Arrange a hydrogen generator A (Fig. 37) and fit it with a delivery tube B. Fill a test tube C with water and invert it in a beaker of water, placing a slender rubber band around it about 5 cm. from the closed end. Weigh accurately 2 g. of granulated zinc and place it in A. Through the funnel tube add 10 cc. of twice normal hydrochloric acid, to which 2 drops of cupric sulfate has been added (why ?), and bring the end of the delivery tube B under C so as to collect the evolved hydrogen. At once note the exact time to the second. Keep the rubber band at the level of the water in the beaker, and when the hydrogen has displaced the water in C to this level, again read the time to the second. Repeat the entire experiment, substituting twice normal sulfuric acid for the hydrochloric acid. Again repeat, using twice normal acetic acid. Compare the time required in the three experiments. What do you conclude as to the relative strengths of the three acids? c. Repeat the three experiments in b, using pieces of magnesite (MgCO,) of equal size instead of the zinc. Gaseous carbon dioxide (CO) is evolved in this reaction. What is now the order of the three acids in strength? 75. Bases and salts from the standpoint of the ionization theory. a. Bases. Test the conductivity of dilute solutions of the following typical bases: sodium hydroxide, calcium hydroxide, potassium hydroxide. Discuss the results. b. Salts. Test the conductivity of dilute solutions of the following typical salts: sodium sulfate, ammonium chloride, sodium acetate. Discuss the results. 76. Color of ions and of molecules. Prepare a small amount of dilute solutions of each of the following: potassium bromide, potassium sulfate, potassium chloride. From the standpoint of the theory of ionization, what ions are present in each of the solutions? What is the color of each of these ions? Place in a test tube about one-half gram of each of the following substances: cupric bromide, cupric sulfate, cupric chloride. Note the color of each. Now dissolve each in the Note the color of the solu smallest possible amount of water. tions. Now gradually dilute each until the tubes are nearly filled with water. Mix each thoroughly and note the color of the solutions. Give an explanation of the results. 77. Electrochemical series. a. Dissolve about 1 g. of copper sulfate in a test tube two thirds full of water and dip an iron nail into the solution, first rubbing the nail bright with sandpaper (R). Reverse the experiment by dipping a bright piece of copper foil into a solution of ferrous sulfate. Is the reaction reversible? Why? b. Suspend a piece of mossy zinc or tin by a thread in a solution of lead acetate, and let it stay undisturbed, noting the appearance of the metal from time to time. Would you expect a piece of silver or bismuth to give similar results? c. Polish a piece of aluminium foil and dip it for a time in pure water. Is hydrogen evolved? Would you expect it to be? Now dip the foil into a dilute solution of mercuric chloride and watch for evidences of evolution of hydrogen. Is mercury deposited on the foil? Would it evolve hydrogen? A deposit of mercury prevents any oxide or hydroxide from sticking to the aluminium. Does this help in explaining the evolution of hydrogen? CHAPTER XIV COMPOUNDS OF NITROGEN 78. Preparation of ammonia. a. Dissolve 1 g. of ammonium chloride in a little water in a test tube and heat to boiling. Can you detect the odor of ammonia? Add a few drops of a solution of sodium hydroxide. Is the ammonia liberated (R)? Moisten a small strip of red litmus paper and hold it at the mouth of the tube. Note the result. Hold the end of a glass rod moistened with concentrated hydrochloric acid in the mouth of the tube. What is formed (R)? b. Usual laboratory method. This differs from the method used in a only in the fact that the less expensive calcium hydroxide (slaked lime) is sub stituted for sodium hydroxide. Fig. 38 shows the form of apparatus used. A represents a 250-cc. flask; B and C are 250-cc. wide-mouthed bottles partly filled with water. Notice that the glass tubes extending into the bottles do not quite touch the surface of the water. Why? Two dry bottles will also be needed for collecting some of the gas. Put into the flask A an intimate mixture of about 30 g. of finely powdered slaked lime B FIG. 38 E and 15 g. of ammonium chloride; place it on a sand bath and heat gently with a small flame. A wire gauze may be |