Le Châtelier's Principle CPR Essay

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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.

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_uwmadison_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 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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Guidance for Writing Your Text

1. Explain how temperature changes affect the equilibrium system in Part I. Indicate the reactants and products and their 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).

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).

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.