Chem Lab 3

Name: Lab Partners:
Experiment Date: 02/09/2013 Due Date: 02/23/2013

Lab Exercise No. 2
Determination of Kc Values Using UV Absorption

Objectives * To determine the equilibrium constant for a given reaction * To understand the concept of Le Chatelier’s Principle * To gain experience in the use of a UV Spectrophotometer

Background/Concepts * A chemical equilibrium is the state reached by a reaction mixture when the forward reaction and the reverse reaction occur at equal rate, resulting in constant values for the concentrations of the reactants and products. * The equilibrium constant Kc for a reversible reaction is the ratio of the concentrations of the products to the concentrations of the reactants, with each concentration raised to the power of their coefficient in the chemical equation. At constant temperature and pressure, Kc will remain constant regardless of the concentrations of products and reactants. * The Le Chatelier’s Principle states that any change in concentration, partial pressure, or temperature applied to a chemical system at equilibrium will cause the system to shift its equilibrium composition to counteract the change. * Light is transmitted through a sample measured by a spectrophotometer and calculates the transmittance percentage. The lower the concentration of a solution, the higher the transmittance percentage will become, because less light will be absorbed while passing through the solution. * Beer’s Law says that there is a logarithmic relationship between the transmittance and the absorbance of a solution. The absorbance value of the samples can be calculated from the measured transmittance values using Beer’s Law. Then the absorbance values would be used to find the equilibrium constant Kc of the reaction. The absorbance of a solution is directly proportional to its concentration.
Procedure
The spectrophotometer was first warmed up for fifteen minutes and set to zero with a wavelength of 450. The spectrophotometer was then calibrated using a 2 M solution of HNO3. 10 ml of 0.0020 M KSCN, 25 ml of 2.0 M HNO3, 65 ml of DI Water were measured using a 10-ml calibrated cylinder, a 25-ml calibrated cylinder and a 100-ml calibrated cylinder; they were then added to a clean 250-ml calibrated beaker.
1.0 ml of 0.10 M Fe(NO3)3 was added to the same beaker using a 1-ml transfer pipette. The solution in the beaker was mixed with a clean stirring rod. A clean cuvette was placed in an empty 100-ml beaker to keep it from falling over. The cuvette was then filled halfway with the solution using another transfer pipette. The cuvette was held by the upper portion and placed inside the spectrophotometer’s sample holder. The holder’s lid was closed and the reading for % Transmittance was recorded after the fluctuation had stopped. The solution in the cuvette was poured back into the orginal solution in the 250-ml beaker.
The above steps were repeated for another nine times in order to have a final volume of 110 ml for the solution. Calculations were performed to find the values for the absorbance, and the value for the equilibrium constant Kc.

Data and results

Solution No.Measured Quantities | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | % Transmittance | 68.8 | 52.2 | 41.2 | 35.0 | 30.0 | 26.6 | 24.0 | 22.6 | 21.0 | 19.8 | Absorbance(2-Log T) | 1.62•10-1 | 2.82•10-1 | 3.85•10-1 | 4.56•10-1 | 5.23•10-1 | 5.75•10-1 | 6.20•10-1 | 6.46•10-1 | 6.78•10-1 | 7.03•10-1 | [Fe+3] M | 9.90•10-4 | 1.96•10-3 | 2.91•10-3 | 3.85•10-3 | 4.76•10-3 | 5.66•10-3 | 6.54•10-3 | 7.41•10-3 | 8.26•10-3 | 9.09•10-3 | [SCN-] M | 1.98•10-4 | 1.96•10-4 | 1.94•10-4 | 1.92•10-4 | 1.90•10-4 | 1.89•10-4 | 1.87•10-4 | 1.85•10-4 | 1.83•10-4 | 1.82•10-4 | ([Fe+3]i+[SCN-]i) | 1.19•10-3 | 2.16•10-3 | 3.11•10-3 | 4.04•10-3 | 4.95•10-3 | 5.85•10-3 | 6.73•10-3 | 7.59•10-3 | 8.44•10-3 | 9.27•10-3 | ([Fe+3]i•[SCN-]i) | 1.96•10-7 | 3.84•10-7 | 5.66•10-7 | 7.40•10-7 | 9.07•10-7 | 1.07•10-6 | 1.22•10-6 | 1.37•10-6 | 1.52•10-6 | 1.65•10-6 | A{([Fe+3]i+[SCN-]i) | 1.93•10-4 | 6.09•10-4 | 1.20•10-3 | 1.84•10-3 | 2.59•10-3 | 3.36•10-3 | 4.17•10-3 | 4.90•10-3 | 5.72•10-3 | 6.52•10-3 | X=A([Fe+3]i+[SCN-]i)([Fe+3]i∙[SCN-]i) | 9.84•102 | 1.58•103 | 2.12•103 | 2.49•103 | 2.85•103 | 3.15•103 | 3.41•103 | 3.58•103 | 3.78•103 | 3.95•103 | Y=A([Fe+3]i•[SCN-]i) | 8.28•105 | 7.34•105 | 6.81•105 | 6.16•105 | 5.76•105 | 5.39•105 | 5.07•105 | 4.71•105 | 4.47•105 | 4.26•105 |

Calculations of Results with Discussion

* Absorbance=2-Log % Transmittance * Sample 1: A=2-Log 68.8= 1.62×10-1 * Sample 2: A=2-Log 52.2= 2.82×10-1 * Sample 3: A=2-Log 41.2= 3.85×10-1

* [Fe+3]= Molarity of Fe(NO3)3(Volume of Fe+3 solution used)Total Volume of Reaction Solution * Sample 1: [Fe+3]=1 ml ×(0.1M)100 ml+1 ml=9.90×10-4M * Sample 2: [Fe+3]= 2 ml ×(0.1M)100 ml+2 ml=1.96×10-3M * Sample 3: [Fe+3]= 3 ml ×(0.1M)100 ml+3 ml=2.91×10-3M

* [SCN-]=Molarity of SCN-(Volume of SCN- solution used)Total Volume of Reaction Solution * Sample 1: [SCN-]=10 ml ×(0.002M)100 ml+1 ml=1.98×10-4M * Sample 2: [SCN-]=10 ml ×(0.002M)100 ml+2 ml=1.96×10-4M * Sample 3: [SCN-]=10 ml ×(0.002M)100 ml+2 ml=1.94×10-4M

* [Fe+3]i+[SCN-]i * Sample 1: [Fe+3]i+[SCN-]i=9.90×10-4+1.98×10-4=1.19×10-3 * Sample 2: [Fe+3]i+[SCN-]i=1.96×10-4+1.96×10-4=2.16×10-3 * Sample 3: [Fe+3]i+[SCN-]i=2.91×10-3+1.94×10-4=3.11×10-3

* [Fe+3]i• [SCN-]i * Sample 1: [Fe+3]i• [SCN-]i=9.90×10-4•1.98×10-4=1.96×10-7 * Sample 2: [Fe+3]i• [SCN-]i=1.96×10-4•1.96×10-4=3.84×10-7 * Sample 3: [Fe+3]i• [SCN-]i=2.91×10-3•1.94×10-4=5.66×10-7

* A([Fe+3]i+[SCN-]i) * Sample 1: A([Fe+3]i+[SCN-]i)=1.62×10-1•1.19×10-3=1.93×10-4 * Sample 2: A([Fe+3]i+[SCN-]i)=2.82×10-1•2.16×10-3=6.09×10-4 * Sample 3: A([Fe+3]i+[SCN-]i)=3.85×10-1•3.11×10-3=1.20×10-3

* X=A(Fe+3i+SCN-i)Fe+3i∙SCN-i * Sample 1: 1.93×10-41.96×10-7=9.84×102 * Sample 2: 6.09×10-43.84×10-7=1.58×103 * Sample 3: 1.20×10-35.66×10-7=2.12×103

* Y=AFe+3i∙SCN-i * Sample 1: 1.62×10-11.96×10-7=8.28×105 * Sample 2: 2.82×10-13.84×10-7=7.34×105 * Sample 3: 3.85×10-15.66×10-7=6.81×105

* The results were extremely close. All the points lie near the trend line, indicating a linear relationship between X and Y, as predicted by the given equation:
AFe+3i∙SCN-i=-KcAFe+3i+SCN-iFe+3i∙SCN-i+b
OR
Y=-KcX+b

* Using the two points 1 & 8 to determine slope (X,Y values of samples 1 and 8):
-Kc=Slope=△Y△X=Y8-Y1X8-X1=4.47×105-8.28×1053.78×103-9.84×102=-136
Kc=136

* The Theoretical Kc given was 120.
Percent Error=Theoretical Kc-(Calculated Kc)Theoretical Kcx 100

Percent Error=136-120120x 100

Percent Error = 13.3%
Conclusion
* Our results supported the theory that the concentrations of the products and the reactants of a system at equilibrium remain constant, shown by the unchanging transmittance reading of each of the samples after they were allowed time to reach equilibrium. * The errors in our results could have happened because of the difference between actual concentrations and theoretical concentrations. When the solution from the cuvette was poured back into the beaker, some drops of the solution were left in the inner wall of the cuvette. When the new solution with higher concentration was transferred to the cuvette, it was mixed with the droplets of the old concentration, causing the cuvette solution’s concentration to be less than the beaker solution’s concentration. * The results show that as the concentration of the solution increased, so did the absorbance verifying Beer’s Law. * The results also support Le Chatelier’s Principle, proving that when a system at equilibrium is disturbed by a change in concentration of its products and reactants, it will shift the reaction in a way to counteract the change and result in a new equilibrium. Every time Fe(NO3)3 was added to the solution, its transmittance reading changed, meaning a change in the equilibrium composition. * The value for the equilibrium constant Kc was found by finding the negative value of the slope graph of X vs Y.
AFe+3i∙SCN-i=-KcAFe+3i+SCN-iFe+3i∙SCN-i+b

* All the points on the graph were to form a straight line to show that Kc is constant. This supports the fact that given constant temperature and pressure, the equilibrium constant for concentration will remain the same regardless of concentrations. The experiment was in room temperature and pressure making the value of Kc constant in each sample. * When the spectrophotometer transmits UV light through the samples, some of the UV light will become absorbed by the [FeSCN+2] ions, for UV light has shorter wavelength than red light. [FeSCN+2] samples were red because of the ions absorbed light of wavelengths outside the red spectrum and transmitted light of wavelengths within the red spectrum. The spectrometer measured the amount of light able to pass through the sample and gave the results in transmittance percentage. This shows how much [FeSCN+2] is present. The higher the concentration, the more UV light is absorbed giving a lower transmittance percentage.