Equilibrium Pre-lab

Procedure and Instructions

Experimental Design

Fe3+(aq) + SCN1-

(aq) FeSCN2+(aq)

1. In the first reaction the concentration of Fe3+ will be 100 times more concentrated than the SCN1- ion. Under these conditions the reaction will essentially go 100% to the right forming what we will call the standard complex of FeSCN2+.

2. Through a sequence of serial dilutions we will reduce the [Fe3+] added to each new system. We will find the [FeSCN2+] for each equilibrium using a visual comparison technique. The [FeSCN2+] at equilibrium we will set into motion our TFD and CM strategies to find [Fe2+] and [SCN1-] at equilibrium.

3. Using an excel spreadsheet we will compare several mathematical possibilities for the equilibrium state to the one predicted by the law of chemical equilibrium

Lab Station Assets

Each station will have a wooden test tube rack with 4 wide mouth test tubes, a white rack with 5 narrow mouth test tubes and a blue marked narrow mouth test tube, and a special measuring rule.

Propipette Bulb

The propipette bulb has three marked valves. The A valve is used when you want to produce suction potential. The S valve is used to suck up the fluid and the E valve is used to slowly deliver the contents of the pipette.

Station Two Activities

At station #2 you will deliver to precisely 10.00 ml of the [.20] Fe3+ reactant. Notice that the test tube is angled but the pipette is perpendicular to the ground. At the end of the station a red line marks the substation where the distilled water is located. As noted in the photo to the right, the test tube is once again angled. You will use the burette to deliver precisely 15.00 ml of distilled water to the 10.00 ml of Fe3+ reagent. We will refer to this event as dilution 1. By your action you have made a 10.00 ml to 25.00 ml dilution of the reagent.

Station One Activity

At station #1 you will deliver precisely 5.00 ml of the [.00200] KSCN. At the end of the station table you will cross a red line to enter the substation. Here you will deliver precisely 5.00 ml of the [.200] Fe(NO3)3 reagent. The dark red complex will form immediately (see photo to the right). This test tube will be called the standard (std.).

Dilution Stations

Stations 3-5 are organized exactly alike. When you first get to the station use the labeled pipette to immediately draw from your “active” wide mouth test tube precisely 5.00 ml of the diluted Fe3+ reagent. Deliver this volume to the 5.00 ml of SCN1- reagent that is already in the narrow mouth test tube. Then transfer precisely 10.00 ml of the diluted Fe3+ reagent to a clean wide mouth test tube. Finally, deliver precisely 15.00 ml of distilled water to this test tube to complete the second dilution (now called dilution 2). This new dilution will be the “active” wide mouth test tube for the next station. Now move to the next station and repeat this procedure.

Station Six

Station six does not require a dilution. When you get there you should use the labeled pipette to deliver precisely 5.00 ml of the contents from the “active” wide mouth test tube that you made at the last dilution station. When this is done you move to the reading station.

Results

In the above photo, sample results are displayed. There is a definite progression of shade from the darker “standard” (located on the left next to the blue marked test tube) to the most diluted system )located on the far right). All of the systems are at equilibrium but the equilibrium concentrations are all different. We will use the known concentration of the standard to find the equilibrium concentrations of the other systems.

Reading Station At the reading station you first measure the height of the “standard” test tube. You will use sleeves to cover both the standard test tube and the narrow mouth test tube that has achieved equilibrium and you want to test. Looking down the shaft of both test tubes you will draw out reagent from the “standard” tube until it appears “lighter” than the other sample. Then you should carefully add drops of the standard back to the standard until they have the same shade. When this is done, measure the new height of the standard test tube. After you record the measurement you should repeat the process for the remaining samples.

Sample Reading

Consider the two test tubes to the right. The original height of the “standard” was 5.40 cm. After drawing out the necessary volume to achieve the same shade of color the height of the standard was reduced to 2.10 cm. From this data we will compute the concentration of the test tube to the right of the standard.

[standard] x (2.1/5.4) =[standard] x (.389) =

In other words the concentration of the unknown was 38.9% of the concentration of the “standard.”

Final Destination

After all the readings have been taken a team representative should provide the various measurements for your group. Mr. Zachmann will insert the data into a prepared spreadsheet. We will do a team analysis on the following day. Before coming to class consider whether you want to average data or use box and whisker plots.