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sulphuric acid interact to produce zinc sulphate (ZnSO,) and
hydrogen. The initial and final compositions of the whole
changing system may be approximately represented thus:-
(3Zn + 4H ̧SO ̧Aq + Na2SO2Aq
J3Zn

=

3ZnSO,Aq+ Na ̧SO Aq + 3H ̧0 + H‚S ; Zn + H2SO1Aq = ZnSO ̧Aq + 2H.

A portion of the hydrogen evolved by the interaction of the zinc and sulphuric acid is employed in reducing the sodium sulphite, and the rest of the hydrogen is evolved as gas.

Sulphur dioxide is commonly produced by partially deoxidising (or reducing) sulphuric acid (H,SO,): for this purpose a concentrated aqueous solution of the acid is heated with copper, or carbon, or sulphur; the chemical changes which occur are these,—

(1) 2H,SO, + Cu = CuSO, + 2H,O + SO ̧ ;

(2) 2H2SO + C = 2H2O + CO2 + 2SO ̧ ;

(3)

3

4

2

2H2SO + S = 2H2O + 3SO2.

4

Sulphur dioxide (SO,) is gaseous at ordinary temperatures and pressures, but is easily liquefied; the other oxides of the group are solids. The oxides MO, and MO, are all acidic; SO, and SO, dissolve in water with production of heat and formation of solutions of the acids H2SO, and H,SO,, respectively; SeO, also dissolves in water to form a solution of the acid H.Se TeO, and TeO, are nearly insoluble in water; they do not interact with water to produce acids; the acids H.TeO, and H.TeO, however exist as definite solids, which when heated yield TeO, and TeO,, (and water) respectively. The oxides MŎ, and MỎ, all interact with solutions of alkalis, or, when heated, with solid alkalis, to produce salts; thus,

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Some of the oxides MO, and MO, combine with acidic oxides to form compounds, but these compounds are usually unstable, and are more or less easily decomposed by heat; thus compounds of SO, with P,O,, As,O,, and B,O,, are known; TeO, also combines with hydrogen chloride to form TeO 2HCl and TeO..3HCI.

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Compounds with the halogens. M2X2, MX2, and MX. 176 The following compounds have been prepared by the direct union of M with halogen :

Sulphur.

177

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Compounds with oxygen and the halogens; oxyhaloid compounds. MOX2 and MO2X2 (X=Cl, Br, I). Several of these compounds have been prepared: as examples we shall take SOCI, and SO,Cl. The former is obtained by reacting on sodium sulphite with phosphoric chloride, the latter by a similar reaction between sulphuric acid and phosphoric chloride. The compositions of the systems before and after the changes may be approximately represented in equations as follows ;

(1) Na,SO, + 2PCI, SOCI, + 2POCI, + 2NaCl ;

=

(2) H2SO + 2PC1 ̧ = SO,C1, + 2POCI ̧ + 2HCl.

2

The compounds SOCI, and SO,Cl, interact with water to produce hydrochloric acid, and sulphurous or sulphuric acid;

thus

(1) SOCI ̧ + 2H ̧O + Aq = 2HCl Aq + H2SO ̧Aq;

(2) SOCI ̧ + 2H2O + Aq = 2HClAq + H2SO ̧Aq.

Certain relations between the oxychloride SOCI, and the acid H.SO, and between the oxychloride SO,CI, and the acid H2SO are established by the foregoing reactions: these relations are perhaps better suggested if the reactions are stated as follows ;—

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(SOCI, is not directly obtained from H,SO,, because this acid only exists in aqueous solution.)

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If the formulae of sulphurous and sulphuric acids are written as SO.O.H, [or SO(OH),], and SO,.O,H, [or SO,(OH),], respectively, the reactions which occur between these acids and phosphoric chloride are suggested by the formulae. (s. chap. XVII.)

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Compounds with oxygen and hydrogen; Acids. Several of 178 these acids are known; the most important are those whose compositions are expressed by the general formulae H,MO and H,MO The acids HMO, where M-S or Se are produced by dissolving the oxides MO, in water; H,TeО, is formed by oxidising tellurium by nitric acid in presence of water (s. par. 175). The acid H.SO, is produced by dissolving the oxide SO, in water; H,SeO, is formed by oxidising selenion in presence of water by the interaction between water and chlorine (8. par. 175); H,TeO, is formed by decomposing the barium salt of this acid by the proper quantity of sulphuric acid, and removing water by evaporation; thus

BaTeO Aq + H2SO Aq = BaSO4 + H2TeO2Aq.

2

3

Of the acids HMO, and HMO, H.SO, is known only in aqueous solutions; when such a solution is evaporated water and sulphur dioxide (SO2) are produced; the other acids have been isolated. H2SO, and H2SO, are thick oily liquids at ordinary temperatures; H,TeO, and H,TeO, are solids. These acids are all decomposed by heat into water and the corresponding oxides; thus

3

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HMO, heated gives H2O + MO2;
H2O+MO2.

H2MO

4

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This change occurs at a higher temperature when M = Te than when M = Se, and at a higher temperature when M Se than when MS; in other words, the stability of the acids towards heat increases as the combining weight of M in

creases.

There is an oxide corresponding to each of these acids: this oxide, except in the cases of TeO, and TeO,, interacts with water to produce the acid; it is also produced, along with water, when the acid is heated.

The nomenclature of these oxides and their corresponding acids is exhibited in the following table.

Oxide.

MO 2.
Sulphur, selenion, or tellurium,
dioxide; or sulphurous, selenious,
or tellurous, anhydride.

MO3. Sulphur, or tellurium, trioxide;
or sulphuric, or telluric, anhydride

Corresponding acid.

H2MO,. Sulphurous, selenious,
or tellurous, acid.

H,MO4. Sulphuric, selenic, or
telluric, acid.

179

180

181

Compounds with electro-positive or metallic elements. The three elements combine with many metals to form sulphides, selenides, and tellurides, of similar compositions. The following are a few examples: CuM, ZnM, FeM, Bi,M,, &c. Most of these compounds interact with aqueous solutions of acids to give salts of the metal and a hydride of M; thus

CuM+ 2HCl Aq = CuCl ̧Aq + H ̧M.

The elements sulphur, selenion, and tellurium, are evidently similar in their properties. They all combine directly with hydrogen to form hydrides MH,, two of which are slightly acidic. They combine with oxygen to form oxides MO, and MO,, which are more or less markedly acidic. They form several haloid compounds which are not acidic; the stability of these towards heat increases as the combining weight of M increases. They form oxyhaloid compounds, each of which interacts with water to form an acid of M, and a haloid acid HX (where X = Cl, Br, or I). They do not interact with acids to form salts.

Some of the prominent properties of the elements of the sulphur group are compared with the more prominent properties of the halogens, and of the alkali metals, in the following table. The sulphur group of elements is evidently much more allied to the halogens than to the alkali metals.

Sulphur group. Sulphur, Selenion, Tellurium.

to

All solids; more or less
brittle; fairly high
melting points.
Electronegative
many other elements.
Form gaseous com-
pounds with hydrogen,
MH; soluble in water
giving feebly acid, or,
in case of TeH2,
neutral solutions.

Form oxides by direct
union with oxygen;
most of these dissolve
in water forming fairly
stable acids.

Unite with many
positive, or metallic,
elements; compounds
with negative elements,

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As regards the variations of properties in each of these 182 three groups we have found that as the combining weights increase the elements become more positive. In the sulphur and halogen group the elements become heavier and their melting points increase; the oxides, oxyacids, and compounds with chlorine, become more solid, and more stable towards heat; the hydrides become less stable towards heat.

In the halogen group the interaction with water takes place more completely and at a lower temperature, the smaller is the combining weight of the element; in the alkali-metals group the reverse of this holds good. In the sulphur group, tellurium is more metal-like in appearance and general physical properties than sulphur or selenion; its oxides do not directly interact with water to form acids; its hydride is easily decomposed by heat, and its aqueous solution is not acidic; it does not exhibit allotropy: in a word, tellurium is more metallic than either sulphur or selenion.

The terms reduction and oxidation, also reducers or reducing 183 agents and oxidisers or oxidising agents, have been used in describing some of the chemical changes exhibited by the elements and compounds considered in preceding paragraphs. When chlorine is passed over mercuric oxide the chlorine is oxidised and the mercuric oxide is simultaneously reduced. (s. par. 154.) When nitric acid is heated with tellurium the element is oxidised and the acid is simultaneously reduced. (s. par. 175.) When chlorine is passed into concentrated caustic potash solution potassium hypochlorite (KClO) is formed, when this is heated oxygen is evolved and potassium chloride (KCl) remains; when chlorine is passed into warm potash solution holding bismuthous oxide (Bi,O,) in suspension bismuthic oxide (BIO) and potassium chloride are formed; the bismuthous oxide is oxidised and the potassium hypo

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