+2 UNIT 9 PAGE-3
ZINC
Occurrence
Zinc does not occur in native form because it is a reactive metal. The main ores of zinc are :
(i) Zinc blende ZnS (iv) Calamine ZnCO3
(ii) Zincite ZnO (v) Franklinite ZnO Fe2O3
(iii) Willemite Zn2SiO4
Extraction
The chief ore employed for the extraction of zinc is the sulphide, which is concentrated by froth floatation process. The carbonate ore requires no enrichment. The concentrated sulphide ore is roasted on sintering machine to give an oxide sinter. The sulphur dioxide produced is utilized for manufacture of sulphuric acid.
2 ZnS + 3 O2 ® 2 ZnO + 2 SO2
ZnCO3 ® ZnO + CO2
The oxide is then made brickettes with coke and clay and heated by producer gas in vertical retorts at 1673 K. Zinc ( boiling point 1183 K )distills off and is collected by rapid chilling. The crude metal is further purified by distillation or electrolytically.
Zinc is largely used for protecting iron from rusting (galvanisation and electrogalvanisation). It is also used as a constituent of many alloys, e.g., brass( Cu 60%, Zn 40%) and german silver (Cu 25-30%, Zn 25 – 30% , Ni 40 – 50%)
MERCURY
Occurrence
Mercury ( 1 x 10-5 % of the earth’s crust) is some times found native in rocks. The most important ore is cinnabar (HgS)
Extraction
The extraction is done in the following steps :
1. Concentration : The ore is finely powdered and is then concentrated by froth floatation process.
2. Roasting : The concentrated ore is roasted in excess of air when the vapours of mercury are formed. These are condensed to form liquid mercury.
In some cases , cinnabar is heated with lime (CaO) when mercury vapours are produced.
The volatilised mercury is condensed to liquid mercury metal. It is about 99.5% pure and contains some impurities of lead, zinc, tin, copper etc.
3. Refining : The mercury is refined by the following steps :
(a) Treatment with dil nitric acid : Impure mercury is passed slowly through a long tube containing 5% nitric acid. Some of it react with nitric acid to form mercury(I)nitrate which then reacts with metals present as impurities (such as Zn, Pb, Sn etc) which pass into solution leaving pure mercury.
6 Hg + 8 HNO3 (dil) ® 3 Hg2(NO3)2 + 4 H2O + 2 NO
Hg2(NO3)2 + Zn ® Zn(NO3) 2 + 2 Hg
Hg2(NO3)2 + Pb ® Pb(NO3) 2 + 2 Hg
Hg2(NO3)2 + Sn ® Sn(NO3) 2 + 2 Hg
(b) Distillation under reduced pressure : It is finally purified by distillation under reduced pressure. The vapours collected are condensed to give mercury in the liquid state.
STEEL AND IMPORTANT ALLOYS
Alloys are homogeneous mixtures of two or more metals and a metal and a non-metal. Transition metals have a good tendency to form alloys.
Steel obtained by addition of some other one or more metals such as Cr, V, Ti, Mo, Mn, Co or Ni to carbon steel are called alloy steels. The metals are added to obtain some special properties to steel. Some special alloy steel , their composition , properties and uses are given below.
Uses
Amalgam is an alloy of a metal with mercury. Sodium amalgam is used as a reducing agent. An amalgam of tin is used for coating mirror and mercury-silver-tin alloy is employed for dental filling. Other uses of mercury include the production of mercury drugs and detonators. It is used in thermometers, botton cells , vaccum pumps and fluorescent lamps.
Some important alloy steels
Name | Percentage composition | Poperties | Uses |
1. Stainless steel | Fe = 73 , Cr = 18, Ni = 8 | Resistance to rusting | Utensils, cycle and automobile parts, cutlery |
2. Nickel steel | Fe=96-98, Ni = 2 - 4 | Hard, elastic and rust proof | Cables, automobiles and aeroplane parts, armour plates, gears |
3. Invar | Fe = 64, Ni = 36 | Low expansion on heating | Meter scales, measuring instruments, clock pendulms |
4. Chrome steel | Fe = 98 , Cr = 1.5 – 2.0 | High tensile strength | Cutting tools such as files, cutlery |
5. Tungsten steel | Fe = 94 , W = 5 and C | Hard , resistant to corrosion | High speed cutting tools, springs |
6. Silicon steel | Fe = 85 , Si = 15 | Hard and resistant to acids | Pumps and pipes for carrying acids |
7. Alnico | Fe = 60 , Al = 12, Ni = 20, Co = 5 | Strongly magnetic | Permanent magnets |
8. Manganese steel | Fe = 86 , Mn = 13 and C | Extremely hard, resistant to wear and tear | Rock crushers, burglar proof safes, rail road tracks |
Alloys of Transition Metals
Alloy | Percentage composition | Uses |
Alloys of copper | ||
Brass | Cu = 60 , Zn = 40 | Utensils, condenser tubes, catridge caps |
Aluminium bronze | Cu = 90, Al = 10 | Coins, picture frames, golden powder for paints, cheap jewellery |
Bronze | Cu = 90 , Sn= 10 | Coins, statues, control valves |
Bell metal | Cu = 80, Sn = 20 | Bells, gongs |
Gunmetal | Cu = 88, Sn = 10, Zn = 2 | Gears, bearings, castings |
German silver | Cu = 25 – 50, Zn = 25 – 35, Ni = 10 - 35 | Utensils, resistence wire |
Phosphor bronze | Cu = 95, Sn = 4.8, P = 0.2 | Springs, electrical equipments |
Monel metal | Cu = 30, Ni = 67, Fe and Mn = 3 | Acid pumps and acid containers |
Gold copper alloy | Au = 90 , Cu = 10 | Gold coins, jewellery, watch cases, spectacle rims |
Constantan | Cu = 60, Ni = 40 | Electrical apparatus |
Alloys of silver | ||
Coinage silver | Ag = 90, Cu = 10 | Silver coins |
Silver solder | Ag = 63, Cu = 30, Zn = 7 | Soldering and for joining metals |
Dental alloy (dental amalgam) | Hg = 52, Sn = 12.5, Cu = 2, Zn = 0.5 | For filling teeth |
Palladium silver | Ag = 40, Pd = 60 | Potentiometer and winding of some special instruments |
SOME IMPORTANT COMPOUNDS OF TRANSITION ELEMENTS
Oxides
The transition metals generally react with oxygen at high temperatures to form oxides. The general formulae of the oxides are : MO, M3O4, MO2, M2O3 and MO3. The important oxides of the elements of first transition series are given in the following TABLE. These oxides exhibit acidic, basic and amphoteric character.
Important Oxides of first transition series.
Sc | Sc2O3(b) |
Ti | TiO(b), Ti2O3 (b), TiO2 ( c) |
V | VO(b), V2O3 (b), VO2 ( c), V2O5 (a) |
Cr | CrO(b), Cr2O3 ( c), CrO2 ( c) , CrO3 (a) |
Mn | MnO(b), Mn2O3 (b), Mn3O4 ( c) |
Fe | FeO(b), Fe2O3 ( c), Fe3O4 (b) |
Co | CoO(b) |
Ni | NiO(b) |
Cu | Cu2O(b), CuO( c) |
Zn | ZnO (c ) where a = acidic ; b= basic ; c = amphoteric |
It is observed that when the metal in its highest oxidation state, its oxide is acidic, for low oxidation state, its oxide is basic, while for intermediate oxidation state, the oxides show amphoteric behaviour. For example, oxides of manganese have the behaviour as :
Among the oxides of transition metals, potassium dichromate, potassium chromate and potassium permanganate are very important and are discussed below.
POTASSIUM DICHROMATE , K2Cr2O7
Preparation : It is prepared from chrome iron ore (FeCr2O4). The preparation of potassium dichromate from chromite ore involves the following steps :
i) Conversion of chromite ore to sodium chromate: The chromite ore is fused with sodium hydroxide or sodium carbonate in the presence of air.
ii) Conversion of sodium chromate to sodium dichromate : The solution of sodium chromate is filtered and acidified with dilute sulphuric acid giving its dichromate.
2 Na2CrO4 + 2 H+ ® Na2Cr2O7 + 2 Na+ + H2O
iii) Conversion of Sodium dichromate into potassium dichromate : Potassium dichromate is prepared by mixing a hot solution of sodium dichromate and potassium chloride in equimolar proportions.
Sodium chloride being least soluble, precipitates from the hot solution and is removed by filtration. Orange crystals of potassium dichromate separate out from the mother liquor on cooling.
Properties
The important properties of K2Cr2O7 are
1. It is an orange red crystalline solid having melting point 670 K.
2. Solubility : It is moderately soluble in cold water but readily soluble in hot water.
3. Action of heat : It decompose on heating to form potassium chromate, chromic oxide and oxygen.
4. Action with alkalies : On heating with alkalies, the orange colour of dichromate solution changes to yellow due to the formation of chromate ions.
On acidification the colour of the yellow solution , again changes to orange red due to the reversible reaction.
Thus, the dichromate ion (Cr2O72- ) and Chromate ion (CrO42- ) exist in equilibrium which are inter-convertible by changing the pH of the solution.
In alkaline solution, chromate ions are present, while in acidic solution dichromate ions are present.
5. Chromyl chloride test : When potassium dichromate is heated with Con. Sulphuric acid and a soluble metal chloride(e.g. NaCl) orange red vapours of chromyl chloride are evolved. This is chromyl chloride test.
6. Action with hydrochloric acid : Potassium dichromate reacts with hydrochloric acid and evolves chlorine.
7. Oxidising character : Potassium dichromate acts as a powerful oxidising agent in acidic medium. In the presence of dilute sulphuric acid, K2Cr2O7 liberates nascent oxygen and therefore acts as an oxidising agent.
In terms of electronic concept, the Cr2O72- ion takes up electrons in the acid medium and hence acts as an oxidising agent.
Cr2O72- + 14H+ +6 e- ® 2 Cr3+ + 7 H2O
Both sodium dichromate and potassium dichromate are oxidising agents. But potassium dichromate is preferred since it is not hygroscopic and can be used as primary standard.
Some of the oxidising reactions of potassium dichromate are :
i) It oxidises iodides to iodine.
Cr2O72- + 14H+ +6 e- ® 2 Cr3+ + 7 H2O
6 I - ® 3 I2 + 6 e-
--------------------------------------------------------
Cr2O72- + 14H+ + 6 I- ® 2 Cr3+ + 7 H2O + 3 I2
ii) It oxidises ferrous sulphate to ferric sulphate.
Cr2O72- + 14H+ +6 e- ® 2 Cr3+ + 7 H2O
6 Fe2+ ® 6 Fe3++ 6 e-
--------------------------------------------------------
Cr2O72- + 14H+ + 6 Fe2+ ® 2 Cr3+ + 7 H2O + 6 Fe3+
iii) It oxidises Hydrogen sulphide to sulphur.
Cr2O72- + 14H+ +6 e- ® 2 Cr3+ + 7 H2O
3 H2S ® 6 H+ +3 S + 6 e-
--------------------------------------------------------
Cr2O72- + 8 H+ + 3 H2S ® 2 Cr3+ + 7 H2O + 3 S
iv) It oxidises SnCl2 to SnCl4.
Cr2O72- + 14H+ +6 e- ® 2 Cr3+ + 7 H2O
3 Sn2+ ® 3Sn4+ + 6 e-
-------------------------------------------------------
Cr2O72- + 14H+ + 3 Sn2+ ® 2 Cr3+ + 7 H2O + 3Sn4+
Structure of Chromate and dichromate ions
The chromate ion has tetrahedral structure in which four oxygen atoms around chromium atom. The structures of chromate and dichromate ions are given below :
Uses of Potassium dichromate
i) For volumetric estimation of ferrous salts, iodides and sulphites.
ii) For the preparation of other chromium compounds such as chrome alum, K2SO4.Cr2(SO4)3.24H2O, Chrome yellow(PbCrO4) and chrome red (PbCrO4PbO)
iii) Used in the photography for hardening gelatin film.
iv) In dyeing for providing Cr(OH)3 as a mordant.
v) Chromic acid mixture used for cleaning glassware, consists of K2Cr2O7 and Con. H2SO4.
vi) Potassium dichromate is used in chrome tanning in leather industry.
vii) It is used as an oxidising agent.
Potassium Permanganate , KMnO4
Preparation
Potassium permanganate is prepared from mineral pyrolusite (MnO2). The preparation of KMnO4 involves the following steps:
i) Conversion of pyrolusite ore to potassium manganate: The pyrolusite (MnO2) is fused with caustic potash (KOH) or potassium carbonate in the presence of air to give a green mass due to the formation of potassium manganate.
ii) Oxidation of potassium manganate to potassium permanganate : The green mass is extracted with water resulting in green solution of potassium manganate. The solution is then treated with a current of chlorine or ozone or carbon dioxide to oxidise potassium manganate to potassium permanganate. The solution is concentrated and dark purple crystals of potassium permanganate separate out.
2 K2MnO4 + Cl2 ® 2 KCl + 2 KMnO4
2 K2MnO4 + O3 + H2O ® 2 KMnO4 + 2 KOH + O2
Alternatively, the alkaline potassium manganate solution is electrolytically oxidised. The potassium manganate solution is taken in an electrolytic cell which contains iron cathode and nickel anode. The potassium manganate solution is taken in anode compartment, while dilute alkali solution is added in the cathode compartment. When current is passed, the maganate ion is oxidised to permanganate ion at anode and hydrogen is liberated at cathode.
Properties
The important properties of potassium permanganate are :
1. It is dark violet crystalline solid having a metallic lustre.
It has m.p. 523 K.
2. Solubility : It is fairly soluble in water giving purple solution.
3. Action of heat : When heated strongly, it decomposes
to give oxygen.
4. Oxidising character : Potassium permanganate is a powerful oxidising agent in neutral, alkaline or acidic solution because it liberates nascent oxygen as :
In acidic medium :
or, MnO4- + 8 H+ + 5e-® Mn2++ 4 H2O
In alkaline or neutral medium
2 KMnO4 + H2O ® 2 KOH + 2 MnO2 + 3 [O]
or MnO4- + 2 H2O + 3 e- ® MnO2 + 4 OH-
In acidic medium , KMnO4 oxidises:
a) Ferrous salts to ferric salts :
MnO4- + 8 H+ + 5e-® Mn2++ 4 H2O
[Fe2+ ® Fe3+ + e- ] x 5
------------------------------------------------------------
MnO4- + 8 H+ + 5 Fe2+ ® Mn2++ 4 H2O + 5 Fe3+
b) Oxalates to carbon dioxide
[MnO4- + 8 H+ + 5e-® Mn2++ 4 H2O] x 2
[C2O42- ] ® 2 CO2 +2 e- ] x 5
---------------------------------------------------------------
2 MnO4- +16 H+ + 5 C2O42- ® 2Mn2++ 8 H2O+10 CO2
c) Iodides to iodine: It oxidises KI to iodine.
[MnO4- + 8 H+ + 5e-® Mn2++ 4 H2O] x 2
[2I- ® I2 +2 e- ] x 5
---------------------------------------------------------------
2 MnO4- +16 H+ + 10 I- ® 2Mn2++ 8 H2O + 5 I2
In alkaline medium, KMnO4 oxidises iodides to iodates.
2 KMnO4 + H2O ® 2 KOH + 2 MnO2 + 3 [O]
KI + 3 [O] ® KIO3
-------------------------------------------------------
2KMnO4 + KI + H2O ® 2 MnO2 + 2 KOH + KIO3
or 2MnO4- + I- + H2O ® 2 MnO2 + 2 OH- + I O3-
Uses
i) As a n oxidising agent in the laboratory and industry.
ii) As a disinfectant for well water.
iii) In qualitative and quantitative analysis.
iv) Alkaline potassium permanganate is used in organic chemistry under the name Bayer's reagent.
Halides
Transition elements react with halogens at high temperatures to form transition metal halides. The halogens react in the order :
F2 > Cl2 > Br2 > I2
Metals are usually oxidised to their highest oxidation states to form fluorides. The lower oxidation states are stabilized in iodides. Bonding in fluorides is usually ionic. But the ionic character decreases in chlorides, bromides and iodides with increasing mass of halogen. A few halides of silver and mercury are discussed below.
Silver halides
All the halides of silver(I)are known. The fluoride is soluble whereas the chloride , bromide and iodide are insoluble in water. Silver flouride may be prepared by the action of hydrofluoric acid on silver(I) oxide. The white chloride and yellow bromide are most conveniently prepared by double decomposition :
Ag+(aq) + X- ® AgX(s) ( X = Cl, Br or I )
The chloride and bromide are soluble in ammonia to give a solution containing the linear complex ion , [Ag(NH3)2]+.
AgCl + 2 NH3 ® [Ag(NH3)2]Cl
Diamminesilver(I)chloride
Silver halide darken in light owing to photochemical decomposition, a property which is primarily responsible for their use in photography.
2 AgBr ® 2 Ag + Br2
All silver halides dissolves thiosulphate and cyanide solutions give thiocyanato and dicyano complexes of silver(I)
Silver chloride is used in photography chiefly for printing paper for lantern slides, the bromide is used in large quantities for the production of photographic films and the iodide chiefly for the production of colloidal emulsion plates in photography.
Mercury halides
Mercury forms halides in the two oxidation states +1 and +2.
Mercury(I) halides
All the mercury(I) halides are known. The most common among these is white mercury (I) chloride or calomel (Hg2Cl2)
Preparation
(i) It is prepared by heating a mixture of mercury(II)chloride and mercury.
HgCl2 + Hg ® Hg2Cl2
(ii) It is prepared as a white precipitate by mixing solutions of mercury(I) salt and sodium chloride.
Hg2(NO3)2 + 2 NaCl ® Hg2Cl2 + 2 NaNO3
Properties
1. It is a white amorphous solid insoluble in cold water but soluble in hot water.
2. When heated, mercury(I)chlorides decomposes into mercury(II)chloride and mercury.
Hg2Cl2 ® HgCl2 + Hg
3. The action of aqueous ammonia on solid mercury(I) chloride gives a mixture of black finely divided mercury and white mercury aminochloride, i.e., disproportionation occurs :
Hg2 Cl2 + 2 NH3 ® Hg + Hg(NH2) 2Cl + NH4Cl
Uses
Mercurous chloride is used :
(i) As a purgative in medicine
(ii) In making standard calomel electrodes.
Mercury (II) halides
Mercury(II) chloride or Corrosive sublimate (HgCl2)
Preparation
1. It is prepared by passing chlorine gas over mercury.
Hg + Cl2 ® Hg Cl2
2. On a commercial scale, it is prepared by hating a mixture of mercuric sulphate and sodium chloride in presence of traces MnO2 which acts as an oxidising agent. Manganese dioxide prevents the formation of mercury(I)chloride
HgSO4 + 2 NaCl ® HgCl2 + Na2SO4
Properties
1. It is a white crystalline mass but form aqueous solution it crystallises into colourless needles.
2. It is a covalent compound, sparingly soluble in water, the solubility being increased by the addition of chloride ions, when soluble tetrachloromercurate(II)complex ion is formed.
HgCl2 + 2 Cl-® [HgCl4 ] 2-
3. Mercury(II) chloride reacts with aqueous ammonia to form a white precipitate of mercuryaminochloride (i.e., infusible white precipitate).
HgCl2 + 2 NH3(aq) ® Hg(NH3)2Cl + NH4Cl
4. The infusible white precipitate decomposes without melting on heating.
5. Gaseous ammonia or ammonium chloride on reaction with mercury(II) chloride produces a fusible white precipitate of diamminemercury(II)chloride.
HgCl2 + 2 NH3 ® Hg(NH3)Cl + NH4Cl
6. Mercury(II)chloride solution is reduced by many reducing agents , e.g., formaldehyde, tin(II)chloride, sulphur dioxide, etc., precipitating white mercury(I)chloride first, which with excess of reducing agent turns black owing to the formation of metallic mercury.
2HgCl2 + SnCl2 ® Hg2 Cl2 + SnCl4
white
Hg2 Cl2 + SnCl2 ® 2 Hg + SnCl4
black
Uses
1. A very dilute solution of the salt (0.1%) can be used for sterilizing surgical instruments.
2. It is used as a preservative for wood.
3. It is also used in the preparation of calomel electrodes.
Mercury(II) iodide or mercuric iodide , HgI2
Preparation
1. Mercuric(II)iodide is prepared by the action of potassium chloride solution on mercuric chloride when a yellow precipitate of mercuric iodide is formed.
2 KI + Hg Cl2 ® HgI2 + 2 KCl
3. It can also be obtained by grinding mercury with appropriate amount of iodine.
Properties
Mercury(II) iodide , although sparingly soluble in water, dissolves readily in a solution of potassium iodide due to the formation of iodo complex, e.g.,
HgI2 + 2 KI ® K2[HgI4]
Potassium tetraiodomercurate(II)
The tetraiodo complex forms light yellow crystals , K2[HgI4] 2 H2O, freely soluble in water and alcohol. The complex dissolves in potassium hydroxide solution to give Nessler’s reagent which forms a brown precipitate or colouration with ammonia owing to the formation of of iodide of Millon’s base, Hg2NI H2O having the following structure.
Uses
Mercury(II)iodide finds application for the treatment of skin infection and for the preparation of Nessler’s reagent.
Salts of Oxoacids
The higher elements of first transition series have at least one stable oxidation state each of which furnishes stable hydrated cations and forms salts of oxoacids. Metals of the second and third transition series forms few simple cations and hence few oxosalts. Also the large number of complexes which exist for the first transition series are almost entirely absent with heavier metals. Some oxosalts are discussed below
Copper sulphate , Blue vitriol CuSO4 5 H2O
Preparation
1. In the laboratory, copper sulphate is prepared by dissolving copper oxide, copper hydroxide or copper carbonate in dilute sulphuric acid.
The solution upon concentration and cooling gives needle shaped crystals of hydrated copper sulphate (CuSO4 5H2O) or blue vitriol.
2. On commnercial scale, it can also be prepared by the action of hot dilute H2SO4 on copper scrap in presence of air.
2 Cu + 2 H2SO4 + O2 ® 2 CuSO4 + 2 H2O
In this method, scrap copper is packed in tall cylindrical tower lined inside with lead. Dilute sulphuric acid is sprayed from the top of tower whereas a stream of air along with steam is introduced from the base of the tower. The dilute solution of copper sulphate obtained at the base of the tower. The dilute solution of copper sulphate obtained at the base is recirculated through the tower until it becomes sufficiently concentrated. The concentrated solution is cooled to get the crystals of copper sulphate.
Properties
(i) Copper sulphate is readily soluble in water.
(ii) The salt, CuSO4 5 H2O is insoluble in alcohol and is precipitated on adding alcohol to the aqueous solution.
(iii) When heated slowly. The pentahydrate decomposes in the following stages.
(iv) Copper(II) sulphate CuSO4 5 H2O, has the structure in which four water molecules are co-ordinated to the central copper cation at the centre of a square but the fifth water molecule is held by hydrogen bonds between a sulphate ion and co-ordinated water molecules. The fifth hydrogen bonded water molecule is deeply embeded in the crystal lattice and hence not easily removed.
(v) Copper(II)sulphate forms well-defined crystalline double salts with sulphates of strongly electropositive metals which generally correspond to the type M2SO4. CuSO4 6 H2O (eg (NH4)2SO4 CuSO4 6 H2O )and are greenish blue in colour. These double salts are isomorphous with double salts of bivalent metals, Fe, Co, Ni. The aqueous solutions of copper(II)sulphate is slowly hydrolysed forming basic copper sulphate
(vi) When treated with excess of ammonia solution, it gives a deep blue solution due to the formation of tetraamminecopper(II)sulphate complex.
CuSO4 + 4 NH3 ® [Cu(NH3)4]SO4
(vii) Copper sulphate liberates iodine from potassium iodide solution.
The liberation of iodine is quantitative and the reaction is used for the estimation of cupric salt in a solution.
Uses
(i) Anhydrous salt is used to test the presence of moisture.
(ii) As a mordant in dyeing and in calicoprinting.
(iii) In electroplating and electrorefining of metals.
(iv) Mixed with lime , it is used as germicide and fungicide under the name Bordeaux mixture.
Silver nitrate, AgNO3
It is commercially called Lunar caustic.
Preparation
It is prepared by dissolving silver in warm dilute nitric acid and then crystallising the solution.
3 Ag + 4 HNO3 ® 3 AgNO3 + 2 H2O + NO
Properties
1. It forms large colourless rhombic crystals.
2. It is not hygroscopic and very soluble in water. The solubility increases with increasing temperature. An aqueous solution is susceptible to decomposition by light.
3. When heated it decomposes in two stages.
4. When treated with halides, it gives precipitates of corresponding silver halides.
5. Silver nitrate reacts with sodium thiosulphate forming white precipitate which changes to yellow, orange and finally black.
2 AgNO3 + Na2S2O3 ® Ag2S2O3 + 2 NaNO3
white ppt
2 Ag2S2O3 + H2O ® Ag2S + H2SO4
Black ppt
6. Silver nitrate is decomposed by organic matter, such as glucose, paper, skin and cork. It has also a caustic and destructive effect on organic tissues.
Silvering of mirrors
The process of depositing a uniform and thin layer of silver on a clean glass surface is called silvering of mirror. It is used for making looking glasses, concave mirrors and reflecting surfaces. The process is based on the reduction of ammoniacal solution of silver nitrate by some reducing agent such as formaldehyde, tartrate, glucose etc.
2 AgNO3 + 2 NH4OH ® Ag2O + 2 NH4NO3 + H2O
Ag2O + HCHO ® 2 Ag + HCOOH
Silver mirror
Uses
(i) Large quantities of silver nitrate are used in the production of light sensitive plates, films and papers.
(ii) In the laboratories it is extensively used as a group reagent for the detection of halide ions.
(iii) In small doses , silver nitrate is used as a medicine in nervous diseases.
(iv) A solution of silver nitrate is used in marking linen.
(v) Silver nitrate is also used in silvering of mirror.