keq experiment

7/28/2019 Keq Experiment

1/6

Burhan Riaz

11/08/2011

The Ratio between Products and Reactants at Equilibrium

Beginning Ideas:

We are given a reversible reaction of Fe + SCN which produces the product Fe(SCN). To find a

the Keq one must know the concentrations and coefficients of the reactants and products. We

know the initial concentrations of the reactants and by using a spectrometer we can find the final

concentration of the product. To find the final concentrations of the reactants, we can use an

algebraic "ICE" table.

Tests:

5 trials were completed using different concentrations of reactants. We prepared the spectrometer

by using water as a blank and setting the transmittance to 100%. We prepared the solutions by

measuring Fe(NO3)3, KSCN, and H2O in separate graduated cylinders and poured the measured

amounts into the cuvette. The solution always added up to 10mL. The resultant solution was then

placed into the spectrometer and 2 minutes were given for the machine to adjust its transmittance

measurement. The transmittance was then noted and absorbency was calculated. By using the

properties of absorbency, the final concentration of Fe(SCN) was calculated. By plugging this

value into an ICE table, the rest of the final concentrations were calculated. The Keq was then

calculated.

Observations:

Initial Observational Data


2/6

Trial Fe(NO3)3(mL) KSCN(mL) H2O(mL) Total Volume(mL) %Transmittance

1 3.0 3.0 4.0 10.0 48%

2 3.5 3.5 3.0 10.0 44%

3 4.0 4.0 2.0 10.0 39%

4 4.5 4.5 1.0 10.0 35%

5 5.0 5.0 0.0 10.0 31%

As expected, increasing the amounts of reactants made a darker colored product as shown by the

decreasing amount of transmittance.

Evidence:

Calculated Data

Trial Absorbency(Ab) Fe(SCN) Concentration

1 0.32 7.25x10-5

2 0.36 8.11x10-5

3 0.40 9.30x10-5

4 0.46 1.40x10-4

5 0.51 1.57x10-4

The absorbance was calculated by plugging the %Transmittance into A=2-log(%T). Once

absorbency was found, Beer's law can be used to find concentration, C=A/Eb. E is 4400 and b is

1cm. You can then use an ICE table to algebraically find the final concentrations of the

reactants. Simply find x by subtracting the initial concentration of the product, 0, by the final

concentration of the product. Then subtract x from the initial concentrations of the reactants.

Fe(NO3)3

Concentration(moles/L)

KSCN


FeSCN

Concentration(moles/L)I 2x10

-22.0x10

-20

C -x -x +x

E 2.93x10-3

2.93x10-3

7.25x10-5


3/6

Now that all the final concentrations have been found Keq can be calculated. Keq is the product

raised to the power of its coefficient divided by the product of the reactants raised to the power

of their respective coefficients.

Trial

Fe(NO3)3


KSCN


Fe(SCN)

Concentration(moles/L) Keq

1 1.93x10-3

1.93x10-3

7.25x10-5

19.51

2 1.92x10-3

1.92x10-3

8.11x10-5

22.03

3 1.91x10-3

1.91x10-3

9.30x10-5

25.49

4 1.90x10-3

1.90x10-3

1.40x10-4

28.51

5 1.88x10-3

1.88x10-3

1.57x10-4

32.59

The average Keq is 25.6

Claim:

We can experimentally find Keq by knowing the initial concentrations of the reactants and

finding the final concentration of the product by using a spectrometer. We can then algebraically

find the rest of the information needed to compute a ratio.

Reading:

A key component in understanding this experiment is knowing the relationship between %

transmittance and absorbance. According to Beer's Law, Absorbance=Ebc1. The extinction

coefficient in our case is 447 and the path length is 1cm1. This means we can find the

concentration if we know the absorbency. By using a spectrometer, we can find % transmittance

and absorbency1. We noted down % transmittance since it is linear while absorbency can be

infinite1. The relationship between the two is A=log(100) -log(%T)

1. Once the concentration has


4/6

been found one can plug that information into an ICE table to get the final concentrations of the

reactants1.

A constant value can be applied to all reactions which can predict where an equilibrium rests2.

The Keq is calculated by the product raised to the power of its coefficient divided by the product

of the reactants raised to the power of their respective coefficients1. A high Keq can indicate that

the reaction is product-favored2. In this experiment it is important to note that Fe(NO3)3poses a

safety hazard and it is recommended that you take precaution1.

Reflection:

Qualitatively, the addition of higher volumes of reactants produced a less transparent solution.

This matches with our data of decreasing transparency from trials 1-5. As a consequence, a

greater absorbency meant a higher concentration of the final product. This is why the Keq

increased per trial. What this means is that the more reactants added (and less H2O) the more

product-favored our reactions. This makes perfect sense with what we have so far learned. The

average Keq was 25.6 which means this reaction is greatly product favored.

News Brief:

How do people use trace analysis to find arsenic poising of a victim on your favorite CSI show?

And how does the EPA know when there are harmful chemicals in the lake nearby your house?

According to Beer's law and the Law of Mass Action, one can find the concentrations of a

chemical even if it is minutely small. For example, if you bring a sample into a lab and use a


5/6

spectrometer you can get data that shows how much of that chemical is in the sample by using

Beer's law. You can even predict future reactions of chemicals once you have found a constant

that can predict how reaction will take place.

Citations:

1Chemistry 2046L Laboratory Manual for Chemistry Fundamentals II

2Tro, Nivaldo J. Chemistry: a Molecular Approach. Upper Saddle River, NJ: Pearson Prentice

Hall, 2011. Print.


6/6

Burhan Riaz

11/08/2011

News Brief:

How do people use trace analysis to find arsenic poising of a victim on your favorite CSI show?

And how does the EPA know when there are harmful chemicals in the lake nearby your house?

According to Beer's law and the Law of Mass Action, one can find the concentrations of a

chemical even if it is minutely small. For example, if you bring a sample into a lab and use a

spectrometer you can get data that shows how much of that chemical is in the sample by using

Beer's law. You can even predict future reactions of chemicals once you have found a constant

that can predict how reaction will take place.

keq experiment

Documents