Lab #13 – LeChatelier’s Principle
Name
Background
LeChatelier’s Principle says that when a system at equilibrium is subjected to a stress, the system will shift to
relieve the stress. It provides an easy qualitative way to explain existing chemical phenomena and to predict the
results of future chemical phenomena. Understanding LeChatelier’s Principle is really a matter of
understanding the terms system, stress, and, shift. A system is a chemical or physical process that can be
represented by a balanced equation. A stress is one of several conditions that we can change to manipulate the
results of the process. The four stresses are a change in concentration of a reactant or a product, a change in
temperature, a change in pressure, and the addition of a catalyst. A shift is described as either left or right. A
shift right means the reaction will consume reactants and produce products (thus increasing the value of the K
expression), while a shift left means the reaction will consume products and produce reactants (thus decreasing
the value of the K expression).
The predictions made qualitatively with LeChatelier’s Principle can also be made quantitatively by using the
Keq equation. LeChatelier is generally easier and is preferred if quantitative predictions are not needed.
In this lab you will apply several stresses to four different chemical systems, each chosen for the ease with
which a shift can be detected. It will be your job to explain the changes you observe in terms of LeChatelier’s
Principle. In your explanations, you will identify the specific stress and describe the resulting shift.
Procedure
System A: Methyl Violet, an acid-base indicator
HMV(aq) <==> MV-(aq) + H+(aq)
yellow
violet
1. Place 5 mL of distilled water in a medium test tube and add a few drops of methyl violet indicator. Record
the color in the data table below.
2. Select a reagent that will shift the equilibrium to the other color. Your choices are 6 M HCl, NaOH, or NH3.
Add the reagent dropwise until the color change occurs. Record the new color and the reagent used to cause
the shift.
3. Select a reagent to shift the equilibrium back to the original color. Add it dropwise to test your hypothesis.
If your first choice fails, try another reagent. [Note: you may have to start over at Step 1] Record the new
color and the reagent used to cause the reversal. Complete the table before going on to System B.
Step
1
Original
color
Final
color
Shift (left
or right)
X
X
Reagent
causing
shift
X X
2
3
a. What color is methyl violet in an acidic environment?
b. What color is methyl violet in a basic environment?
c. How is the change in color change related to the K equation?
Detailed explanation of stress
(How does [H+] change?)
System B: Solubility equilibrium
1 PbCl2(s) <==> 1 Pb+2(aq) + 2 Cl-(aq)
d. Write the Keq expression for this process:
Keq =
e. PbCl2 is only slightly soluble in water, so what is true about the value of Keq for the process above?
1. Set up a hot water bath with a 400 mL beaker. Begin heating the water as you proceed to Step 2.
2. Add 5.0 mL of 0.3 M Pb(NO3)2(aq) to a medium test tube. f. What is [Pb+2]?
Add 5.0 mL of 0.3 M HCl to a small graduated cylinder. g. What is [Cl-]?
Add 1.0 mL HCl to the test tube containing the Pb(NO3)2(aq). Swirl to mix, record observations in table.
3. Add HCl in 1 mL increments until a noticeable amount of white precipitate is present after swirling. Record
the volume of HCl needed in the table.
4. Place the test tube with the PbCl2 precipitate in the hot water bath for a few minutes. Record observations.
Place the test tube under a cold water tap for a few minutes. Record observations.
5. Add water in 1 mL increments until the precipitate dissolves completely. Record the volume of water
needed.
Step
mL and ID of
added reagent
2
1 mL HCl
Observations
Shift
(right or left)
Specific stress
3
4-hot
NA
4-cold
NA
5
h. Is the dissolving of PbCl2 endothermic or exothermic? Explain how you can tell.
1 Co(H2O)6+2(aq) + 4 Cl-(aq) <==> 1 CoCl4-2(aq) + 6 H2O(l)
blue
1. Put a few crystals of CoCl2• 6 H2O(s) in a medium test tube. In the fume hood, add 2 mL of 12 M HCl. Stir
to dissolve and record the color in the table on the next page.
System C: Cobalt complexes
2. Add 2 mL portions of water, stirring after each addition, until no further color change occurs. Record the
color.
3. Place the test tube in the hot water bath and record any color change. Place the test tube under a cold water
tap and record any color change.
Original
color
Step
Final
color
Shift
(left
or right)
Stress
Detailed explanation of stress, with balanced equation
1
2
3-hot
3-cold
i. Is the formation of the blue CoCl4-2 complex endothermic or exothermic? Explain how you can tell.
System D: Complexes and Precipitates
Transition metals are capable of forming appropriately named complex ions, or complexes. As you might
guess, transition metals use d electrons in forming a complex. The CoCl4-2 in System C is a complex ion, as
compared to the simple ion, Cl-. The fun begins when a transition metal can form a complex and a precipitate
with the same agent, depending how much of that agent is present. For example, consider the two possible
reactions between Zn+2 cations and OH- ions:
1 Zn+2(aq) + 2 OH-(aq) <==> 1 Zn(OH)2(s) where the zinc(II)hydroxide precipitates.
1 Zn(OH)2(s)+ 2 OH-(aq) <==> 1 Zn(OH)4-2(aq) where the complex ion, Zn(OH)4-2, is dissolved.
1. Add approximately 2 mL of 0.1 M Zn(NO3)2(aq) to each of three medium test tubes. To each tube add 1 drop
of 6 M NaOH. Record your observations.
2. To the first tube add 6 M HCl dropwise. To the second tube add 6 M NaOH dropwise. To the third tube add
6 M NH3. Record your observations for the three tubes.
Step
Reagent
1
Zn(NO3)2 + NaOH
2
HCl
2
NaOH
2
NH3
Observation
Balanced Equation explaining observation
3. Repeat steps #1 and #2 above using 0.1 M Mg(NO3)2 instead of Zn(NO3)2. Record your observations.
Step
Reagent
3
HCl
3
NaOH
3
NH3
Observation
Balanced Equation explaining observation
Prelab summary
Procedural Tips: Follow directions.
Safety: Diaper protocol is in place. Rinse hands after touching reagent bottles. Don’t touch face or clothes.
All 6 M reagents are skin contact hazards. Rinse thoroughly with water and contact teacher.
6 M NH3 is and inhalation hazard.
12 M HCl is a severe inhalation hazard and must stay in the fume hood.
Disposal: All reagents can be rinsed down the drain.
Prelab Questions
1. Write a balanced equation showing the reaction between H+ ions and OH- ions. What reagent could be
added to a solution to decrease [H+]? What reagent could be added to a solution to decrease [OH-]?
To decrease [H+], add
To decrease [OH-], add
2. Write an equation showing how NH3 increases the [OH-] in a solution.
3. Water is added to a solution of NaOH. What happens to [H2O]? Explain your answer.
How are [Na+] and [OH-] affected by the added water? Explain your answer.
4. Many calcium compounds are less soluble in hot water than in cold. Is the dissolving process for these
compounds endothermic or exothermic? Explain your answer – a balanced equation may help.
5. When OH- ions are added to a Ni(NO3)2 solution, a precipitate forms. When more OH- ions are added, the
precipitate dissolves. Write a balanced equation to explain both observations.