Experiment 3 pre-lab
1. Ken McFarland “Chemical Equilibrium”, CHEM 1130, TA - Nastaran Marzijarani, Section
108, Room 1871, Will Brubacher, Thursday 2:00 pm
2. Purpose of Experiment: This experiment is to determine the equilibrium constant of the
reaction between iron (III) ions and thiocyanate acid.
3. Procedure:
Materials: 25 mL volumetric flask, 25 mL buret, Spectronic 20, five 15 mL test tubes, a 4
in. test tube, 2.5 M NHO3, 2.00 x 10-3 M Fe3+, 2.00 x 10-3- M HSCN, graduated pipettes,
Calculate how much 2.5 M nitric acid is needed to prepare 25.00 mL of 0.500 M nitric acid.
To do this fill your volumetric flask approximately half way with distilled water, then add the
amount of nitric acid needed to the water. Fill the flask the rest of the way with distilled
water. After you have prepared the solution, cap it, and mix it well. Rinse your buret with the
nitric acid, and then fill it, insuring that there is no air bubble in the tip. Record how full the
buret is.
Label the large test tubes 1 through 5, and add the proper amount of Fe3+ and HSCN to each.
Tube mL Fe+ mL HSCN mL H+ (nitric acid)
1
5.00
1.00
4.00
2
5.00
2.00
3.00
3
5.00
3.00
2.00
4
5.00
4.00
1.00
5
5.00
5.00
0.00
To mix each test tube, hold it firmly with your fingers extended and flick the bottom with
your finger, making sure you point it away from your face.
Use a 4 in. test tube that has no scratches or cracks, making sure to insert it into the Spec. 20
the same way each time to reduce error. We will check the absorbance reading of each of the
solutions at 447 nm in the Spec. 20. Adjust the left control know on the Sepc. 20 so that the
absorbance scale reads infinity with the lid closed, and the sample holder empty. Adjust the
right control know so that the absorbance scale reads 0 when the lid is closed, and the 4 in.
test tube, filled 2/3 of the way with distilled water, is in the sample holder. Prepare the test
tube for first solution to be tested by rising it with that solution, and then filling it 2/3 full
with the solution to be measured. Place the tube in the sample holder, close the lid, and read
the absorbance scale. Repeat this for each of the other four solutions, being sure to rinse the
test tube with each new solution before filling it. It is recommended to go from least
concentrated to most concentrated.
After all data has been recorded, the solutions are to be poured into the inorganic waste tub.
4. Observations, Results and Data Sheets: See attached data sheets.
5. Questions:
1. The color changed relatively gradually. It was not noticeable at the time, but if left alone,
and then checked again, was more noticeable.
2. The amount of product formed would be higher, because the Keq must remain constant,
and therefore more of the product must be formed for the Equilibrium point, and Keq value,
to be reached.
3. If the solution in the test tube was a significantly different temperature, either warmer or
cooler, for one of the reactions than the other four, the Keq value for that tube would very
significantly.
4. a) Keq=1
b) Our Keq was greater than one.
c) This equilibrium favors the products.