03MANKeq - Chemical Equilibrium Revised: 4/21/10 CHEMICAL...

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Chemical Equilibrium Revised: 4/21/10 1 CHEMICAL EQUILIBRIUM: CALCULATING K C Adapted from "Chemistry with Computers" Vernier Software, Portland OR, 1997 & from “Journal of Chemical Education” Vol. 75, No. 12, December 1998 http://jchemed.chem.wisc.edu/Journal/Issues/1998/Dec/abs1628.html OBJECTIVES For you, the student, to write…. SAFETY PRECAUTIONS Safety goggles and aprons must be worn at all times. Part B: Potassium thiocyanate is moderately toxic by ingestion; emits toxic fumes of cyanide if strongly heated or in contact with concentrated acids. Iron(III) nitrate is a strong oxidizer; skin and tissue irritant. Nitric acid is corrosive and can cause burns. INTRODUCTION When Fe 3 + and SCN are combined, equilibrium is established between these two ions and their product, the FeSCN 2 + ion: F e 3 + (aq) + SCN (aq) FeNCS 2+ (aq) (1) (Note: The thiocyanate ion acts as a isothiocyanate ligand to Fe 3+ , in other words, the iron binds to the nitrogen atom of the ligand not the sulfur atom. This is the reason that the order of SCN is
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Chemical Equilibrium Revised: 4/21/10 2 changed to NCS for the product.) You will use Spartan to more fully understand the structure and properties of the SCN - reagent and the FeNCS 2+ product. To calculate the equilibrium constant, K c, for the reaction, the concentrations of all ions at equilibrium must be known: [FeNCS 2 + ] eq , [SCN ] eq , and [Fe 3 + ] eq . In this experiment, four equilibrium systems containing different concentrations of these three ions will be prepared. Then, the equilibrium concentrations of the three ions will be experimentally determined and substituted into the equilibrium constant expression to determine the value of the equilibrium constant, K c . Beer’s Law (A = C l ) must be employed to determine [FeNCS 2+ ] eq . A Vis ible to N ear I nfraR ed Spectrometer (VIS-NIR) will be used to measure absorbance of solutions containing the FeNCS 2 + ion. By invoking Beer’s Law, absorbance can be assumed equivalent to concentration. Red solutions, like FeNCS 2 + (aq), absorb blue light, therefore, the max setting should be expected to be around 480 nm. By comparing the absorbance of each equilibrium system, A eq , to the absorbance of a standard FeNCS 2 + solution, A std , [FeNCS 2 + ] eq can be determined. The standard solution created will contain a very large concentration of Fe 3 + and a small initial concentration of SCN ([SCN ] i ). (The [Fe 3 + ] in the standard solution is 100 times larger than [Fe 3 + ] in the equilibrium mixtures.) According to LeChatelier's principle, the high [Fe 3 + ] forces the reaction far to the right, using up nearly 100% of the SCN ions. Therefore, for every one mole of SCN reacted, one mole of FeNCS 2 + is produced and [FeNCS 2 + ] std can be assumed equal to [SCN ] i , the initial concentration of SCN in the standard solution.
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03MANKeq - Chemical Equilibrium Revised: 4/21/10 CHEMICAL...

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