Lab 21 Chemical Equilibrium: Qualitative Lab

LAB 21 CHEMICAL EQUILIBRIUM: QUALITATIVE LAB

BACKGROUND:

Do you remember when we talked about going up the down escalators? That was an example of equilibrium. We also talked about equilibrium between the liquid and gas phase of a substance. That was an example of physical equilibrium; today you will explore chemical equilibrium. This lab is qualitative, which means you will just be good observers and describe chemical equilibrium. You will start off with a system at equilibrium and then attempt to change the equilibrium by changing the concentration of the ions. The reaction chosen for this equilibrium system has a colored product so it will be easy to see if the equilibrium changes because the color intensity will change.

PROCEDURE

1. Put about 50 ml of .0020 M potassium thiocyanate solution, KSCN, and 50 ml of distilled water into a 150-250 ml beaker. Mix well, describe and record the solution.

2. Obtain a solution of .20 M iron III nitrate, Fe(NO3)3, describe and record.

3. Add 10-12 drops of the Fe(NO3)3 solution to the beaker, swirl and describe the results.

4. Obtain a bottle of potassium nitrate, KNO3 and describe its appearance.

5. Place 4 Petri dishes on a piece of white paper and label the paper next to each dish 1,2,3,4. Pour equal amounts of the solution from the beaker into each dish. Just approximate, do not use a graduated cylinder. The dish labeled #1 will be the control; don’t ever add anything to this dish. It will now be your reference dish. You can compare changes in color intensity back to this original.

6. Add 4-5 small crystals of solid KSCN to dish #2. Describe and record the results.

7. Add 5-6 drops of Fe(NO3)3 solution to dish #3. Describe and record the results.

8. To dish #4 add 10-12 small crystals of sodium hypophosphate, Na2HPO4, a few at a time. Observe carefully the solution that is in the region around each crystal, then gently swirl. Describe and record the results.

QUESTIONS

1. What ions are present in KSCN and Fe(NO3)3 ?

2. What would be the formulae for the two new possible combinations?

3. What ions are present in the KNO3 ?

4. Could the ions in KNO3 be responsible for the change noted in step 3?

5. What must be the identity of the colored product in step 3?

6. Other experiments have shown the identity of the colored ion is FeSCN+2(aq). Write the equation for its formation.

7. When KSCN(s) was added in step 6, what happened to the concentration of the SCN-(aq) ?

8. What effect does the addition of KSCN(s) in step 6 have on the concentration of the FeSCN+2(aq) ?

9. How does the addition of the Fe(NO3)3 solution in step 7 affect the concentration of the Fe+3(aq) ?

10. What effect does the addition of the Fe(NO3)3 solution in step 7 have on the concentration of the FeSCN+2(aq) ?

11. Other experiments have shown that the Fe+3(aq) combines with HPO4-2(aq), which comes from the Na2HPO4.

A. What effect does the addition of HPO4-2(aq) in

step 8 have on the concentration of the Fe+3(aq)?

B. What effect does it have on the concentration of

the FeSCN+2(aq) ?