EXERCISE 64

GENERAL METHODS FOR THE PREPARATION OF THE COMPOUNDS OF THE METALS

Apparatus. 6 test tubes.

Materials. 0.1 g. of each of the following salts, dissolved in 5 cc. water: (1) calcium chloride, (2) lead nitrate, (3) barium chloride, (4) ferric chloride, (5) silver nitrate (solutions of any of these on the reagent shelf may be used); ammonium carbonate solution (R. S.); 0.1 g. potassium iodide dissolved in 5 cc. water; hydrochloric acid.

a. By the direct union of the elements. Recall the formation of sulfides of copper and of iron (Exercise 37); of chlorides of copper and antimony (Exercise 25). Write the equations for the reactions involved.

b. By dissolving a metal or its hydroxide in appropriate acids. Recall the formation of zinc sulfate (Exercise 8); of sodium chloride (Exercise 28); of copper nitrate (Exercise 33). Write the equations for the reactions. When a metal or its hydroxide is acted upon by an acid, what becomes of the metal?

c. By acting upon a salt of an acid with an acid having a higher boiling point. Recall the action of sulfuric acid upon sodium nitrate (Exercise 33); of hydrochloric acid upon iron sulfide (Exercise 38); of sulfuric acid upon fluorides (Exercise 44); of sulfuric acid on chlorides (Exercise 26); of hydrochloric acid on carbonates (Exercise 50). Write the equations for each reaction and show in what respects they are all similar.

d. By the decomposition of a compound. Recall the action of heat upon potassium chlorate (Exercise 7); upon copper nitrate and lead nitrate (Exercise 34). Write the equations for the reactions involved.

e. The following compounds are insoluble (see Appendix, Table of Solubility of Various Solids): calcium carbonate (CaCO), lead sulfate (PbSO), barium carbonate (BaCO), ferric hydroxide (Fe(OH),), silver chloride (AgCl), lead iodide (PbI). Prepare a small amount of each in a test tube (R) and state the general principle involved in each reaction.

EXERCISE 65

THE COMPOUNDS OF SODIUM

Apparatus. Beaker; hydrogen generator for generating carbon dioxide (Fig. 30); 250-cc. bottle; hard-glass test tube; platinum wire; burner; piece of cobalt glass (10 cm. square).

Materials. Litmus paper (red and blue); hydrochloric acid; 5 g. sodium chloride; 5 g. sodium carbonate; sulfuric acid; 2 or 3 crystals of Glauber's salt; 10 g. ammonium carbonate; marble for generating carbon dioxide; 3 g. sodium bicarbonate; limewater (R. S.).

a. Recall experiments with sodium, Exercise 27.

b. Dissolve 5 g. of sodium chloride in as little water as possible. Set the solution aside overnight until some of the salt crystallizes out. Examine the shape of the crystals (use magnifying glass) (?).

c. Dissolve 5 g. of sodium carbonate in 20 cc. of water. Test the solution with red and with blue litmus paper (?).

Now convert the sodium carbonate present into common salt (R). How can you be sure that the product contains no unchanged sodium carbonate? Describe the method (R). Treat some of the salt so prepared with sulfuric acid. What gas is evolved (R)?

d. What changes does Glauber's salt undergo when exposed to air (refer to Exercise 43)?

e. Dissolve 10 g. of finely powdered ammonium carbonate in 100 cc. of cold ammonium hydroxide, shaking or stirring the mixture to secure solution. Saturate this solution with sodium chloride by shaking it with the finely

pulverized salt. Pour off the clear solution into a 250-cc. bottle and pass a slow current of carbon dioxide through it until a precipitate (sodium bicarbonate) forms. Filter off the precipitate, dry between two pieces of filter paper, and examine (R). If a precipitate does not form by the end of the laboratory period, cork the bottle, and set it aside until the next laboratory period.

f. Fill a hard-glass test tube about one-fourth full of sodium bicarbonate and heat gently. Prove that carbon dioxide is evolved. What liquid condenses in the colder part of the tube? Write equations for the reactions by which sodium carbonate is converted into the bicarbonate, and vice versa.

g. Bend the end of a platinum wire into the form of a small loop and hold it in the Bunsen flame until it ceases to give any color to the flame; then dip it into a solution of a compound of sodium so that a drop of the solution is suspended in the loop; then hold it in the outer film of the base of a Bunsen flame. Note the color. Note the appearance of the sodium flame when viewed through a piece of cobalt glass.

EXERCISE 66

* THE DETERMINATION OF THE WEIGHT OF COMMON SALT OBTAINED BY ADDING HYDROCHLORIC ACID TO A DEFINITE WEIGHT OF SODIUM BICARBONATE

(QUANTITATIVE)

Apparatus. Evaporating-dish and watch-glass cover; ring stand; burner; balance.

Materials. 1 g. sodium bicarbonate; hydrochloric acid.

Carefully weigh the evaporating-dish and watch glass. Transfer to the dish about 1 g. of sodium bicarbonate and reweigh. Pour 4 or 5 cc. of water on the bicarbonate, and place the watch glass on the dish so that only the lip of the dish remains uncovered. Now pour down the lip of the dish

2 or 3 drops of hydrochloric acid. Wait until the effervescence caused by the escape of the carbon dioxide ceases, then add a few drops more of the acid. Repeat until the addition of the acid no longer causes any effervescence. Now hold the watch glass in the hand just above the dish and with a little water carefully rinse back into the dish the liquid which has collected on the undersurface of it. Remove the watch glass and slowly evaporate the solution (Fig. 33).

When the solution has evaporated nearly to dryness, cover the dish with the watch glass and heat the dish with the tip of the flame. Continue the heating until there is no more liquid left in the dish or clinging to the undersurface of the glass. Then withdraw the heat and, after the dish is cool, reweigh.

From your results calculate the amount of salt formed from 1 g. of the bicarbonate. Compare your result with those obtained by other members of the class. How does the average of the results obtained compare with the theoretical results?

EXERCISE 67

THE TEST FOR POTASSIUM; THE PREPARATION OF POTASSIUM NITRATE

Apparatus. Platinum wire for flame test; cobalt glass; burner; small beaker; stirring-rod; funnel.

Materials. 0.1 g. potassium nitrate and 0.1 g. sodium chloride, each dissolved in a little water; 17 g. sodium nitrate; 15 g. potassium chloride; filter paper.

a. Test for potassium. Repeat g, Exercise 65, using a solution of a compound of potassium (?). Note the appearance of the flame through a piece of cobalt glass (?). Repeat, using a solution containing a compound of sodium as well as one of potassium. How could you detect both sodium and potassium if they were present in the same solution?

*b. Preparation of potassium nitrate. Dissolve 17 g. of sodium nitrate in 15 cc. of boiling water; also 15 g. of potassium chloride in 30 cc. of boiling water. Mix the two solutions in a small beaker and evaporate (stirring the mixture) to about 20 cc.; then quickly filter the hot solution and set the filtrate (filtrate A) aside until cold.

The reaction between potassium chloride and sodium nitrate is reversible, and the number of grams of each of the four compounds involved which dissolve in 100 g. of water at 15° and 100° is as follows:

From a study of these solubilities, what compound should you expect would separate when the hot solutions of sodium nitrate and potassium chloride are mixed together? Test the solid on the filter paper. (Examine the crystals with a magnifying glass and compare with b, Exercise 65, to see if your conclusion is correct. Taste the crystals.)

What solid should you expect would separate from filtrate A when it is cooled? Should you expect it to be pure? Examine it with a magnifying glass. Can you detect crystals of sodium chloride in this solid?

Dissolve the solid in as little hot water as possible, cool the solution, and again filter off the solid. Repeat until no crystals of sodium chloride can be detected. Prove the identity of this compound (?).