Infraestructuras y equipamientos .1 Red viaria

General. As explained in a previous chapter, when ammonia is passed into water the two compounds

combine to form the base NH4OH, known as ammonium hydroxide. When this base is neutralized with acids

there are formed the corresponding salts, known as the ammonium salts. Since the ammonium group is univalent, ammonium salts resemble those of the alkali metals in formulas; they also resemble the latter salts very much in their chemical properties, and may be conveniently described in connection with them. Among the ammonium salts the chloride, sulphate, carbonate, and sulphide are the most familiar.

Ammonium chloride (sal ammoniac) (NH4Cl). This substance is obtained by neutralizing ammonium

hydroxide with hydrochloric acid. It is a colorless substance crystallizing in fine needles, and, like most ammonium salts, is very soluble in water. When placed in a tube and heated strongly it decomposes into hydrochloric acid and ammonia. When these gases reach a cooler portion of the tube they[Pg 295] at once recombine, and the resulting ammonium chloride is deposited on the sides of the tube. In this way the salt can be separated from nonvolatile impurities. Ammonium chloride is sometimes used in preparation of ammonia; it is also used in making dry batteries and in the laboratory as a chemical reagent.

Ammonium sulphate ((NH4)2SO4). This salt resembles the chloride very closely, and, being cheaper, is used

in place of it when possible. It is used in large quantity as a fertilizer, the nitrogen which it contains being a very valuable food for plants.

Ammonium carbonate ((NH4)2CO3). This salt, as well as the acid carbonate (NH4HCO3), is used as a

chemical reagent. They are colorless solids, freely soluble in water. The normal carbonate is made by heating ammonium chloride with powdered limestone (calcium carbonate), the ammonium carbonate being obtained as a sublimate in compact hard masses:

2NH4Cl + CaCO3 = (NH4)2CO3 + CaCl2.

The salt always smells of ammonia, since it slowly decomposes, as shown in the equation (NH4)2CO3 = NH4HCO3 + NH3.

The acid carbonate, or bicarbonate, is prepared by saturating a solution of ammonium hydroxide with carbon dioxide:

NH4OH + CO2 = NH4HCO3.

It is a well-crystallized stable substance.

Ammonium sulphide ((NH4)2S). Ammonium sulphide is prepared by the action of hydrosulphuric acid upon

ammonium hydroxide:

2NH4OH + H2S = (NH4)2S + 2H2O.

[Pg 296]

If the action is allowed to continue until no more hydrosulphuric acid is absorbed, the product is the acid sulphide, sometimes called the hydrosulphide:

NH4OH + H2S = NH4HS + H2O.

If equal amounts of ammonium hydroxide and ammonium acid sulphide are brought together, the normal sulphide is formed:

NH4OH + NH4HS = (NH4)2S + H2O

It has been obtained in the solid state, but only with great difficulty. As used in the laboratory it is always in the form of a solution. It is much used in the process of chemical analysis because it is a soluble sulphide and easily prepared. On exposure to the air ammonium sulphide slowly decomposes, being converted into ammonia, water, and sulphur:

(NH4)2S + O = 2NH3 + H2O + S.

As fast as the sulphur is liberated it combines with the unchanged sulphide to form several different ammonium sulphides in which there are from two to five sulphur atoms in the molecule, thus: (NH4)2S2,

(NH4)2S3, (NH4)2S5. These sulphides in turn decompose by further action of oxygen, so that the final

products of the reaction are those given in the equation. A solution of these compounds is yellow and is sometimes called yellow ammonium sulphide.

FLAME REACTION—SPECTROSCOPE

When compounds of either sodium or potassium are brought into the non-luminous flame of a Bunsen burner the flame becomes colored. Sodium compounds color it intensely yellow, while those of potassium color it pale violet. When only one of these elements is present it is[Pg 297] easy to identify it by this simple test, but when both are present the intense color of the sodium flame entirely conceals the pale tint characteristic of

potassium compounds.

It is possible to detect the potassium flame in such cases, however, in the following way. When light is allowed to shine through a very small hole or slit in some kind of a screen, such as a piece of metal, upon a triangular prism of glass, the light is bent or refracted out of its course instead of passing straight through the glass. It thus comes out of the prism at some angle to the line at which it entered. Yellow light is bent more than red, and violet more than yellow. When light made up of the yellow of sodium and the violet of potassium shines through a slit upon such a prism, the yellow and the violet lights come out at somewhat different angles, and so two colored lines of light—a yellow line and a violet line—are seen on looking into the prism in the proper direction. The instrument used for separating the rays of light in this way is called a spectroscope (Fig. 79). The material to be tested is placed on a platinum wire and held in the colorless Bunsen flame. The resulting light passes through the slit in the end of tube B, and then through B to the prism. The resulting lines of light are seen by looking into the tube A, which contains a magnifying lens. Most elements give more than one image of the slit, each having a different color, and the series of colored lines due to an element is called its spectrum.

Fig. 79 [Pg 298]

The spectra of the known elements have been carefully studied, and any element which imparts a

characteristic color to a flame, or has a spectrum of its own, can be identified even when other elements are present. Through the spectroscopic examination of certain minerals a number of elements have been

discovered by the observation of lines which did not belong to any known element. A study of the substance then brought to light the new element. Rubidium and cæsium were discovered in this way, rubidium having bright red lines and cæsium a very intense blue line. Lithium colors the flame deep red, and has a bright red line in its spectrum.

EXERCISES

1. What is an alkali? Can a metal itself be an alkali?

2. Write equations showing how the following changes may be brought about, giving the general principle involved in each change: NaCl --> Na2SO3, Na2SO3 --> NaCl, NaCl --> NaBr, Na2SO4 --> NaNO3, NaNO3

--> NaHCO3.

3. What carbonates are soluble?

4. State the conditions under which the reaction represented by the following equation can be made to go in either direction:

Na2CO3 + H2O + CO2 <--> 2 NaHCO3.

5. Account for the fact that solutions of sodium carbonate and potassium carbonate are alkaline.

6. What non-metallic element is obtained from the deposits of Chili saltpeter?

7. Supposing concentrated hydrochloric acid (den. = 1.2) to be worth six cents a pound, what is the value of the acid generated in the preparation of 1 ton of sodium carbonate by the Le Blanc process?

8. What weight of sodium carbonate crystals will 1 kg. of the anhydrous salt yield?

9. Write equations for the preparation of potassium hydroxide by three different methods.

10. What would take place if a bit of potassium hydroxide were left exposed to the air?

11. Write the equations for the reactions between sodium hydroxide and bromine; between potassium hydroxide and iodine.

12. Write equations for the preparation of potassium sulphate; of potassium acid carbonate.

ROBERT WILHELM BUNSEN (German) (1811-1899)

Invented many lecture-room and laboratory appliances (Bunsen burner); invented the spectroscope and with it discovered rubidium and cæsium; greatly perfected methods of electrolysis, inventing a new battery; made many investigations among metallic and organic substances

[Pg 299]

13. What weight of carnallite would be necessary in the preparation of 1 ton of potassium carbonate?

14. Write the equations showing how ammonium chloride, ammonium sulphate, ammonium carbonate, and ammonium nitrate may be prepared from ammonium hydroxide.

15. Write an equation to represent the reaction involved in the preparation of ammonia from ammonium chloride.

16. What substances already studied are prepared from the following compounds? ammonium chloride; ammonium nitrate; ammonium nitrite; sodium nitrate; sodium chloride.

17. How could you prove that the water in crystals of common salt is not water of crystallization?

18. How could you distinguish between potassium chloride and potassium iodide? between sodium chloride and ammonium chloride? between sodium nitrate and potassium nitrate?

[Pg 300]

CHAPTER XXIV

THE ALKALINE-EARTH FAMILY