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If NaX is heated with sulphuric acid only, hydrogen chloride, bromide, or iodide is formed; if one of these compounds is heated with manganese dioxide, chlorine, bromine, or iodine is formed along with manganese chloride, bromide, or iodide hence the change represented in the above equation probably takes place in two parts occurring simultaneously. Thus
(2) 2HX + MnO ̧ + H2SO1 = MnSO1 + 2H2O + 2X.
Chemical properties. Compounds with hydrogen. The elements chlorine, bromine, and iodine, combine directly with hydrogen, to form the compounds HX, where X = Cl, Br, or I. Hydrogen chloride, HCl, is formed by exposing a mixture of equal volumes of hydrogen and chlorine to diffused daylight; hydrogen bromide is formed by passing a mixture of hydrogen and bromine vapour through a hot tube; hydrogen iodide is formed, but only in small quantities, by passing a mixture of hydrogen and iodine vapour through a tube containing powdered glass or finely divided platinum, heated to 300° or 400°. These compounds, HX, are also produced by decomposing NaX or KX by an acid; e.g.
2NaCl + H2SO1 = Na2SO + 2HCl.
They are also formed, along with phosphorous acid (H.PO,), when water interacts with chloride, bromide, or iodide, of phosphorus; thus PX, + 3H ̧0 = 3HX + PO2H ̧
The compounds HX are gaseous at ordinary temperatures and pressures; they are very soluble in water and the solution in each case is markedly acid (s. chap. IX.). Hydrogen iodide is decomposed by heat into hydrogen and iodine; the change begins at about 180°; hydrogen bromide is also decomposed by heat but the change does not begin until a temperature higher than 180° is reached; hydrogen chloride is not decomposed by heat.
Compounds with oxygen. None of the elements, chlorine, bromine, iodine, combines directly with oxygen. No oxide of bromine has been isolated; one oxide of iodine, and two (or perhaps three) oxides of chlorine are known. The compositions of these oxides are as follows;-Cl,O, CIO,, ( CI,O), 10. The oxides C10 and 1,0, are prepared by carrying out chemical changes in which oxygen is produced in presence of chlorine and iodine, respectively. To prepare C10, mer
curic oxide (HgO) is heated in a stream of dry chlorine. When mercuric oxide is heated it is decomposed into mercury and oxygen; therefore by passing chlorine over heated mercuric oxide we carry out a reaction in which oxygen is produced in presence of chlorine. When a concentrated solution of nitric acid (HNO) is heated, oxygen, water, and oxides of nitrogen are produced. If iodine is heated with concentrated nitric acid, a part of the oxygen coming from the nitric acid combines with the iodine to form iodine pentoxide (IO). The two reactions are represented in equations thus ;
(1) 2HgO+4Cl = HgCl ̧. HgO + CI,O.
(2) 4HNO ̧+21 = I ̧0 ̧ + 2H2O + 3NO + NO„.
In the interaction of water with chlorine oxygen is evolved; if iodine is suspended in water and chlorine is passed into the water hydrated iodine pentoxide is produced; thus
21+ 6H2O + 10Cl + Aq = I ̧O2H ̧0 + 10HClAq.
When the compound IO.. HO thus produced is heated, water is removed and IO, remains.
The other oxide of chlorine, CIO,, is produced by decomposing chloric acid, HCIO,, by heat. Chloric acid is very easily decomposed; when the potassium salt of the acid interacts with sulphuric acid, chloric acid and potassium sulphate are formed, but the heat produced in the reaction suffices to decompose the chloric acid into chlorine dioxide (CIO) and other products. These reactions may be represented thus in equations ;
(1) 3KCIO, +3H,SO, = 3HC10, + 3KHSO1;
(2) 3HClO, = 2ClO,+ HClO,+H,O.
It is still undecided whether the body described as chlorine trioxide, Cl,O,, is a definite compound or a mixture of chlorine dioxide and chlorine.
The oxides of chlorine and iodine are acidic. Each interacts with water to form an acid or acids; thus
(1) Cl2O + H2O + Aq = 2HClOAq;
(2) 2C1O,+H_O + Aq = HClO_Aq+HClO Aq;
(3) 1,0, + H2O+ Aq= 2HIO,Aq.
The three acids hypochlorous (HCIO), chlorous (HC1O,), chloric (HCIO), are known only in aqueous solutions; when these solutions are concentrated to remove water the acids are decomposed into oxides of chlorine and water. Iodic acid
(HIO) is known as a solid body; when heated it decomposes into iodine pentoxide (1,0) and water.
Compounds with non-metallic elements other than hydrogen or oxygen. Chlorine, bromine, and iodine, combine directly with many non-metallic elements. The compositions of these compounds can generally be expressed by formulae in which X represents one combining weight of any one of the three elements. The following table shews the compositions of some of the best known of these compounds.
with arsenic; AsX, where X=Cl or Br only.
antimony; Sbx,: also SbC15.
boron; BX, where X-Cl or Br only.
phosphorus; PX: also PX, where X-Cl or Br only: also P2L. selenion; SeX: also SeX: also SeX, where X=Cl or Br only. silicon; SiX where X=Cl or I only.
tellurium; TeX: also TeX.
A few other compounds of chlorine, bromine, and iodine with non-metallic elements are formed indirectly; e.g. CX, CX, CX, SiBr, Si Cl, &c.
Compounds with metallic elements. The elements we are considering combine with most metals to form compounds with similar compositions. The compositions of some of these are represented in the following formulae
NaCl, KBr, KI; CdX„, CaX„, BaX„ ZnX; BiX; Cr ̧X ̧;
These compounds are usually produced by heating the metal in contact with chlorine, bromine, or iodine; in some cases however it is necessary to use indirect methods of preparation.
The binary compounds of the three elements we are considering are usually called haloid compounds (i.e. compounds resembling common salt, NaCl); the elements themselves are often called halogens. Many of the haloid compounds of the non-metals other than hydrogen and oxygen, and some of those of the metals, can be gasified without decomposition. Most of the non-metallic haloid compounds interact with water to produce solutions of HX (where X = Cl, Br, or I), and generally either an oxide or an oxygen-containing acid of the non-metal formerly combined with halogen. The following equations represent some of these changes :
2AsCl2+ 3H2O + Aq = As2O2Aq + 6HCl Aq.
28 Br2+3HO+ Aq= H SO, Aq+3S+4HBrAq.
In some cases the products of the interaction of a nonmetallic haloid compound and water are HX and a compound of the non-metal with oxygen and halogen; thus
SbI2+H2O + Aq = SbOI + 2HIAq.
Most of the haloid compounds of the metallic elements are chemically unchanged when brought into contact with water; several dissolve in water. In some cases however chemical change occurs; the usual products are haloid compounds of hydrogen (HX) and an oxychloride, oxybromide, or oxyiodide, of the metal:—thus,
BiCl + 2H2O + Aq = BiOCl + 2HClAq;
Interactions with water. The three elements dissolve in 157 water, chlorine very freely, bromine less freely, and iodine only in small quantities. By cooling aqueous solutions of chlorine or bromine crystals separate having the composition Cl. 5H ̧O and Br. 5H,O respectively: no hydrate of iodine-i.e. compound of iodine with water has been obtained. Aqueous solutions of the three elements contain small quantities of hydrochloric, hydrobromic, and hydriodic acids, respectively; i.e. the water and chlorine &c. interact as shewn by the equation
This reaction proceeds more rapidly when X = Cl than when X = Br. When X = I but very little reaction occurs. These reactions are hastened by sunlight. If some easily oxidised substance is dissolved in water and chlorine is passed into the liquid the substance is usually oxidised; thus a solution of sulphur dioxide reacts with chlorine to produce sulphur trioxide, a solution of phosphorous oxide reacts with chlorine to produce phosphoric oxide:-or, in equations
(1) SO2Aq+H ̧O + 2Cl = 2HCl Aq + SO2Aq.
(2) P ̧ ̧Aq+2H ̧O + 4C1 = 4HClAq + P2O¿Aq.
The bleaching action of chlorine depends upon its interacting with water to produce oxygen. Dry chlorine does not bleach a piece of madder-dyed cloth; but if water is present
the cloth is bleached. The colourless bodies produced are the results of the interaction of oxygen with the colouring matter of the cloth; this oxygen is produced from the water by interaction with chlorine as already described. An aqueous solution of bromine bleaches more slowly than a solution of chlorine, and a solution of iodine bleaches very slowly indeed : the bleaching action is more or less rapid according as the element decomposes water rapidly or slowly (v. supra).
Interactions with solutions of alkalis. Chlorine, bromine, and iodine interact with cold aqueous solutions of caustic potash, soda, &c. to produce potassium or sodium (&c.) chloride, bromide, or iodide, and also potassium (&c.) hypochlorite, hypobromite, or (probably) hypoiodite. Thus in equations (X= Cl, Br, or I)
6KOHAq + 6X = 3KXAq + 3KXOAq + 3H2O.
The interaction which occurs between one of the halogens and a hot solution of caustic potash, soda, &c. is expressed thus:
6KOHAq + 6X = 5KXAq + KXО ̧Aq ± 3H ̧0.
The products are potassium (&c.) chloride (bromide or iodide), potassium (&c.) chlorate (bromate or iodate), and
Solutions in water of potassium, sodium, (&c.) hypochlorite or hypobromite are changed by heat into potassium (&c.) chloride or bromide, and chlorate or bromate, thus
3KCIOAq (heated) = 2KC1Aq + KClO,Aq.
If an easily oxidised substance is present it is oxidised and only potassium chloride or bromide is produced. From these facts it follows that if an easily oxidised substance is dissolved in an aqueous solution of caustic potash, the solution is heated and chlorine is passed in, oxidation ought to occur. Experiment shews that this conclusion is correct; experiment further shews that an element or compound which is not soluble in aqueous caustic potash may often be oxidised by suspending it in hot potash solution and passing in chlorine. Examples of such interactions are these :
(1) SeO2+2KOHAq + 2C1 = SeO ̧Aq + 2KClAq + H ̧O, (2) Bi,O,+4KOHAq + 4Cl = Bi„O ̧ + 4KClAq + 2H ̧O, (3) MnSO,Aq + 2KOHAq + 2C1
· MnO ̧ + K2SO̟ ̧Aq + 2HClAq,