mL of the 0.0020M was mixed into the test tube using a pipet.
After the standard solution was made, the five separate equilibrium solutions could be created. This process began by measuring 30mL of 0.0020M into a graduated cylinder. Then five test tubes labeled 1-5 each had 5.0mL of the 0.0020M pipetted into them. That was followed by pipetting 1.0, 2.0, 3.0, 4.0, and 5.0mL of 0.0020M into the each test tube based on the respected labeled number. Afterwards each test tube was then added deionized water to bring the total solution volume to 10.0mL. To ensure thorough mixing of each solution a stirring rod was used, but rinsed with DI water