LAB Report 14
The Determination of an Equilibrium Constant
Zengping Li

Introduction: Our objective is to determine the equilibrium constant for the reaction Fe3++SCN- = FeSCN2+ and too see if the equilibrium is same or not under different conditions. Equilibrium constant can tell us which direction the reaction shift to. If eq less than 1, the reaction shift to left. If eq equal to zero, the reaction is equal. If the eq greater than 1, the reaction shift to right. For beaker 1 the M of FeSCN is mole of FeSCN/ total volume = volume of SCN/1000X0.002/0.050=0.00016 Breaker number FeSCN 1 0.00016 2 0.00012 3 0.00008 4 0.00004
Procedure:
First, mixing the known amount of Fe(NO3)3 in to known amount of SCN.( assume they react compeletely). Second ,fill the cuvette with the FeSCN mixture that we just got, place it in the spectrometer and start the data collection. Then the absorbance of the mixture will show on the spectrophotometer. With it we can calculate the equilibrium constant that we are looking for.
Discussion :
Data table
Part I:
Temperature: 21.9C FeSCN Absorbance
1 0.00016 0.603318
2 0.00012 0.449558
3 0.00008 0.295798
4 0.00004 0.142038
Y=mx+b we have m=3844 and b=-0.011722 and we are looking for absorbance y
For beaker 1: y= 3844x0.00016+(-0.011722)=0.603318
Linear Regression equation: y=mx+b
PART II
Beaker Absorbance FeSCN at eq
A 0.253 0.00006277
B 0.356 0.00008956
C 0.476 0.00012078
For this data, now we have abosorbance and we are looking for the mole mass of FeSCN at equilibrium. By using equation y=mx+b now we have absorbance which is y, m and b.
For beaker A: x=(y-b)/m= (0.253-0.011722)/3844=0.00006277
Data Analysis
Beaker FeSCN Fe SCN Keq
A 0.00006277 0.01996 0.00009722 32.347
B 0.00008956 0.0199 0.00007522 59.83
C 0.00012078 0.0193961 0.00041039 76.61
For beaker A: the initial concentration of Fe(No3)3 is 5/1000x0.2/0.050=0.02
The initial concentration of SCN is 0.00016 then we can write down the ice table Fe(No3)3 SCN FeSCN
Initial 0.02 0.00016 0
Change -0.00006277/2 -0.00006277/2 0.00006277
EQ 0.01996 0.00009722 0.00006277
Now we can calculate the equilibrium constant
Keq= FeSCN/FexSCN=0.00006277/0.01996x0.00009722=32.347
Calculating equilibrium concentrations:
Beaker A Fe SCN FeSCN
Initial 0.02 0.00016 0
Change -0.00006277/2 -0.00006277/2 0.00006277
Equilibrium 0.01996 0.00009722 0.00006277
Beaker B Fe SCN FeSCN
Initial 0.02 0.00012 0
Change -0.00008956/2 -0.00008956/2 0.00008956
Eq 0.0199 0.00007522 0.00008956
Beaker C Fe SCN FeSCN
Initial 0.02 0.00008 0
Change -0.00012078/2 -0.00012078/2 0.00012078 eq 0.0193961 0.00041039` 0.00012078