7 Determination of an Equilibrium Constant Introduction When chemical substances react the reaction typically does not go to completion Rather the system goes to some intermediate state in which the rates of the forward and reverse reactions are equal In this state the reactants and the products have concentrations which do not change with time Such a system is said to be in chemical equilibrium When in equilibrium at a particular temperature a reaction mixture obeys the Law of Chemical Equilibrium which imposes a condition on the concentrations of reactants and products This condition is expressed in the equilibrium constant K c for the reaction In this experiment we will study the equilibrium properties of the reaction between iron III ion and thiocyanate ion 3 Fe aq SCN aq 3 FeSCN 2 aq When solutions containing Fe ion and thiocyanate ion are mixed they react to some 2 extent forming the FeSCN complex ion which has a deep red color As a result of the 3 reaction the equilibrium amounts of Fe and SCN will be less than they would have 2 3 been if no reaction had occurred for every mole of FeSCN formed one mole of Fe and one mole of SCN will react The equilibrium constant expression Kc for Reaction 1 is Chemistry 1B Experiment 7 21 22 Chemistry 1B Experiment 7 The value of Kc is constant at a given temperature This means that mixtures 3 containing Fe and SCN will come to equilibrium with the same value of K c no matter 3 what initial amounts of Fe and SCN were used Our purpose in this experiment will be to find Kc for this reaction for several mixtures that have been made up in different ways and to show that Kc indeed has the same value in each of the mixtures The reaction is a particularly good one to study because Kc is of a convenient magnitude and 2 the color of FeSCN ion makes for an easy analysis of the equilibrium mixture The mixtures will be prepared by mixing solutions containing known concentrations of iron III nitrate Fe NO 3 3 and potassium thiocyanate KSCN The 2 color of the FeSCN ion formed will allow us to determine its equilibrium concentration Knowing the initial composition of a mixture and the equilibrium 2 concentration of FeSCN we can calculate the equilibrium concentrations of the rest of the pertinent species and then determine Kc 2 Method for Determining FeSCN 2 In this experiment you will use a spectrophotometer to determine FeSCN in the equilibrium mixtures Instructions for use of the Spectronic 20D spectophotometer are attached The spectrophotometer measures absorbance the amount of light absorbed by the complex at a given wavelength Beer s law expresses the relationship between absorbance A and concentration of a colored species c A bc Here is a constant that depends on the wavelength of light and on the substance that is absorbing the light b is the distance that the light travels through the sample of the absorbing substance 2 The FeSCN complex absorbs blue light That is why it has a reddish orange color It absorbs the most light at a wavelength of 447 nm Therefore at this wavelength 2 absorbance measurements will have the highest sensitivity to FeSCN In this experiment you will measure the absorbance of all solutions at 447 nm In the first part of the experiment you will determine the relationship between the 2 absorbance and FeSCN at 447 nm You will do this by measuring the absorbance of 2 three standard solutions in which FeSCN is known There is a problem here how 2 2 can known concentrations of FeSCN be obtained FeSCN participates in the 3 equilibrium with Fe and SCN ions Known amounts of the reactants will not necessarily yield a known amount of the product This difficulty can be avoided According to Le Ch telier s principle a net reaction from left to right that is in the forward direction occurs when more of a reactant is added As more and more of the same reactant is added more and more of the product will be formed It is possible to add so much of this reactant that essentially all of the other reactant will be converted to product You will use limiting quantities of 3 2 SCN and overwhelming amounts of Fe to produce known amounts of FeSCN in Chemistry 1B Experiment 7 23 2 your standard solutions The amount of FeSCN that is formed will then be essentially equal to the starting amount of the limiting reactant Once you have measured the absorbances of the standard solutions you will plot 2 the absorbances against the concentrations of FeSCN on a graph or calibration curve The points on the calibration curve will fall on a straight line which has a slope of b the constant of proportionality between absorbance and concentration You can 2 use this calibration curve to find FeSCN in other solutions In the second part of the experiment you will measure the absorbance of a different set of solutions in which substantial amounts of both reactants and the product are present You will use your calibration curve to convert the measured absorbance to 2 the equilibrium concentration of FeSCN in each solution From the initial concentrations of the reactants and the equilibrium concentration of the product you will calculate the equilibrium constant for the reaction In preparing each of the mixtures in this experiment you will maintain the concentration of H ion at 0 5 M The hydrogen ion does not participate directly in the reaction but its presence is necessary to avoid the formation of brown colored species 2 2 such as FeOH which would interfere with the analysis of FeSCN Experimental Procedure SAFETY PRECAUTIONS Wear your SAFETY GOGGLES Nitric acid HNO 3 is corrosive If any solutions containing nitric acid splash onto your hands or clothing wash them off immediately with copious amounts of running water WASTE DISPOSAL All waste from this experiment should be poured into the INORGANIC WASTE containers in the fume hood 2 Part 1 Finding the relationship between absorbance and FeSCN Label 3 medium sized test tubes Table 7 1 shows the amounts of the 3 concentrated 0 20 M Fe NO3 3 in 1 M HNO3 solution 2 00 10 M KSCN solution and purified water that should be added to each tube Pipet the approximate amount of each solution into each tube Record the exact amount of each solution that you actually add You will need to use these actual amounts in your calculations Obtain three separate small pieces of parafilm Close the top of each test tube with the parafilm Mix each solution thoroughly by inverting the test tube several times Record your observations How do the