Chemistry

d and f Block Element - 2

Session

Session Objectives

1. Metallurgy of Zn

2. Metallurgy of Hg

3. Potassium dichromate

4. Potassium permanganate

5. Some halides of transition elements

6. Copper sulphate

7. Silver nitrate

8. Photography

Metallurgy of Zinc

Ore: Zinc blende (ZnS)

Roasting: Sulphide ore is converted to oxide.

Conversion of ZnO to Zn: Zinc oxide is mixed with coke and packed

into fire clay retorts. The furnace is heated to 1400-1500° C by

producer gas and air.

distillationZnO C Zn CO

Vapours of Zn are collected after distillation.

Concentration of ore: Zinc blend is concentrated first by froth floatation process.

2 22ZnS 3O 2ZnO 2SO

Extraction of MercuryConcentration of ore by froth-floatation process.Roasting of concentrated ore

2 2impure metal

HgS O Hg SO

The flue dust from the furnace is mixed with quicklime and distilled in iron retorts

4impure metal

4HgS 4CaO 4Hg CaSO 3CaS

Impure mercury thus obtained has impurities of Zn, Cd, Pb and Cu, it is purified by treatment with dilute HNO3 which contains a little mercury(I)nitrates.

2 22

pure metalZn Hg 2Hg Zn

Further purification of mercury is carried out by distillation under reduced pressure.

Illustrative Example

Why Zn, Cd and Hg are volatile metals? Explain.

Solution

Zn, Cd, Hg are volatile due to weak metallic bond which is due to the bigger atomic size.

Potassium dichromate

Hexavalent chromium compounds are generally more toxic than trivalent chromium compounds. May be fatal if absorbed through the skin, if swallowed or inhaled. Contains chromium (VI), a known cancer hazard,allergen, corrosive, skin eye and respiratory irritant,may act as a sensitizer.

Potassium dichromate (K2Cr2O7)

Potassium dichromate is prepared from chromates which in turn are prepared from chrome iron, FeCr2O4

2 4 2 3 2 2 4 2 3 24FeCr O 8Na CO 7O 8Na CrO 2Fe O 8CO

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na2Cr2O7.2H2O can be crystallized.

2 4 2 2 7 22Na CrO 2H Na Cr O 2Na H O

Sodium dichromate is more soluble than potassium dichromate. Potassium dichromate is prepared by treating the solution of sodium dichromate with potassium chloride.

2 2 7 2 2 7Na Cr O KCl K Cr O 2NaCl

Orange crystals of potassium dichromate crystallize out.

Structure of chromate and dichromate ions

Cr

O

OO

O

2–

Chromate ion

O

Cr Cr

O

O

OO

O

O180 pm

131°

161 pm

2–

Dichrom ate ion

Oxidising property of potassium dichromate

In acidic solution, its oxidising action can be represented as follows;

2 3 o2 7 2Cr O 14H 6e 2Cr 7H O E 1.33 V

It liberates I2 from KI.K2Cr2O7 + 7H2SO4 + 6KI 4K2SO4 + Cr2(SO4)3 + 3I2 + 7H2O

It oxidizes ferrous salts to ferric salts.K2Cr2O7 + 7H2SO4 + 6FeSO4 K2SO4 + Cr2(SO4)3 + 3Fe2 (SO4)3 + 7H2O

It oxidizes H2S to sulphur.K2Cr2O7 + 4H2SO4 + 3H2S K2SO4 + Cr2(SO4)3 + 3S + 7H2O

Oxidising property of potassium dichromate

It oxidizes SO2 to sulphuric acid.K2Cr2O7 + H2SO4 + 3SO2 K2SO4 + Cr2(SO4)3 + H2O

It oxidizes sulphites to sulphates.K2Cr2O7 + 4H2SO4 + 3Na2SO3 K2SO4 + Cr2(SO4)3 + 4H2O + 3Na2SO4

Action of heat:4K2Cr2O7 4K2CrO4 + 2Cr2O3 + 3O2

Chromyl chloride test

When potassium dichromate is treated with a strong (concentrated) sulphuric acid and a chloride, reddish  brown vapours of chromyl chloride are formed.

K2Cr2O7 + 4KCl + 6H2SO4 2CrO2Cl2 + 6KHSO4 + 3H2O

This test is used in detection of chloride ions in qualitative analysis.

Illustrative Example

Write balanced equation for the reaction between K2Cr2O7 and acidified solution of (a) ferrous sulphate (b) KI solution.

(a). 2 2 3 32 7 2Cr O 14H 6Fe 2Cr 6Fe 7H O

2 32 7 2 2Cr O 14H 6I 2Cr I 7H O (b).

Solution

Potassium permanganate (KMnO4)

Prepared by fusion of MnO2, with alkali metal hydroxide and an oxidising agent like KNO3.

This produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permangnate.

2MnO2 + 4KOH + O2 2K2MnO4 + 2H2O3MnO4

2- + 4H+ 2MnO4- + MnO2 + 2H2O

Commercially it is prepared by the alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of mangnate(VI).

3

fuse with KOH, oxidisewith air or KNO 2

2 4MnO MnO

electrolytic oxidation inalkaline solution2

4 4MnO MnO

Structure of permanganate ion

The four oxygen atoms are arranged tetrahedrically around manganese in MnO4

–

O

O

O

O –Mn

Permanganate ion

Properties of potassium permanganate

Action of heat: Potassium permanganate decomposes to oxygen, potassium manganate and manganese dioxide when heated to 746 K.2KMnO4 K2MnO4 + MnO2 + O2

Action of concentrated sulphuric acid: When treated with cold concentrated sulphuric acid, potassium permanganate is converted to Mn2O7 (green oil) which decomposes on warming to MnO2 (it is highly explosive).2KMnO4 + 2H2SO4 Mn2O7 + 2KHSO4 + H2O2Mn2O7 4MnO2 + 3O2

Oxidising nature of potassium permanganate

In acidic solution 24 2MnO 8H 5e Mn 4H O

Oxidation of H2S to S2KMnO4 + 3H2SO4 + 5H2S K2SO4 + 2MnSO4 + 8H2O + 5S

Oxidation of ferrous sulphate to ferric sulphate2KMnO4 + 8H2SO4 + 10FeSO4 K2SO4 + 2MnSO4 + 5Fe2 (SO4)3 + 8H2O

Oxidation of potassium iodide to iodine:2KMnO4 + 3H2SO4 + 10KI K2SO4 + 2MnSO4 + 8H2O + 5I2

To provide acidic medium, sulphuric acid is used instead of HCl as it is oxidised to chlorine.

Oxidizes oxalate to carbon dioxide5(COO-)2 10CO2 + 10e-

Strong oxidising agent, both in acidic and alkaline medium.

Oxidising nature of potassium permanganate

In alkaline solution

4 2 2MnO 2H O 3e MnO 4OH

Potassium iodide is oxidized to potassium iodate2KMnO4 + H2O + KI 2MnO2 + 2KOH + KIO3

Oxidising nature of potassium permanganate

In neutral medium:

In neutral medium, potassium permanganate is weakly oxidizing and the reaction involved is

MnO4– + 2H2O + 3e– MnO2 + 4OH–

2KMnO4 + H2O 2KOH + 2MnO2 + 3O

The alkali (KOH) produced renders the solution basic as the reaction proceeds and the reaction given above is then essentially same as that for alkaline medium.

Uses of potassium permangnate

Potassium permanganate is used in volumetric analysis for estimation of ferrous salts, oxalates, iodide and hydrogen peroxide.

It is used as a strong oxidizing agent in the laboratory.

It is also used as a disinfectant and germicide.

High-grade potassium permanganate is used in rural areas to remove iron and hydrogen sulfide(rotten egg smell) from well water.

Illustrative Example

Describe with chemical reactions what happens when(i) Dilute NaOH solution is added to K2Cr2O7 solution in water.(ii) Pyrolusite ore is fused with KOH in presence of air.(iii) Chromite ore is fused with molten NaOH in presence of air.(iv) Acidified KMnO4 solution is added to the ferrous sulphate solution.

2 22 7 4 2Cr O 2OH 2CrO H O

(orange) (Yellow)

(i)

2 2 2 4 2MnO 4KOH O 2K MnO 2H O (ii)

2 4 2 2 3 2 4 24FeCr O 16NaOH 7O 2Fe O 8Na CrO 8H O (iii)

2 2 34 2MnO 5Fe 8H Mn 5Fe 4H O (iv)

Solution

Halides

Transition metal reacts with halogens at elevated temperature.

The reactivity of halogens decreases in the order:

F2>Cl2>Br2>I2.

Flourides are stable in higher oxidation state whereas iodide are stable in lower oxidation state.

Bonding in flourides is usually ionic.

Silver halide

Flouride is prepared by the action of hydrofloric acid on siver(I) oxide.

The white chloride and yellow bromide and iodide can be easily prepared.

Ag+(aq) + X-(aq) AgX(s)

All halides are known for silver.Flouride is soluble but chloride, bromide and iodide are insoluble in water.

Silver halides darken in the light owing to photochemical decomposition,So, these are useful in photography.

Preparation

Properties

Chlorides and bromides are soluble in NH3 to give a solution containing the linear complex ion, [Ag(NH3)2]+.AgCl + 2NH3 [Ag(NH3) 2Cl]

All dissolves in thiosulphate and cyanide solutions.AgCl + 2KCN K[Ag(CN) 2] + KClAgCl + 2Na2S2O3 [Ag(S2O3)2 ] + 2NaCl

S A g S

S

O SO

O

O

O O

3 -

D ith iosu lpha toa rgen ta te(I) ion

CN A g NC[ ]D icyanoargen ta te (I) ion

Illustrative Example

When a bright silver object is placed in a solution of gold chloride, it acquires a golden tinge but nothing happen when it is placed in a solution of copper chloride. State reason for this behaviour of silver.

Gold is yellow in colour, therefore gold gets deposited on bright silver object. Ag is more reactive than gold.

3Ag + AuCl3 3AgCl + Au

Ag is less reactive than copper, therefore no reaction occurs with CuCl2.

Solution

Mercurous chloride (Hg2Cl2)

Preparation:

Heating the mercury(II) chloride and mercury.Hg+HgCl2 Hg2Cl2

By the reduction of mercury(II) chloride by a reducing agent, e.g., tin(II) chloride.SnCl2 + 2HgCl2 Hg2Cl2 + SnCl4

Properties

On heating it decomposes into mercury(II) chloride and mercury.

Used in calomel electrodes.

Chemical properties of Hg2Cl2

Hg2Cl2 dissolves in aqua regia, forming mercuric chloride and also to some extent in hot concentrated HCl.

Hg2Cl2 + 2HCl Hg+H2 [HgCl4]

It blackens with ammonia.

Hg2Cl2 + 2NH3 Hg(NH2)Cl + Hg +NH4Cl

Mercurous iodide(Hg2I2)

KI+Hg2(NO3) 2 Hg2I2 + KNO3

Nessler’s reagent: It gives a brown colour with small traces of ammonia, and a brown precipitate with larger amounts — a sensitive reagent for detection and determination of traces of ammonia.

Mercurous iodide dissolves in potassium iodide.

KI + Hg2I2 K2 [HgI4]

Nesseler’s reagent

Mercuric chloride (HgCl2)

It is a corrosive sublimate and prepared from mercuric sulphate with sodium chloride.HgSO4 + 2NaCl Na2SO4 + HgCl2

Mercuric chloride reacts with ammonia which yields chloride of Millon’s base.HgCl2 + 2NH3 Hg(NH2)Cl + NH4ClHg(NH2)Cl + H2O H2N-Hg-O-Hg-Cl + NH4Cl

Million’s base

Mercury(II) chloride solution is reduced by many reducing agents, e.g., formaldehyde, tin(II) chloride, sulfur 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 Hg2Cl2 + SnCl4Hg2Cl2 + SnCl2 2Hg + SnCl4

Illustrative Example

Write the complete chemical equations for each of the following.(a) An alkaline solution of KMnO4 reacts with an iodide(b) An excess of SnCl2 solution is added to a solution of mercury (II) chloride.

Solution:

4 2 3 2Potassium iodide Potassium iodate

2KMnO KI H O KIO 2MnO 2KOH

2 2 2 2 4Mercury(II) chloride Mercurouschloride

(white)

2HgCl SnCl Hg Cl SnCl

2 2 2 4(Black)

Hg Cl SnCl 2Hg SnCl

(a) Alkaline solution of KMnO4 oxidises iodide to iodate

(b) SnCl2 first reduces mercury (II) chloride into mercurous chloride and then into mercury.

Copper Sulphate

Produced by spraying dilute sulphuric acid on to scrap copper packed in a lead lined tower in which a current of air is blown.2Cu + 2H2SO4 + O2 2CuSO4 + 2H2O

Some important chemical reactions of copper sulphate are:2CuSO4 + 4KI 2K2SO4 + 2CuI + I2CuSO4 + 4KCN 2CuCN + (CN) 2 + 2K2SO4

With excess of KCN,CuCN + 3KCN K3[Cu(CN) 4]

Most common is blue vitriol, CuSO4.5H2O

Used for electroplating, and as germicide and fungicide [bordeaux mixture is CuSO4 and Ca(OH)2]

CuSO4.5H2O is insoluble in alcohol and is precipitated on adding alcohol to the aqueous solution.

305 K 373 K4 2 4 2 4 2

573 K 673 K4 3

CuSO .5H O CuSO .3H O CuSO .H O

CuSO CuO SO

Silver nitrate, AgNO3

Prepared by dissolving metallic silver in dilute nitric acid.It is not hygroscopic and is very soluble in water.Aqueous solutions are susceptible to decomposition by light.When heated it decomposes in two stages:

723 K 980 K3 2 2 22AgNO 2AgNO O Ag NO

Silver nitrate is decomposed by organic matter, such as glucose, paper, skin and cork.Caustic and destructive effect on organic tissues.

Used in the production of light sensitive plates, films and papers.In laboratory, it is used as a group reagent for the detection of halide ions.In small doses, it is used as a medicine in nervous diseases.It is also used in silvering of mirror.

AgNO3 dissolves in ammonia, sodium thiosulphate and as well as in conc. HCl. It may easily be reduced to pure metallic silver by the action of Na2CO3.

Photography

A photographic film contains suspension of silver bromide in gelatin. When it is exposed to sun light incipient reduction of silver halide takes place in which light falls. The latent image is formed is developed by immersing in potassium ferrous oxalate.

AgBr + K2Fe(C2O4)2 KFe(C2O4)2 + Ag+KBr

The developed image which contains silver and silver bromide is fixed by putting into the hypo solution and thereby negative is produced.

AgBr + 2Na2S2O3 Na3[Ag(S2O3) 2] + NaBr

A positive print is then finally obtained by laying the negative on a printing paper with an emulsion of silver chloride and gelatin.

Thank you