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4: Chemical Conceptions and Misconceptions

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    60955
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    Chemical Conceptions and Misconceptions

    CHEM 173 Physical Chemistry Laboratory

    Prof. V. Morris

    Purpose

    In this initial laboratory exercise you will shake the rust off of your laboratory investigative skills by using some basic tools of logic and your background knowledge of chemistry to distinguish and classify several unknown solids based on their physical chemical properties. The format of this laboratory report will vary slightly from the standard format for labs because of its open-ended nature. Quite often the need to think independently, to make rational choices based on common observation is frightening to students. This is especially the case when most of your courses have only required you to apply some memorized process. This purpose of this lab is to get your creative juices flowing and additionally promoting nascent knowledge recall and usage.

    I. Observations of Thermodynamic Processes

    Objectives:

    To engage students in applying observational skills and to relate these observations of common physical processes to Thermodynamics lecture topics in Physical Chemistry. This exercise will also examine individual student conceptions (and misconceptions) of diffusion, the zeroth and first laws of thermodynamics, and entropy.

    Several solutions of varying composition and temperature will be mixed. You should predict the results of the experiments, carefully observe the demonstrations, observe the results, and interpret them using appropriate thermodynamic terms. Every attempt should be made to relate what you see to the laws of thermodynamics, state variables and state functions, equilibrium, and energy transfer.

    Procedure:

    1. Observe and record the chemical and physical properties of the solutions that will be used in this demonstration.
    2. Between two and six individual experiments will be conducted. For each experiment, predict what will happen and provide a justification.
    3. Carefully observe each experiment and compare your prediction to the eventual result. Was your prediction correct? Was your justification sound?
    4. What misconceptions can you identify in your original thinking?
    5. Which misconceptions (if any) do you think were addressed by this exercise?

    Elements to address in your report

    1. What thermodynamic principles determined the behavior of the solutions being mixed in these demonstrations?
    2. Briefly explain the results of each demonstration
    3. How does the first law of thermodynamics apply?
    4. Which state variables were changed during the demonstrations?
    5. For each incorrect prediction, explain the misconception.

    II. Physical Chemical Discrimination

    Objective: Use your chemical intuition, your knowledge, and the resources provided in this lab to identify and discriminate the chemical and physical properties of four granular solids. In particular, you need to determine:
    a. Whether the solid is a compound or a mixture
    b. The nature of the bonding in the solid
    c. Your best guess at chemical composition (e.g. an organic acid, an inorganic salt, a plastic)
    d. At least three other chemical or physical properties (preferably thermodynamic)
    e. Explain whether the properties that you determine are extensive or intensive.

    The common substances are some combination of the following that covers metallic bonded, covalently bonded, ionically bonded, inorganic and inorganic materials as well as mixtures and pure materials.

    Procedure:

    1. The class will be divided into groups of three or four students. Each group will work together to develop a single lab report to submit. All members of the group must sign off on the cover page of the lab report.
    2. Each group will be assigned a suite of four granular solids indicated only by a number between 1 and 9.
    3. Physically examine each of your solids and make an exhaustive list of your observations. Try to determine specific physical properties of the substance. What distinguishes it from the other solids? This should be reported in your report as a table. An example is given below.
    SAMPLE OBSERVATIONS TABLE
    Sample Number General Observations Initial Predictions for
    Solvent Interactions
    Results of
    Solvent Interactions
    Initial Predictions
    of Physical Methods
    Results of
    Physical Methods
    7 Bluish color with
    brown specks,
    smells pungent,
    begins to glow
    eerily after a few minutes
    The sample will explode
    upon contact with water
    The sample exploded
    upon contact with water
    The sample will create
    n acidic aqueous solution
    The sample created
    a basic aqueous solution
    1. Based on your physical examination and observations predict the type of bonding in each solid (e.g. ionic, covalent, polar covalent, non-polar covalent, metallic). You will attempt to test these guesses in subsequent steps.
    2. Examine the two carts at the front of the lab. One cart has some standard instrumentation and devices and the other contains a suite of common solvents. Based on your observations and discussions within your group, identify two solvents to use to determine further chemical and physical properties.
    3. Obtain small volumes of the solvents (usually less than 25-ml) and bring back to the bench to sequentially test for chemical and or physical properties of each solid. Do NOT mix the solvents.
    4. Add small amounts of your first solvent dropwise to a small amount of each sample (individually). You may use either a watch glass or a small beaker for this part. Continue until there is no further chemical change, physical change, or complete dissolution. Record your observations.
    5. Repeat this procedure for the other solvent and fresh amounts of your samples.
    6. Return to your initial guesses. How consistent are they with respect to this new information?
    7. Return to the carts and review the contents of the hardware table. Select no more than two devices to use to determine additional chemical or physical properties. Record your observations.
    8. Dispose of the chemical materials (Ask the instructor), clean all glassware, and clean bench area

    Elements to address in your report

    1. Which two solvents did you select and why?
    2. What did you learn about each of your solids using the solvents?
    3. Link your observations to the identification of physical properties
    4. Which two devices did you select and why?
    5. What did you learn about each of your solids using the devices?
    6. What is the identity of each of your solids?
    7. If you had a choice to select one additional solvent or one additional instrument to assist in your determination, what would it be and why?
    8. If you had a choice to select one additional solvent or one additional instrument not present on the carts to assist in your determination, what would it be and why?
    9. Design and present a data table that illustrates and compares what you “know” about each substance based on the chemical and physical evidence and measurements. Imagine that you were part of a forensics competition and need to present your results to a panel of judges. The table should clearly represent your new state of knowledge.

    4: Chemical Conceptions and Misconceptions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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