CHAPTER 15 the Laboratory

CHAPTER 15 The Laboratory These skills are usually tested on the SAT Subject Test in Chemistry. You should be able to . • Name, identify, and explain proper laboratory rules and procedures. • Identify and explain the proper use of laboratory equipment. • Use laboratory data and observations to make proper interpretations and conclusions. This chapter will review and strengthen these skills. Be sure to do the Practice Exercises at the end of the chapter. Laboratory setups vary from school to school depending on whether the lab is equipped with macro- or microscale equipment. Microlabs use specialized equipment that allows lab work to be done on a much smaller scale. The basic principles are the same as when using full-sized equipment, but microscale equipment lowers the cost of materials, results in less waste, and poses less danger. The examples in this book are of macroscale experiments. Along with learning to use microscale equipment, most labs require a student to learn how to use technological tools to assist in experiments. The most common are: Gravimetric balance with direct readings to thousandths of a gram instead of a triple-beam balance pH meters that give pH readings directly instead of using indicators Spectrophotometer, which measures the percentage of light transmitted at specific frequencies so that the molarity of a sample can be determined without doing a titration Computer-assisted labs that use probes to take readings, e.g., temperature and pressure, so that programs available for computers can print out a graph of the relationship of readings taken over time LABORATORY SAFETY RULES The Ten Commandments of Lab Safety The following is a summary of rules you should be well aware of in your own chemistry lab. 1. Dress appropriately for the lab. Wear safety goggles and a lab apron or coat. Tie back long hair. Do not wear open-toed shoes. 2. Know what safety equipment is available and how to use it. This includes the eyewash fountain, fire blanket, fire extinguisher, and emergency shower. 3. Know the dangers of the chemicals in use, and read labels carefully. Do not taste or sniff chemicals. 4. Dispose of chemicals according to instructions. Use designated disposal sites, and follow the rules. Never return unneeded chemicals to the original containers. 5. Always add acids and bases to water slowly to avoid splattering. This is especially important when using strong acids and bases that can generate significant heat, form steam, and splash out of the container. 6. Never point heating test tubes at yourself or others. Be aware of reactions that are occurring so that you can remove them from the heat if necessary before they “shoot” out of the test tube. 7. Do not pipette anything by mouth! Never use your mouth as a suction pump, not even at home with toxic or flammable liquids. 8. Use the fume hood when dealing with toxic fumes! If you can smell them, you are exposing yourself to a dose that can harm you. 9. Do not eat or drink in the lab! It is too easy to take in some dangerous substance accidentally. 10. Follow all directions. Never haphazardly mix chemicals. Pay attention to the order in which chemicals are to be added to each other, and do not deviate! SOME BASIC SETUPS Throughout this book, drawings of laboratory setups that serve specific needs have been presented. You should be familiar with the assembly and use of each of these setups. • Preparation of a gaseous product, nonsoluble in water, by water displacement from solid reactants • Preparation of a gaseous product, nonsoluble in water, by water displacement from at least one reactant in solution • Distillation of a liquid • Titration The following are additional laboratory setups with which you should be familiar: 1. PREPARATION OF A GASEOUS PRODUCT, SOLUBLE IN WATER AND LIGHTER THAN AIR, BY THE DOWNWARD DISPLACEMENT OF AIR. SEE FIGURE 42. EXAMPLE: Preparation of ammonia (NH3). 2NH4Cl(s) + Ca(OH)2(s) → CaCl2(s) + 2H2O(g) 2NH3(g) Figure 42. Preparation of Ammonia 2. SEPARATION OF A MIXTURE BY CHROMATOGRAPHY. SEE FIGURE 43. Figure 43. Chromatography Setup EXAMPLE: Chromatography is a process used to separate parts of a mixture. The component parts separate as the solvent carrier moves past the spot of material to be separated by capillary action. Because of variations in solubility, attraction to the filter paper, and density, each fraction moves at a different rate. Once separation occurs, the fractions are either identified by color or removed for other tests. A usual example is the use of Shaeffer Skrip Ink No. 32, which separates into yellow, red, and blue streaks of dyes. 3. MEASURING POTENTIALS IN ELECTROCHEMICAL CELLS. SEE FIGURE 44. Figure 44. Potentiometer Setup for Measuring Potential EXAMPLE: The voltmeter in this zinc-silver electrochemical cell would read approximately 1.56 V. This means that the Ag to Ag+ half-cell has 1.56 V more electron-attracting ability than the Zn to Zn2+ half-cell. If the potential of the zinc half-cell were known, the potential of the silver half- cell could be determined by adding 1.56 V to the potential of the zinc half-cell. In a setup like this, only the difference in potential between two half-cells can be measured. Notice the use of the salt bridge instead of a porous barrier. 4. REPLACEMENT OF HYDROGEN BY A METAL. SEE FIGURE 45. Figure 45. Eudiometer Apparatus EXAMPLE: Measure the mass of a strip of magnesium with an analytical balance to the nearest 0.001 g. Using a coiled strip with a mass of about 0.040 g produces about 40 mL of H2. Pour 5 mL of concentrated HCl into a eudiometer, and slowly fill the remainder with water. Try to minimize mixing. Lower the coil of Mg strip into the tube, invert it, and lower it to the bottom of the beaker. After the reaction is complete, you can measure the volume of the gas released and calculate the mass of hydrogen replaced by the magnesium. (Refer to Chapter 5 for a discussion of gas laws.) SUMMARY OF QUALITATIVE TESTS I. Identification of Some Common Gases Gas Test Result Ammonia 1. Smell cautiously. 1. Sharp odor. NH3 2. Test with litmus. 2. Red litmus turns blue. 3. Expose to HCl fumes. 3. White fumes form, NH4Cl. Carbon dioxide 1. Pass through limewater, 1. White precipitate forms, CO2 Ca(OH)2 CaCO3 Carbon monoxide 1. Burn it and pass product through 1. White precipitate forms, CaCO3 CO limewater, Ca(OH)2 Hydrogen 1. Allow it to mix with some air, then 1. Gas explodes. H2 ignite. 2. Burns with blue flame—product H2O 2. Burn it—trap product. turns cobalt chloride paper from blue to pink. Hydrogen chloride 1. Smell cautiously. 1. Choking odor. HCl 2. Exhale over the gas. 2. Vapor fumes form. 3. Dissolve in water and test with litmus. 3. Blue litmus turns red. 4. Add AgNO3 to the solution. 4. White precipitate forms. Hydrogen sulfide 1. Smell cautiously. 1. Rotten egg odor. H2S 2. Test with moist lead acetate paper. 2. Turns brown-black (PbS). Oxygen 1. Insert glowing splint. 1. Bursts into flame. O2 2. Add nitric oxide gas. 2. Turns reddish brown. II. Identification of Some Negative Ions Ion Test Result Acetate Add concentrated H2SO4 and warm Odor of vinegar released. − C2H3O2 gently. Carbonate Add HCl acid; pass released gas through − White, cloudy precipitate forms. CO3 limewater. Chloride 1. Add silver nitrate solution. 1. White precipitate forms. 2. Then add nitric acid, later followed by 2. Precipitate insoluble in HNO3 but Cl− ammonium hydroxide. dissolves in NH4OH. Hydroxide Test with red litmus paper. Turns blue. OH− White precipitate forms; insoluble in Sulfate Add solution of BaCl , then HCl. 2 HCl. − SO4 Sulfide Add HCl and test gas released with lead Gas, with rotten egg odor, turns paper S2− acetate paper. brown-black. III. Identification of Some Positive Ions Ion Test Result Ammonium Add strong base (NaOH); Odor of ammonia. + NH4 heat gently. Ferrous Add solution of potassium ferricyanide, Dark blue precipitate forms Fe2+ K3Fe(CN)6 (Turnball’s blue). Ferric Add solution of potassium ferrocyanide, Dark blue precipitate forms Fe3+ K4Fe(CN)6. (Prussian blue). Hydrogen Test with blue litmus paper. Turns red. H+ IV. Qualitative Tests of Some Metals FLAME TESTS. Carefully clean a platinum wire by dipping it into dilute HNO3 and heating in the Bunsen flame. Repeat until the flame is colorless. Dip heated wire into the substance being tested (either solid or solution), and then hold it in the hot outer part of the Bunsen flame. Compound of Color of Flame Sodium (Na) Yellow Potassium (K) Violet (use cobalt-blue glass to screen out Na impurities) Lithium (Li) Crimson Calcium (Ca) Orange-red Barium (Ba) Green Strontium (Sr) Bright red HYDROGEN SULFIDE TESTS. Bubble hydrogen sulfide gas through the solution of a salt of the metal being tested. Check color of the precipitate formed. Compound of Color of Sulfide Precipitate Lead (Pb) Brown-black (PbS) Copper (Cu) Black (CuS) Silver (Ag) Black (Ag2S) Mercury (Hg) Black (HgS) Nickel (Ni) Black (NiS) Iron (Fe) Black (FeS) Cadmium (Cd) Yellow (CdS) Arsenic (As) Light yellow (As2S3) Antimony (Sb) Orange (Sb2S3) Zinc (Zn) White (ZnS) Bismuth (Bi) Brown (Bi2S3) Chapter Summary The following terms summarize all the concepts and ideas that were introduced in this chapter. You should be able to explain their meaning and how you would use them in chemistry. They appear in boldface type in this chapter to draw your attention to them. The boldface type also makes it easier for you to look them up if you need to.

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