Chemistry
Nasra20
OnlineLab_13ALeChatelier.docxLab #13A: Le Chatelier’s Principle
Purpose
In this experiment, you will study the application of Le Chatelier’s principle by seeing the effect of the addition of Fe3+ and SCN- to an equilibrium mixture of Fe3+, SCN-, and Fe(SCN)2+; the effect of the addition of an acid to an equilibrium mixture of Ni2+, NH3, and Ni(NH3)62+; and the effect of temperature on an equilibrium mixture of Co2+, Cl-, and CoCl42-.
Materials
Lab #13A
6
Chemicals
· Fe(NO3)3 solid ( SDS)
· KSCN solid ( SDS)
· Ni(NO3)2 solid ( SDS)
· Household ammonia (NH3)
Equipment from Home
· Distilled water
· Spoon to scoop solids
· White Vinegar
· Cell phone camera
Equipment from Kit
· Goggles
· 3 plastic cups
· 50 mL & 10 mL graduated cylinder
· 150 mL & 250 mL beakers
· Stir rod
· 4 test tubes
· Test tube rack
· pipettes
· scale
· Weighing paper
Introduction
Not all chemical reactions go to completion. In many cases, a chemical equilibrium is reached between the reactants and products, provided none of the reactants or the products are removed from the reaction mixture. Le Chatelier’s principle describes what happens to an equilibrium after it has been disturbed.
Le Chatelier’s Principle
Le Chatelier’s principle can be described in the following way: “If a stress is applied to a reaction mixture at equilibrium, net reaction occurs in the direction that relieves the stress” (OpenStax, Section 13.3). In this experiment you will add reagents to mixtures at equilibrium and determine if the reaction mixture has shifted to the left (towards more reactants) or to the right (towards more product). You will then apply Le Chatelier’s Principle to explain why the reaction has shifted in the direction you observed.
Chemical Equilibria
The first equilibrium is between Fe3+, SCN-, and Fe(SCN)2+, which is a deeply colored complex ion. If the red color gets darker, it indicates a shift to the right.
Red
The second equilibrium is between Ni2+, NH3, and Ni(NH3)62+. If the color is light green, more Ni2+ is present. If the color is blue, more Ni(NH3)62+ is present.
Light green Light Blue
The third equilibrium is between Co2+, Cl-, and CoCl42-, another complex ion. If the color is pink/red, more Co2+ is present. If the color is blue, more CoCl42- is present.
Pink/red Blue
Procedure
Preparing solutions
1. Put on safety goggles.
2. Obtain 3 plastic cups to prepare solutions for this lab.
3. Add 20 mL of distilled water from a graduated cylinder to each cup. Label one for each of the following: Fe(NO3)3, KSCN, and Ni(NO3)2. Use either a permanent marker or pencil on clear Scotch® tape to label.
4. Use the scale and weigh paper to measure out the following amount of each substance and put each in the appropriate cup of water. This will result in 0.1 M solutions of each. Stir each solution with the stir rod, rinsing thoroughly between each to avoid contamination.
|
Fe(NO3)3 |
KSCN |
Ni(NO3)2 |
|
0.5 g |
0.2 g |
0.4 g |
Chemical Equilibrium with Fe(SCN)2+
5. Obtain 3 small test tubes and mark with the numbers 1, 2, and 3.
6. Add 20 mL of distilled water from a graduated cylinder to the 150-mL beaker. Using a pipette, add 20 drops of 0.1 M Fe(NO3)3 and record the color of the solution.
7. Pipet 20 drops of 0.1 M KSCN to the same beaker. Stir the solution thoroughly. Record the color of the solution.
8. Using the 10 mL graduated cylinder, add 3 mL of this solution to each of the test tubes, rinsing the grad between each solution.
9. Add 20 drops of 0.1 M Fe(NO3)3 to test tube 1. Mix by gently shaking.
10. Add 20 drops of 0.1 M KSCN to test tube 2. Mix by gently shaking.
11. Add 20 drops of distilled water to test tube 3 and mix. The color of the contents of this tube will serve as a reference.
12. Compare the colors in test tubes 1 and 2 with the color in test tube 3. The intensity of the color in each test tube will indicate the relative concentration of Fe(SCN)2+ in that test tube. Record your observations.
13. Take a picture
of your test tubes and include with your report.
14. Combine all solutions containing SCN- (i.e. the three test tubes & the 150 mL beaker) in the KSCN cup. Stuff a few paper towels into the cup to mop up the liquid. Throw the KSCN cup into your solid trash. DO NOT DUMP solutions containing SCN- down the drain. Rinse the test tubes and 150 mL beaker with plenty of water. The remaining Fe(NO3)3 solution may be flushed down the drain.
Chemical Equilibrium with Ni(NH3)62+
15. Add 10 drops of 0.1 M Ni(NO3)2 to a clean test tube. Record the color.
|
CAUTION: |
NH3 has a strong odor so make sure you don’t inhale it directly. Place cover on vial immediately after use. |
16. Add 10 drops of NH3 solution. Record the new color.
17. Add vinegar (HC2H3O2) until the color changes, 10-20 drops or so. Record the new color.
18. Take a picture
of your test tube and include with your report.
19. Clean the test tube by disposing the solution in the sink. Make sure to rinse with plenty of water. The remaining Ni(NO3)2 solution may be flushed down the drain with plenty of water. Save the rest of the NH3 solution for Lab #14A.
Chemical Equilibrium with CoCl42-
This equilibrium cannot be done at home due to the strong acid (HCl) being used, but you will watch a video and record your observations.
20. Watch the following video and record your observation for the 4 test tubes: https://www.youtube.com/watch?v=dmOif5MUPcE&t=19s
Name __________________
Pre-laboratory Assignment
1. Define the following terms:
a. Chemical equilibrium
b. Le Chatelier’s principle
2. Write chemical equations that describe the three equilibria that you will observe during this experiment.
3. Consider the hypothetical exothermic equilibrium reaction:
A + B ↔ C + D + heat
Which direction will the equilibrium shift under each of the following conditions:
a. Either A or B is added to the equilibrium system: __________
b. Either A or B is removed from the equilibrium system: __________
c. Either C or D is added to the equilibrium system: __________
d. The equilibrium system is heated: __________
e. The equilibrium system is cooled: __________
4. What safety precautions need to be observed during this experiment?
Results
1. Chemical Equilibrium with Fe(SCN)2+
Initial Color: ________
Color after addition of KSCN: _________
Compare the colors in the following pairs of test tubes. Which is darker?
1 and 3: ________ 2 and 3: ________
2. Chemical Equilibrium with Ni(NH3)62+
Color before addition of NH3: ________
Color after addition of NH3: ________
Color after addition of vinegar: ________
3. Chemical Equilibrium with CoCl42-
Initial color: ________
Color after addition of HCl: ________
Color after addition of AgNO3: ________
Color after cooling: ________
Questions
1. Use Le Chatelier’s principle to explain the different colors found in the following equilibria. (This means your explanation will say either reactant added, reactant removed, product added, product removed.)
2. Show all chemical reactions.
a.
i. Which way does it shift when Fe3+ is added?
ii. Explain why in terms of LeChatelier’s Principle.
iii. Which way does it shift when SCN- is added?
iv. Explain why in terms of LeChatelier’s Principle.
b.
i. Which way does it shift when NH3 is added?
ii. Explain why in terms of LeChatelier’s Principle.
iii. Which way does it shift when vinegar is added?
iv. Explain why in terms of LeChatelier’s Principle.
c.
The formation of CoCl42- from Co2+ and Cl- is endothermic.
i. Which way does it shift when HCl is added?
ii. Explain why in terms of LeChatelier’s Principle.
iii. Which way does it shift when Ag+ is added?
iv. Explain why in terms of LeChatelier’s Principle.
v. Which way does it shift when it is heated?
vi. Explain why in terms of LeChatelier’s Principle.
vii. Which way does it shift when it is cooled?
viii. Explain why in terms of LeChatelier’s Principle.
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