Measurement Of An Equilibrium Constant Essay

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Measurement of an Equilibrium Constant
Camille Kuo

Table of Contents:
Purpose and Hypothesis­3
Safety Considerations­4
Post Lab Calculations­7­8­9­10
Evidence and Reasoning­10­11
Errors and Improvements­11


The purpose of this lab was to discover the K eq of the reaction F e3+ + SCN − → ←
F eSCN 2+ . This was done by measuring the absorbance of the product of the reaction and graphing a Beers Law graph to compare concentration and absorbance. We were then confronted with a series of solutions where we had to find the absorbances and concentration, then use that knowledge to find the concentration of the products at equilibrium. After we had the concentrations of all of the reatants and products at eqilibrium we used an equation for K eq to find K eq . Introduction:
Most chemical reactions can work forward and in reverse, notated as such:
A + B → ← C + D
When the concentrations of A and B are higher the C and D, the reaction works forward, producing C and D. When the concentrations of C and D are much higher, the reaction works in reverse, producing A and B. When enough of the other side of the reaction is formed, the reaction slows down, eventually reaching equilibrium. This is when the ratio of concentrations of the reactants and products remains constant. This ratio is called the equilibrium constant, and is shown as: K eq = [C][D]
Equilibrium for the equation for the formation of F eSCN 2+ is reached very quickly through the equation F e3+ + SCN − → ← F eSCN 2+
The equilibrium constant for this equation is: K eq = [Fe][SCN]


In order to find equilibrium we must know the equilibrium concentrations of the reactants and products. This can be found by knowing the exact initial concentrations of the reactants and after the solution reaches equilibrium, measuring the concentration of F eSCN 2+ using the spectrophotometer. Using Beers law, we can graph the line of Absorbance vs. Concentration. Since the mole ratio of F e3+ , F eSCN 2+ , and SCN − is one, we know the concentrations of all of the reactants and products at equilibrium if we know the absorbance of F eSCN 2+ . Purpose:
The purpose of this lab is to find the equilibrium concentrations of the reactants and products and the K eq for the reaction F e3+ + SCN − → ← F eSCN 2+ using our knowledge of mole ratio, Beers Law, and a spectrophotometer. Hypothesis:
Although we guess an exact value for K eq , we can estimate that it will be greater than one because F e3+ is positively charged and SCN − is negatively charged so they will react easily. Variables:
Part A and B:
Independent Variables: Volume of 0.002M K SCN
Dependent Variables: Absorbance of Solution
Controlled Variables: External pressure, temperature, wave length of photospectrometer, total volume of solution


Part C:
Independent Variables: Volume of 0.002M F e(NO)3
Dependent Variables: Absorbance Measured
Controlled Variables: Total volume of solution

Safety considerations:
In this lab we will be working with acids that have the potential to cause harm when they come into contact with your skin. To avoid injury wear goggles when handling any solutions and exercise caution. Do not let any of the solution come into contact with your skin. Procedure:
Part A:
1. Obtain and label three beakers, one with ~45 mL 0.00200M KSCN solution, one with
~90 mL 0.050M HNO3 , and one with ~35 mL 0.200M F e(NO3)3 . Record the molarities of each.
2. Obtain 5 large, clean, dry test tubes and label them 1­5/
3. Use pipettes to add the appropriate amounts of solution to each test tube according to the following table:

Total V (mL0

V of 0.002M

V of 0.200M
F e(NO3)3 (mL)

V of 0.50M
HNO3 (mL)