Determination of an Equilibrium Constant Introduction The goals of this experiment are to implement Beer s law to determine concentration of a complex ion and to calculate the equilibrium constant of a complex ion The reaction for which we will find the value of the equilibrium constant is 3 Fe aq 2 HSCN aq H aq FeSCN aq The equation used to determine the equilibrium constant Keq is 2 K eq H FeSCN 3 Fe HSCN This constant is a type is an indicator of the ratio of products to reactant in a solution at equilibrium The units of the items found in brackets are moles per liter Equilibrium is the point at which the forward rate of reaction is equal to the backwards rate of reaction Another concept that is important in this lab is dilution To dilute something is to make it less concentrated through the addition of water or another solvent The final concentration can be calculated using C1 V 1 C 2 V 2 In this equation C1 is the original molarity V2 is the original volume C2 is the final molarity and V2 is the final velocity In this lab we will be using a spectrophotometer This piece of equipment measures the absorbance of a solution with of a concentration over a set distance for a chosen wave length It does this by sending light through a sample of a solution in a cuvette and reading how much light was received at the other end It then uses the following equation to determine the absorbance based on the amount of light transmitted log T 100 Abs After determining the absorbance of a solution Beer s law can be used to determine the molar absorptivity constant This is done by plugging the absorbance A the path length b and the concentration c into the Beer s law equation A b c Another important principle used in this lab is Le Chatelier s principle This principle states that if a large amount of products or reactants are added the reaction will run back towards equilibrium This principle is implemented by using a large about of Fe3 relative to a small amount of HSCN This forces the reaction to essentially run to completion One modern day use of spectrophotometric methods is to determine the amount of anionic surfactants in tap water Because many detergents and cleaning agents contain these they are inadvertently leaked into our waterways The spectrometric methylene process is used in the filtration of these surfactants 1 Procedure 1 The Spectronic 20 spectrophotometer was turned on Next the visible spectrum was set to 300 599 nm The wavelength dial was set to 447nm Once the spectrophotometer warmed up after 15min the spec 20 was adjusted to read T Then the T was set to zero The first sample used in the spectrophotometer was a cuvette filled 3 4 with 0 50 M HNO3 The cuvette holder was removed from the Spec 20 The Spec 20 was then adjusted so that the T read 100 To avoid light being absorbed from the natural oils from fingers finger cots were provided to handle the disposable cuvettes Next using volumetric flasks and pipets solution 1A was prepared With the frosted side facing the front the cuvette with solution 1A was placed in the front The absorbance of solution 1 was read using the Spec 20 This process of making solutions then reading their absorbance was repeated for solution 2A through solution 5A Note that each solution was prepared then immediately measured and discarded before the proceeding solution was made The initial molarity of HSCN was calculated using the values given in the chart on page 34 The Spec 20 was blanked again Once again a 0 50M HNO3 cuvette filled 3 4 s was placed in the Spec 20 and it was set to read 100 0 T Solution 1B was made and then measured Jurado E Fern dez Serrano M N nez Olea J Luz n G Lechuga M Simplified spectrophotometric method Chemosphere2006 65 278 285 The process of making then measuring solutions from the A chart of solutions was repeated using the solutions specification for part B for solutions 2B 5B Clean up 2 Table of Reagents3 Molar Mass g mol Chemical Formula Iron III Nitrate Fe NO3 3 Thiocyanic Acid HSCN Nitric Acid Ferrothiocyanate HNO3 FeSCN2 Density g mL 241 86 1 68 59 09 1 13 63 01 113 92 1 51 1 87 Hazards Oxidizer can cause skin burns Do not ingest Acid Harmful in contact with skin by inhalation or if swallowed strong acid so is very corrosive in high concentration Toxic if ingested Questions to consider 1 We are trying to determine the equilibrium constant Keq of this reaction 3 Fe aq 2 HSCN aq H aq FeSCN aq We are also trying to determine the molar extinction coefficient for FeSCN2 2 A b c A is the absorbance is the molar extinction coefficient b is the path length and c is the concentration in moles per liter 3 We will be able to measure the initial concentrations of Fe3 and HSCN through dilution calculations Since we will know the molarities and the volumes of the stock Thiocyanic Acid Iron III Nitrate and Nitric Acid we can measure the final volume of our solutions and can use the following equation to solve for the initial concentrations of Fe3 HSCN and H C1 V 1 C 2 V 2 Then we can discover the equilibrium concentration of FeSCN2 by using the spectrophotometer and Beer s law Note this can only be done after finding molar extinction coefficient Since the mole ratio of FeSCN2 is one to one with Fe3 HSCN and H you know the magnitude of the change in FeSCN2 is equal to the change in Fe3 HSCN and H Since you know the initial concentration of FeSCN 2 is zero the 2 Hayden McNiel 2011 2012 Chemistry 203 205 Lab Manual Combined Chemical Dictionary Chem Net Base Web 03 Feb 2012 http ccd chemnetbase com proxy2 library illinois edu dictionarysearch do jsessionid 6E1679C7A29B0DB0A07AD53F365E3E2F method view 3 change in concentrations and the initial concentrations of the rest of the reagents you can determine the equilibrium concentrations 4 a The graph of absorbance vs concentration can be used to discover the molar extinction coefficient More specifically the slope of the linear regression is the value of this coefficient b Once the value of the molar extinction coefficient is determined you can use the spectrophotometer and Beer s law to determine the concentration of FeSCN2 at equilibrium Once the equilibrium concentration of FeSCN2 is known you can use that in turn to determine the equilibrium constant 5 3 Fe aq 2 HSCN aq H aq FeSCN aq 2 K eq H FeSCN 3 Fe HSCN