Laboratory Activity: Teacher Notes

Activity 2: Temperature Effects

Major Chemical Concept

This activity is designed to develop the concept that a change in temperature can affect the equilibrium position of a chemical system.

Level

Basic, general or honors students

Expected Student Background

General knowledge of solutions and concentrations, formula writing, equation writing, balancing equations, effect of indicators in a solution, and acids and bases in solution.

Time

Completing the activity and answering questions requires one 50-min class period.

Safety

No safety precautions beyond those that apply to normal chemical laboratory work need to be stressed for this activity. Some students may be concerned with the superficial resemblance between SCN ­ and CN ­ . Although SCN ­ should be treated with the same respect as other chemicals, it does not have the level of toxicity of CN ­ . Soluble chemicals should be washed down the drain with plenty of water. Solid chemicals should be disposed of in a solid waste jar.

Materials (For 24 students working in pairs)

A hand-held ultraviolet lamp. Preferred: 4 watt long and short wavelength ultraviolet light. (Model Mineralight UVSL-25 available from Ultraviolet Products, Inc., 5100 Walnut Grove Avenue, San Gabriel, CA 91778 or Model UVGL-25 from Central Scientific Company, 11222 Melrose Avenue, Franklin Park, IL 60131-1364)

Thin-wall, open-end capillary tubing (1 mm) for micropipets; alternatively, use toothpicks

Filter paper, 12 squares for liners

24 Wide mouth, clear screw cap jars (Mason jars will work),16 oz, or 600-mL beaker with aluminum foil cover also work well

Iodine, several crystals

Teacher-Student Interaction

Walk around laboratory and ask students if they understand what is happening. Discuss how to calculate R f values. Caution students about inhaling solvent vapor.

Anticipated Student Results

  • Salicylamide and acetaminophen produce dark spots. Caffeine produces a light spot. Aspirin does not produce a spot

    • .

  • Yes. The approximate order of migration is salicylamide, followed by acetaminophen, caffeine, then aspirin.
  • Yes. The iodine permanently darkens some of the spots.
  • Thedifferencesinthe rateofmovementof substancesdependuponsolubility of the substance and adsorption properties of the backing on the plate. Because the solvent is typically nonpolar and the backing is strongly polar, the backing holds a polar substance more tightly than a nonpolar substance. These polar substances move considerably more slowly than nonpolar substances in the nonpolar solvent.

    • Data Analysis

    1. Canultravioletlightbe usedtodistinguishthe substancesfromoneanother?
    2. Can iodine be used to distinguish the substances from one another?
    3. Can the R f value be used to distinguish one substance from another?
    4. Is it important that the sheets be examined under ultraviolet light before the iodine visualization?
    5. Why do some substances move along the TLC plate at a rate different from other substances?

    Implications and Applications

    The analysis of an unknown analgesic drug can be performed by TLC. Specific components of a developed TLC sheet can be identified by the visualization method (under ultraviolet light or in iodine vapor) and from subsequent calculations of the R f values. Crime laboratory technicians often use TLC to determine the presence of controlled substances (e.g., cocaine) in an unknown sample.

    The TLC technique is often used by forensic scientists to identify pen inks. For example, the need for differentiating inks arises when people prepare fraudulent documents. A person cheating the government may back-date a record to substantiate a false tax claim. The forensic scientist can determine how many inks or pens were used to prepare a document and the year of manufacture for the inks used.

    Lipstick stains on clothing or cigarettes can also give valuable information about the identity of a likely suspect. The dyes that give lipstick its color can be identified by a TLC separation and compared with those used by the friends of the victim.

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