Page images
PDF
EPUB

I. The elements sodium, potassium, lithium, thallium, phosphorus, and some others, combine with oxygen more or less rapidly at ordinary temperatures. II. Antimony, arsenic, carbon, lead, sulphur, and many other elements, combine with oxygen at temperatures above the ordinary. III. Oxides of calcium, bismuth, chromium, copper, &c. &c. are usually prepared by (i) obtaining compounds of these metals with oxygen and hydrogen, and (ii) heating these hydroxides, and so decomposing them into oxides and water. IV. Oxides of lead, manganese, bismuth, and some other metals—composed of much oxygen relatively to the mass of lead &c.-- are obtained by bringing these metals, or oxides of them composed of the metal united with relatively small masses of oxygen, in contact with two or more compounds which interact to produce oxygen.

V. Oxides of nitrogen, sulphur, tellurium, &c. are obtained by decomposing, by heat or otherwise, compounds of these elements with oxygen and some other element or elements.

The following equations present examples of each of the foregoing methods of preparing oxides :I. 2Na+ 0) = Na,0; 2P+50=P,0..

at ordinary temps. II. 2Sb + 30 = Sb,Og;

,

C +20=CO

at higher temps.
III. CaO, H, = CaO + H,0; Bi0 H. = Bi,O, + 3H,0.

by action of heat.
IV. KCIOAq + PbO (heated) = Pb0, +KCIA;

Sb,0, + 2HNO, (heated) = Sb,0, +H,0 + 2NO,.
V. 2HNO3 + P,0, (heated) = N O + 2HPOZ;

H, Teo, (heated) = H,0 + Teo,
Many elements form more than one compound with oxygen.
Thus, five oxides of nitrogen are known, viz. NO, NO, NO,
NO. NO,; four oxides of lead have been prepared, viz. Pbo,
Pb,d, Pbỏ

,,

PbO, Hydrogen combines directly with a few elements; the combination usually occurs at moderately high temperatures : thus, 2H+S (molten)=H S; H+ Br (heated) = HBr; 2C + 2H (by passing electric sparks) = CH ; &c.

[ocr errors]

2

6

2

3

3

4

2

5

5

3

4

2

[ocr errors]

5

2

3

122

3

Compounds of hydrogen with other elements are sometimes formed in chemical interactions between several elements or compounds; thus when phosphorus is heated with an aqueous solution of caustic potash, phosphorus hydride (PH) is one of the products of the reaction ; when a solution of arsenic oxide in water is brought into contact with dilute sulphuric acid and zinc, arsenic hydride (AsHz), zinc sulphate, hydrogen, and water are formed.

Very many compounds of oxygen and hydrogen, each with two or more other elements have been

prepared. Compounds of oxygen and one other element are called 123 oxides ; compounds of hydrogen with one other element are called hydrides.

The physical properties of oxides and hydrides vary much; some are solids, others are liquids, others are gases, at ordinary temperatures and pressures. The chemical properties of these compounds also vary much, but a great many oxides may be placed in one or other of two classes.

As representatives of these classes let us take the oxides of 124 sulphur and magnesium whose compositions are expressed by the formulae So, and Mgo, respectively (S = 32, Mg = 24, 0= 16).

Sulphur trioxide (SO) is a white, crystalline, solid; it dissolves very easily in water forming a colourless solution. This solution has the following (among other) properties : (1) a sour taste; (2) it turns blue litmus solution bright red; (3) it dissolves many metals--e.g. zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c. --with production of two more substances, one of which is hydrogen, and another is a compound of sulphur, oxygen, and the metal used ; (4) it interacts with oxides of many

metals

-e.g.

oxide of zinc, iron, aluminium, magnesium, cadmium, barium, &c. &c.—to produce two or more substances one of which is the same compound of sulphur, oxygen, and the metal of the oxide used, which was produced in reaction (3), and another of which is water. Further, if the solution of sulphur trioxide in water is evaporated considerably a thick oily liquid is obtained ; if this liquid is cooled below 0° crystals separate having the composition H,SO. This compound is called sulphuric acid. If sulphuric acid is dissolved in water the solution exhibits the same properties as a solution in water of sulphur trioxide. It seems therefore fair to conclude that an aqueous solution of this oxide contains the compound H SO,.

or

4

4

2

2

4

4

[ocr errors]

=

2
+

3

2

[ocr errors]

Assuming that this is so, the chemical changes enumerated above may be represented in equations as follows :

SO, +H, O+ Aq=H. SO, Aq.
(3) Zn + H SO, Aq = ZnSO, Aq + 2H;

Fe + H SO Aq = FeSO Aq + 2H;
2Al + 3H SO, Aq=A1,3SO, Aq + 6H;
Mg + H 50 Aq = MgSO Aq + 2H ;
Cd + H SO Aq=CASO, Aq + 2H;

Ba+H SO, Aq = BaSO + 2H+ Aq.
(4) ZnO + H SO, Aq= ZnSO, Aq+H,0;

FeO + H, SO Aq= FeSO Aq +H,0);
A1,0, + 3H ŠO, Aq = A1,380, Aq + 3H,0;
Mg0o+H SO, Aq = Mg50 Ağ + H20;
CdO + H2SO Aq=CdSO, Aq + H,O;

BaO + HNO Aq= BaSO4 + H2O + Aq. The compounds of zinc, iron, &c. produced in these reactions are called salts. If the composition of each of these salts is compared with that of the sulphuric acid, H,80, by the interaction of an aqueous solution of which with a metal or the oxide of a metal the salt is produced, it is seen that the salt is composed of a metal together with all the sulphur and oxygen which were combined, before the chemical change began, with hydrogen, forming the compound H,80,The compound H,SO, is called an acid. The solution of this acid in water has a sour taste; turns blue litmus red; reacts with zinc, iron, aluminium, and many other metals, to produce salts, and hydrogen; and reacts with oxides of zinc, iron, aluminium, magnesium, and other metals, to produce salts and water.

The oxide so, is an acidic, or acid-forming, oxide; that is to say, it reacts with water to produce a compound which is characterised by the properties enumerated in the preceding sentence.

Many oxides resemble sulphur trioxide in that they react with water to produce compounds of oxygen, hydrogen, and the other element of the oxide, which compounds have a sour taste, turn blue litmus red, and interact with metals and metallic oxides to produce salts. These oxides are placed in one class and are called acid-forming, or acidic, oxides. Thus, the oxides whose compositions are represented by the formulae P.O., CrO2, N,05, Seo,, respectively, are acidic oxides. The interactions of these oxides with water may be thus represented in equations :

4

3

3

2

5

3

3

5

2

Acidic oxide.

Acid.
P.O. +3H 0=2H PO
Cro, + HO=H,Cro,
NO + H2O=2ẮNO

SeO + HO=H,SeO. Let us now turn to the other oxide-Mgo. Magnesium 125 oxide is a white solid; it dissolves in a large quantity of water; this solution has not a sour taste; it turns red litmus blue. Magnesium oxide interacts with sulphuric acid, and with other acids, to produce salts and water (s. par. 124).

Many oxides resemble magnesium oxide in that they interact with acids to form salts; some of these oxides further resemble magnesium oxide in being more or less soluble in water and thus forming solutions which turn red litmus blue.

Those oxides which interact with acids to produce salts are placed in one class and are called basic oxides, or sometimes salt-forming oxides. Those basic oxides which easily dissolve in water producing liquids which turn red litmus blue are usually placed in a sub-class to which the name alkaline, or alkali-forming, oxides is given. Thus the oxides A1,0,, ZnO, Cdo, FeO, BaO are basic oxides (s. reactions represented in equations in par. 124).

The oxides of boron, chlorine, iodine, nitrogen, phosphorus, 126 selenion, sulphur, and several other elements, are acidic oxides.

The oxides of aluminium, barium, beryllium, cadmium, copper, iron, lithium, magnesium, mercury, nickel, palladium, silver, sodium, and many other elements, are basic oxides.

Some of the oxides of chromium, molybdenum, tin, tungsten, uranium, vanadium, and a few other elements, are basic, while other oxides of the same elements are acidic.

Can we classify the hydrides by a method similar to that 127 by which we have roughly arranged the oxides in classes ?

The only element which forms many compounds with hydrogen is carbon. Some of the hydrides, other than those of carbon, interact with water to produce acids; among these are H Br, HCI, HF, HI, H.S. One or two hydrides interact with water to produce compounds which again react with acids to form salts; the best marked hydride of this class is ammonia, NH . Several hydrides are either unchanged by

, water, or dissolve in it without producing either an acid or a salt-forming compound; e.g. H Sb, H, As, H Cu, HP, H, Te.

Hydrides cannot therefore be wholĩy classified by arranging them as acidic (or acid-forming), and basic (or salt-forming), M. E. C.

7

3

2)

128

[ocr errors]

MH,

[ocr errors]

2

3

3

2

2'

hydrides ; but nevertheless several hydrides belong to one or other of these classes.

But there may of course be other properties of the hydrides on which a classification might be based.

We might, for instance, classify the hydrides by looking to the composition of their reacting weights, and arranging them in classes according as the reacting weight is composed of one, two, or more, combining weights of hydrogen, and one, two, or more, combining weights of the other element. Thus the formula M,H, would represent the composition of all the hydrides, where H represents one combining weight of hydrogen, M one combining weight of the element other than hydrogen, and x and y vary.

The following table represents a classification of the greater number of the hydrides, excepting those of carbon, framed on this basis.

Hydrides; M H,
МН.

MH.
BrH, CIH, FH, IH. OH,, SeH,, SH,, TeH,.
MH,

MH,
SbH,, AsH , NH3, PH2. SiH Cu H,, O H.

This classification is evidently based altogether on composition. Before deciding whether it is, or is not, a good classification we should study the properties of the hydrides, with the view of finding whether those placed in each class, MH, MH,, &c., are marked by some common property which cuts them off from those in each of the other classes.

A slight examination of the hydrides in the foregoing table shews that many are gaseous at ordinary temperatures and pressures ; some of these are easily decomposed when mixed with air or oxygen and heated. A classification might be based on the study of these decompositions. In most cases the products of the decomposition are oxide of hydrogen (water; HO), and oxide, or oxides, of the other element. The following table summarises this method of classifying gaseous hydrides.

Gaseous Hydrides; M,H, Easily decomposed, by mixing with Not easily decomposed by oxygen and heating, into M,0, mixing with oxygen and and H.O.

heating.
SeH, SH, Tevi;

BrH, CIH, FH, IH; NH,
SbHg, AsH ,
SiH.; Cu, H,, 0,H,.

129

[ocr errors]
« PreviousContinue »