Solubility Equilibrium and the Effect of Temperature

Introduction

A chemical reaction usually starts with reactants which react to yield products. Many times the reactants are completely used up to make products. However, the reactants sometimes do not completely turn into products. There is an equilibrium between the concentration of reactants and products. At equilibrium, the reactants turn into product and the products decompose into reactants at the same rate. This ratio of the products to reactants at equilibrium is represented by the equilibrium constant, or K. K is found by taking the concentration and order of the products and dividing by the concentration and order of the reactants. In this experiment, iron(III) ion reacts with thiocyanate ion to produce thiocyanatoiron (III). The reaction is represented by the following equation: Fe3+ + SCN– <—-> FeSCN2+. Using a spectrophotometer, the absorbance of FeSCN2+ is measured at different concentrations. The absorbance in then to find concentration and ultimately the equilibrium constant.

Procedure

First, a clean cuvette was obtained, rinsed, and filled three-fourths full with 0.5 M HNO3 solution. This was used as a the blank solution for the spectrophotometer, which was set at 447 nm. Next, a 100 mL volumetric flask was obtained and rinsed with distilled water. Then I will make 4 different [FeSCN2+] concentration of solution to test the absorbance, 0.00004M, 0.00008M, 0.00012M, and 0.00016M. After writing all the data for absorbance, I would start the second part of the experiment.

In the second part, I mark 5 different test tube as 1 to 5. In test tube 1, I add 5 ml of 0.002M Fe(NO3)3, 1ml of 0.002M HSCN and 4 ml of 0.5M HNO3. In the test tube 2, I add 5ml of 0.002M Fe(NO3)3, 2ml of 0.002M HSCN and 3ml of 0.5M HNO3. In test tube 3, I add 5 ml of 0.002M Fe(NO3)3, 3ml of 0.002M HSCN and 2 ml of 0.5M HNO3. In test tube 4, I add 5 ml of 0.002M Fe(NO3)3,41ml of 0.002M HSCN and 1 ml of 0.5M HNO3. In test tube 5, I add 5 ml of 0.002M Fe(NO3)3 and 5ml of 0.002M HSCN. After making these test tube, I write the absorbance from the machine.

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