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Title: Le Chatelier's Principle W2021
» Learning Goals
» Source Materials
» Hyperlink Resources
» Guidance for Studying Source Materials
» Guidance for Writing Your Text
» Writing Prompt
» Calibration Essays and Answer Keys
Learning Goals
To describe shifts in equilibrium when conditions, such as
concentration and temperature, are changed in the General
Chemistry II “Le Châtelier’s Principle” laboratory experiment.
To explain the observed changes in terms of Le Châtelier’s
Principle.
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Source Materials
You should refer to the observations recorded in your lab
notebook for the CHEM 162 "Le Châtelier's Principle"
experiment to
complete this assignment.

2. You may also refer to the appropriate sections in your General
Chemistry II lecture notes and in the OpenStax textbook.
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Hyperlink Resources
University of Wisconsin-Madison's "Twelve Common Errors: A
Student Self-Editing Guide"
- The resources contains brief examples and explanations of
common grammatical errors intended to help students edit
essays
and papers.
URL: https://writing.wisc.edu/wp-
content/uploads/sites/535/2018/07/twelve_common_errors_uwm
adison_writingcenter_rev_sept2012.pdf
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Guidance for Studying Source Materials
Use the observed color changes for the equilibrium systems in
the Le Châtelier's Principle experiment and the corresponding
explanations recorded in your lab notebook to answer the
Guiding Questions and prepare your CPR essay. The Writing
Prompt will
give you specific guidelines regarding what should be included
in your essay. Next, organize the answers to prepare a clear and
cohesive discussion addressing all of the key points indicated in
the Guiding Questions and the Writing Prompt.
Also, note the following:
Your essay should be cohesive and organized. Do NOT simply

3. list all of your explanations and all of your answers to the
questions. Be sure to include a topic sentence that effectively
introduces the essay's subject.
There is a 500 word limit for the essay, so write clearly and
concisely.
All sections of a laboratory report for a scientific experiment,
including discussions and conclusions such as this essay, are
examples of technical writing. Thus, the essay should be
impersonal and written in the third person with no first person
references—i.e., do not use "I", "we", "my", "our", etc., in the
essay.
Make sure that superscripts and subscripts in chemical formulas
are formatted correctly.
Refer to University of Wisconsin-Madison's "Twelve Common
Errors: A Student Self-Editing Guide".
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https://writing.wisc.edu/wp-
content/uploads/sites/535/2018/07/twelve_common_errors_uwm
adison_writingcenter_rev_sept2012.pdf
https://writing.wisc.edu/wp-
content/uploads/sites/535/2018/07/twelve_common_errors_uwm
adison_writingcenter_rev_sept2012.pdf
/
Guidance for Writing Your Text
1. Explain how temperature changes affect the equilibrium
system in Part I. Indicate the reactants and products and their

4. corresponding colors. What is the initial color of the
equilibrium solution observed in the test tubes in Part I? Based
on the
observed color changes, consider if heat acts as a reactant or
product in the reaction. Is the reaction in Part I endothermic or
exothermic in the forward direction?
2. Explain how changes in reactant or product concentration
affect the equilibrium system in Part II. Indicate the reactants
and
products and their corresponding colors.
3. Based on the observations explain how and why the
equilibrium shifts when solid NH4Cl is added to the equilibrium
system in
Part III.
4. Based on the observations explain how and why the
equilibrium shifts when solid NaC2H3O2 is added to the
equilibrium
system in Part IV.
In addition address the characteristics as described for each
reaction below:
Consider what happens when the temperature is changed for the
following system at equilibrium:
[Co(H2O)6]
2+(aq) + 4 Cl–(aq) ⇄ [CoCl4]
2–(aq) + 6 H2O(l).

5. Explain the observed color changes when the solution is heated
and when it is cooled. Based on your observations,
indicate if the reaction is endothermic or exothermic in the
forward direction.
Consider the following equilibrium system, Fe3+(aq) +
SCN–(aq) ⇄ [FeSCN]2+(aq).
Explain the change in the depth of color for the equilibrium
solution upon addition of each of the following: Fe(NO3)3,
KSCN, and solid Na2SO3.
Consider what happens when solid NH4Cl is added to the
following system at equilibrium:
NH3(aq) + H2O(l) ⇄ NH4
+(aq) + OH–(aq).
Indicate the color of the phenolphthalein indicator at different
pH ranges, then explain based on the change in color for the
indicator whether the pH increases or decreases, whether the
solution becomes more acidic or more basic, and whether
the hydroxide ion concentration increases or decreases.
Consider what happens when solid NaC2H3O2 is added to the
following system at equilibrium:
HC2H3O2(aq) + H2O(l) ⇄ H3O
+(aq) + C2H3O2
–(aq).

6. Indicate the color of the phenolphthalein indicator at different
pH ranges, then explain based on the change in color for the
indicator whether the pH increases or decreases, whether the
solution becomes more acidic or more basic, and whether
the hydronium ion concentration increases or decreases.
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Writing Prompt
First, state Le Châtelier's Principle. For each system, describe
the observations for each change imposed, and explain your
observations. The explanations should include the following
information: 1) the change imposed on the system (i.e., the
chemical
added and the resulting reactant or product concentration that
was affected and how, or if heat was added or removed); 2)
indicate if the equilibrium shifts right (in the forward direction)
or shifts left (in the reverse direction); and 3) indicate the
concentration changes for reactants or products that lead to the
observed color change or the change in the indicator color based
on pH.
[文件標題]
Le Chatelier’s Priciple Lab Report
Propose:
This experiment is to offer the observation of shifts in
equilibrium systems with changes in concentration and
temperature, and to use Le Chatelier Principle to explain these
shifts.
Procedure:
There will be four systems used in this experiment. For the first
two systems, the amounts of reactants and products will be

7. monitored by colored transition-metal complexs, and the
direction of equilibrium shifts will be determined based on
color changes. For the last two systems, weak-acid and weak-
base equilibrium systems and the common ion effects will be
used to monitor shifts in pH value, and changes in the
concentrations of hydronium or hydroxide ions will explain the
shifts in the equilibrium systems.
Experiment:
Part I – The [CoCl4]2- Equilibrium System
[Co(H2O)6]2+(aq) + 4Cl-(aq) [CoCl4]2-(aq) + 6H2O(l)
pink
blue
Solution
Observation
CoCl4 solution in six test tubes