化学专业英语之第一和第二副族元素

化学专业英语之副族元素

GROUPS IB AND IIB ELEMENTS

Physical properties of Group IB and IIB

These elements have a greater bulk use as metals than in compounds, and their physical properties vary widely.

Gold is the most malleable and ductile of the metals. It can be hammered into sheets of 0.00001 inch in thickness; one gram of the metal can be drawn into a wire 1.8 mi in length1. Copper and silver are also metals that are mechanically easy to work. Zinc is a little brittle at ordinary temperatures, but may be rolled into sheets at between 120° to 150℃; it becomes brittle again about 200℃-The low-melting temperatures of zinc contribute to the preparation of zinc-coated iron .galvanized iron; clean iron sheet may be dipped into vats of liquid zinc in its preparation. A different procedure is to sprinkle or air blast zinc dust onto hot iron sheeting for a zinc melt and then coating.

Cadmium has specific uses because of its low-melting temperature in a number of alloys. Cadmium rods are used in nuclear reactors because the metal is a good neutron absorber.

Mercury vapor and its salts are poisonous, though the free metal may be taken internally under certain conditions. Because of its relatively low boiling point and hence volatile nature, free mercury should never be allowed to stand in an open container in the laboratory. Evidence shows that inhalation of its vapors is injurious.

The metal alloys readily with most of the metals (except iron and platinum) to form amalgams, the name given to any alloy of mercury.

Copper sulfate, or blue vitriol (CuSO4? 5H2O) is the most important and widely used salt of copper. On heating, the salt slowly loses water to form first the trihydrate (CuSO4? 3H z O), then the monohydrate (CuSO4? H2O), and finally the white

anhydrous salt. The anhydrous salt is often used to test for the presence of water in organic liquids. For example, some of the anhydrous copper salt added to alcohol (which contains water) will turn blue because of the hydration of the salt.

Copper sulfate is used in electroplating. Fishermen dip their nets in copper sulfate solution to inhibit the growth of organisms that would rot the fabric. Paints specifically formulated for use on the bottoms of marine craft contain copper compounds to inhibit the growth of barnacles and other organisms.

When dilute ammonium hydroxide is added" to a solution of copper (I) ions, a greenish precipitate of Cu(OH)2 or a basic copper(I) salt is formed. This dissolves as more ammonium hydroxide is added. The excess ammonia forms an ammoniated complex with the copper (I) ion of the composition, Cu(NH3)42+. This ion is only slightly dissociated; hence in an ammoniacal solution very few copper (I) ions are present. Insoluble copper compounds, execpt copper sulfide, are dissolved by ammonium hydroxids. The formation of the copper (I) ammonia ion is often used as a test for Cu2+because of its deep, intense blue color.

Copper (I) ferrocyanide [Cu2Fe(CN)6] is obtained as a reddish-brown precipitate on the addition of a soluble ferrocyanide to a solution of copper ( I )ions. The formation of this salt is also used as a test for the presence of copper (I) ions.

Compounds of Silver and Gold

Silver nitrate, sometimes called lunar caustic, is the most important salt of silver. It melts readily and may be cast into sticks for use in cauterizing wounds. The salt is prepared by dissolving silver in nitric acid and evaporating the solution.

3Ag + 4HNO3—3AgNO3 + NO + 2H2O

The salt is the starting material for most of the compounds of silver, including the halides used in photography. It is readily reduced by organic reducing agents, with the formation of a black deposit of finely divided silver; this action is

responsible for black spots left on the fingers from the handling of the salt. Indelible marking inks and pencils take advantage of this property of silver nitrate.

The halides of silver, except the fluoride, are very insoluble compounds and may be precipitated by the addition of a solution of silver salt to a solution containing chloride, bromide, or iodide ions.

The addition of a strong base to a solution of a silver salt precipitates brown silver oxide (Ag2G). One might expect the hydroxide of silver to precipitate, but it seems likely that silver hydroxide is very unstable and breaks down into the oxide and water — if, indeed, it is ever formed at all3. However, since a solution of silver oxide js definitely basic, there must be hydroxide ions present in solution.

Ag2O + H2O = 2Ag+ + 2OH-

Because of its inactivity, gold forms relatively few compounds. Two series of compounds are known —monovalent and trivalent. Monovalent (aurous) compounds resemble silver compounds (aurous chloride is water insoluble and light sensitive), while the higher valence (auric) compounds tend to form complexes. Gold is resistant to the action of most chemicals —air, oxygen, and water have no effect. The common acids do not attack the metal, but a mixture of hydrochloric and nitric acids (aqua regia) dissolves it to form gold( I ) chloride or chloroauric acid. The action is probably due to free chlorine present in the aqua regia.

3HCl + HNO3----→ NOCl+Cl2 + 2H2O

2Au + 3Cl2 ----→ 2AuCl3

AuCl3+HCl----→ HAuCl4

chloroauric acid (HAuCl4-H2O crystallizes from solution).

Compounds of Zinc

Zinc is fairly high in the activity series. It reacts readily with acids to produce hydrogen and displaces less active metals

from their salts. 1 he action of acids on impure zinc is much more rapid than on pure zinc, since bubbles of hydrogen gas collect on the surface of pure zinc and slow down the action. If another metal is present as an impurity, the hydrogen is liberated from the surface of the contaminating metal rather than from the zinc. An electric couple to facilitate the action is probably Set up between the two metals.

Zn + 2H+----→ Zn2+ + H2

Zinc oxide (ZnO), the most widely used zinc compound, is a white powder at ordinary temperatures, but changes to yellow on heating. When cooled, it again becomes white. Zinc oxide is obtained by burning zinc in air, by heating the basic carbonate, or by roasting the sulfide. The principal use of ZnO is as a filler in rubber manufacture, particularly in automobile tires. As a body for paints it has the advantage over white lead of not darkening on exposure to an atmosphere containing hydrogen sulfide. Its covering power, however, is inferior to that of white lead.