Principles Related to Practical Chemistry (Part-3) | Chemistry Notes for IITJEE/NEET

PRINCIPLES RELATED TO PRACTICAL CHEMISTRY (PART-3)

DETECTION OF BASIC RADICALS - GROUP III

Members : Fe+++, Al+++, Cr+++

Group reagent :  NH4OH in presence of NH4Cl (excess)

Precautions

• Before oxidation with HNO3, H2S must be completely removed otherwise it will be partly oxidised to H2SO4 which will precipitate the sulphates of Ba, Sr, etc.
• The soln is boiled with conc HNO3 to oxidise Fe++ ions to Fe+++  to ensure complete precipitation of iron as Fe(OH)3. Fe(OH)2 is not completely precipitated by NH4OH in presence of NH4Cl and will pass in the filtrate of IV gp

2FeCl2+  8HNO3  2Fe(NO3)3 + 4HCl + 2NO2 + 2H2O    

• Mn is usually precipitated in gp IV but sometimes due to insufficient quantity of NH4Cl and presence of air bubbles in soln, it is precipitated with the radicals of Group III as manganic hydroxide MnO.OH
• The precipitation should be carried out in hot soln otherwise Cr may be incompletely precipitated due to the formation of violet soluble complex chromamine in presence of excess of NH4Cl. The complex may be decomposed by boiling.
• Large excess of NH4OH should be avoided because Al(OH)3 dissolves giving complex compound.
• Addition of NH4Cl is absolutely essential otherwise the basic radicals of subsequent gps ppt out as hydroxides in this gp

NH4Cl suppresses the ionisation of NH4OH and prevents the precipitation of higher group hydroxides in Group III.

Function of NH4Cl

• The hydroxides of Fe, Al and Cr have a strong tendency to pass into colloidal state. These are coagulated by NH4Cl which is a strong electrolyte
• The hydroxides of Al, Cr, and Zn  are amphoteric. They supply negative ions in soln which form insoluble salts with other metallic ions (eg CrO2, Co(AlO2)3, Fe (ZnO2)3 etc. This entrapment of metallic ions by these hydroxides is known as “carrying down” . It is prevented by NH4Cl.
• The hydroxides of III group absorb other metal ions in the form of hydroxides eg [XFe(OH)3.YNi(OH)2] such hydroxide formation is prevented by NH4Cl separation.

Separation

By treating with Na2O2 or NaOH and Br2 water the hydroxides of Cr and Al dissolve as sodium chromate and aluminate respectively. While Fe(OH)3 and manganic hydroxide remain unaffected. MnO.OH is oxidised by PbO2 and HNO3 to permanganic acid HMnO4 which gives violet or pink colour. Fe(OH)3 dissolves in HCl forming FeCl3.

IRON (Fe+++)

Fe++ salts are easily oxidised when heated with HNO3

6FeSO4 + 3H2SO4 + 2HNO3 3Fe2(SO4)3 + 2NO + 4H2O

• NH4OH
  FeCl3 + 3NH4OHFe(OH)3 ↓ + 3NH4Cl

       Reddish brown

Fe(OH)3 + 3HCl   FeCl3 + 3H2O

• K4[Fe(CN)6]

4FeCl3 + 3K4[Fe(CN)6] Fe4[Fe(CN)6]3 ↓ + 12KCl

              Prussian Blue

FeCl3 + K4[Fe(CN)6] KFe[Fe(CN)6]   + 3KCl

  K. ferric ferrocyanide

• NH4CNS or KCNS

FeCl3 + KCNS  Fe(CNS)Cl2 ↓   + KCl

         Blood red colouration

others believe that complex ferric sulphocyanide is formed

2FeCl3 + 6NH4CNS  Fe[Fe(CNS)6] + 6NH4Cl

• Sodium acetate solution

FeCl3 + 3CH3COONa 3NaCl +  Fe(CH3COO)3 ↓

   reddish brown colouration

ALUMINIUM (Al+++)

• NH4OH
  Al2(SO4)3 + 6NH4OH 2Al(OH)3 ↓+ (NH4)2SO4

  white gelatinous ppt

Al(OH)3 + 3HCl  AlCl3+ 3H2O

Soluble

Al(OH)3 + NaOH  NaAlO2 + 2H2O

sod. aluminate (soluble)

• NH4Cl

NaAlO2 + NH4Cl + H2O  Al(OH)3 + NH3 + NaCl

NaAlO2 + HCl + H2O  Al(OH)3 + H2O + NaCl

Al(OH)3 + 3HCl   AlCl3 + 3H2O

CHROMIUM (Cr+++)

• NH4OH

CrCl3 + 3NH4OH  Cr(OH)3      + 3NH4Cl

Bluish green gelatinous ppt

Cr(OH)3 + 6NH4OH [Cr(NH3)6](OH)2 + 6H2O

 Violet pink soln

 (chrome - ammine)

• NaOH and Br2 water

2Na2O2 + H2O  4NaOH + 2O

Br2 + NaOH  NaBr + HBr + O

2Cr(OH)3 + 4NaOH + 3O  2Na2CrO4 + 5H2O

Sod. chromate (yellow)

• Lead acetate

Na2CrO4 + (CH3COO)2Pb PbCrO4 + 2CH3COONa

Yellow

PbCrO4 + 4NaOH Na2PbO2 + Na2CrO4 + 2H2O

             Soluble

DETECTION OF BASIC RADICALS - GROUP IV

The ppt consists NiS, CoS, ZnS and MnS

ZINC (Zn++) AND MANGANESE (Mn++)

• Separation : ppt + dil HCl ZnS and MnS dissolve as ZnCl2 and MnCl2 and NiS and CoS remain insoluble

ZnS + 2HCl  ZnCl2 + H2S

MnS + 2HCl MnCl2 + H2S

The soln is heated with NaOH

ZnCl2 + 2NaOH  Zn(OH)2 + 2NaCl ppt

MnCl2 + 2NaOH  Mn(OH)2 + 2NaCl ppt

• Excess NaOH :

Zn(OH)2 + 2NaOH   Na2ZnO2 +  2H2O

Sod. Zincate soluble

Na2ZnO2 + H2S   ZnS  + 2NaOH

White ppt

Na2ZnO2 + 2CH3COOH (CH3COO)2Zn + 2NaOH

2(CH3COO)2 Zn + K4[Fe(CN)6] Zn2[Fe(CN)6] + 4CH3COOK

            Bluish white

• Residue + Conc HNO3 +PbO2 or Pb3O4 Pink Coloured permanganic acid

Mn(OH)2 + 2HNO3  Mn(NO3)2 + 2H2O

Pb3O4 + 4HNO3 2Pb(NO3)2 + PbO2 + 2H2O

5PbO2 + 2Mn(NO3)2 + 6HNO32KMnO4 + 5Pb(NO3)2
permanganic acid (pink)

• With oxidising fusion mixture green mass of Na2MnO4

2KNO3 + 2NaOH + Mn(OH)2 2KNO2+  Na2MnO4 + H2O

          sod. manganate (green)

NICKEL (Ni++) AND COBALT (Co++)

NiS and CoS soluble in aqua regia

3NiS + 3HNO3 + 6HCl 3NiCl2 + 2NO + 3S + 4H2O

3CoS + 3HNO3 + 6HCl  3CoCl2 + 2NO + 3S + 4H2O

• Method I

Nickel salt with DMG in ammoniacal medium gives red ppt. of NDMG

  + NiCl2 + 2 NH4OH 2NH4Cl + 2H2O +

Cobaltous (Co++) salt with amm. thio cyanate gives blue colour due to

CoCl2+  4NH4CNS (NH4)2Co(CNS)4  +  2NH4Cl
Amm. Cobalto thiocyanate blue

• Method II
• Cobaltous salts form Sod. cobalti carbonate with NaHCO3. Br2 oxidises this complex to Sod. Cobalt carbonate  which is apple green and stable towards heat.

CoCl2 + 2NaHCO3   Co(HCO3)2 + 2 NaCl

Co(HCO3)2  CoCO3 + CO2 + H2O

CoCO3 + 2NaHCO3   Na4[Co(CO3)3] +   2CO2   + 2H2O

Sod. cobalto carbonate

Br2 + H2O 2HBr + O

2Na4[Co(CO3)3] + 2NaHCO3 + O  2Na3[Co(CO3)3] + 2Na2CO3 + 2H2O
(apple green Sod. cobalti carbonate)

• Nickel : Does not form any complex with NaHCO3 but forms black Ni2O3 in alkaline soln of  Br2

NiCl2 + 2NaHCO3  Ni(HCO3)2 + 2NaCl

Ni(HCO3)2 NiCO3 + H2O + CO2

2NaHCO3 + Br2 2NaBr + NaOBr + 2CO2

2NiCO3 + NaOBr + 3H2O  2Ni(OH)3 + NaBr + 2CO2

2Ni(OH)3  Ni2O3 + 3H2O

• Method III
  Cobalt salts with a-nitro b - naphthol form brown complex

cobalti nitroso b-naphthol Reddish brown

• Method IV

KCN form the following colourless soluble complexes with  Cobalt and Nickel salts.

CoCl2 + 6KCN  K4[Co(CN)6] + 2KCl

K-cobalti cyanide colourless

NiCl2 + 4KCN  K2[Ni(CN)4] + 2KCl

Pot. nickelo cyanide colourless

If the soln is treated with sod. hypobromite (Br2 in presence of NaOH) the Ni cyanide is decomposed and black ppt of Ni2O3 is obtained

Br2 + 2NaOH  NaBr + NaOBr + H2O

NaOBr NaBr + O

2K2[Ni(CN)4] + 4NaOH + ONi2O3 + 4NaCN +4KCN + 2H2O

  Black

And K. cobalto cyanide is oxidised to K.cobalti cyanide which is still colourless

4K4[Co(CN)6] + 2H2O + 2O  4K3[Co(CN)6] +4KOH

K-cobalti cyanide colourless

• Method V
  When KNO2 is added to a cobaltous salt soln in presence of acetic acid a bright yellow ppt of K-cobaltinitrite  is obtained.

CoCl2 + 2KNO2  Co(NO2)2 + KCl

KNO2 + CH3COOH  CH3COOK + HNO2

Co(NO2)2 + 3 KNO2 + 2HNO2 K3[Co(NO2)6] + NO + H2O

      K- cobaltinitrite yellow ppt

The corresponding sod. salt is soluble in H2O and hence reagent for K+ ions. Nickel does not form any ppt in presence of CH3COOH acid

SOME PRECAUTIONS FOR GROUP IV ANALYSIS

• H2S must be passed in hot  soln otherwise ZnS and MnS will form colloidal soln.
• H2S should not be passed for a long time otherwise NiS will turn to colloidal solution
• The NH4Cl checks the formation of colloidal soln.
• KCNS cannot be used in place of NH4CNS in test of Co(see method I above)
• After dissolving sulphides in dil HCl  the H2S should be completely removed by boiling the solution before adding NaOH soln otherwise ZnS will be ppted and may be confused for Mn(OH)2
• When both Co and Ni are present then the soln is first treated with little NH4OH and green Co(OH)2 is removed by filtration. The test for Ni with DMG should be performed in the filtrate because cobalt interferes in the Ni test reacting with DMG to form brown coloured complex ion.

DETECTION OF BASIC RADICALS - GROUP V

Member :  Ba++, Sr++ Ca++ ppted as carbonates

Group reagent : (NH4)2CO3 in presence of NH4Cl and NH4OH

• High concentration of NH4Cl  prevents complete precipitation of the carbonates of this group.
• Before adding (NH4)2CO3 the volume of the solution must be reduced to one-third by evaporation because the solution becomes very dilute upto Vth group.
• Barium chromate  can not be precipitated in presence of an appreciable hydrogen ion concentration, therefore acetic acid  is used and that too in minimum possible amount.
• It is better to use saturated solution of (NH4)2SO4 to detect Sr++ radical. After addition of the reagent, wait for few minutes to see if a precipitate of SrSO4 is obtained.
• The solubility product of SrSO4 is sufficiently large which may result in its incomplete precipitation. Sr++ ions may, therefore, co-precipitate as SrC2O4 along with CaC2O4
• From concentrated solutions calcium may get co-precipitated as calcium sulphate with strontium sulphate.
• Sometimes calcium does not get precipitated in its group due to the formation of Ca(HCO3)2 on addition of (NH4)2CO3 to the concentrated filtrate of Vth group as Ca(HCO3)2 is soluble and so it passes into the filtrate of Vth group. Hence filtrate of Vth group must be tested for calcium before proceeding to VIth group of the analysis.
• BaCrO4 is not sufficiently volatile to give flame test therefore it should be converted to chloride.
• The insoluble carbonate of Ba, Sr and Ca are precipitated on the addition of (NH4)2CO3 to the filtrate of group IV.
• Barium chromate is insoluble in acetic acid while the chromates of Sr and Ca are soluble in it.
• Ammonium sulphate brings about the precipitation of SrSO4 only while calcium is not precipitated by this reagent due to the formation of complex salt
  (NH4)2 [Ca(SO4)2].
• Calcium oxalate being insoluble in acetic acid, is precipitated by the addition of ammonium oxalate.

BARIUM (Ba++)

• BaCl2 + (NH4)2CO3 BaCO3 ¯ + 2NH4Cl white ppt

BaCO3 + 2CH3COOH(CH3COO)2Ba + H2O+CO2

• Pot. chromate :

(CH3COO)2Ba + K2CrO4BaCrO4 ¯ + 2CH3COOK yellow

Soluble in mineral acids but insoluble in CH3COOH.

Chromates of Sr and Ca are soluble in acetic acid

(CH3COO)2Ba + K2CrO4 BaCrO4 + 2CH3COOK

• H2SO4 : BaCl2 + H2SO4  BaSO4 ¯ + 2HCl
• Amm. oxalate : BaCl2 + (NH4)2C2O4BaC2O4 + 2 NH4Cl
  White ppt readily soluble in CH3COOH

STRONTIUM (Sr++)

• (NH4)2CO3 : SrCl2 + (NH4)2CO3SrCO3 + NH4Cl white ppt

SrCO3 + 2CH3COOH (CH3COO)2Sr  + H2O + CO2  soluble

On adding K2CrO4 for the detection of Ba SrCrO4 is not ppted in presence of acetic acid.

Sr(CH3COO)2 + K2CrO4  SrCrO4 + 2CH3COOK soluble

• (NH4)2SO4 : SrCrO4+ (NH4)2SO4 SrSO4 + (NH4)2CrO4
  white ppt, Ca remain in the solution as soluble complex.
• Amm oxalate Soln : SrCrO4+ (NH4)2C2O4 SrC2O4 + (NH4)2CrO4

white ppt insoluble in acetic acid

CALCIUM (Ca++)

• (NH4)2CO3 : CaCl2+ (NH4)2CO3  CaCO3 + 2NH4Cl  white ppt

CaCO3+ 2CH3COOH (CH3COO)2Ca + CO2 + H2O  soluble.

On adding K2CrO4, Calcium chromate is not ppted but remains in the soln

Ca(CH3COO)2 + K2CrO4 CaCrO4 + 2CH3COOK  Soluble

On adding (NH4)2SO4 to CaCrO4;  CaSO4  is not ppted but soluble complex is formed..

CaCrO4 + 2(NH4)2SO4(NH4)2[Ca(SO4)2] +  NH4CrO4 Soluble complex

• Amm oxalate : (NH4)2[Ca(SO4)2]  + (NH4)2C2O4 CaC2O4 + (NH4)2SO4

White ppt insoluble in acetic acid  but soluble in mineral acids.

• K4[Fe(CN)6]Soln : CaCl2 + K4 [Fe(CN)6] Ca.K2[Fe(CN)6] + 2KCl

white ppt

DETECTION OF BASIC RADICALS - GROUP VI

MAGNESIUM (Mg++)

• Sod. hydrogen phosphate soln :  

Mg(NO3)2 + Na2HPO4+ NH4OH Mg(NH4)PO4+ 2NaNO3 + H2O

White ppt.

• NH4OH :

MgCl2 + 2NH4OH Mg(OH)2 + 2NH4Cl

white ppt. soluble in excess of NH4Cl

AMMONIUM (NH4+)

• NaOH :  
  NH4Cl + NaOH  NH3­ + H2O + NaCl

NH3 + HCl  NH4Cl ­ White fumes

• Nessler’s reagent :
  2KI +HgCl2  HgI2 + 2KCl

HgI2 + 2KI K2[HgI4]

2K2[HgI4] + NH4OH + 3NaOH + 4KI + 3NaI + 3H2O

Iodide of millon’s base (Brown ppt)

• Mercurous nitrate : Hg2(NO3)2 + 2NH3 + NH4NO2
• CuSO4 : CuSO4 + 4NH3 [Cu(NH3)4]SO4

deep blue

SODIUM (Na+)

• Pot. dihydrogen antimonate :

KH2SbO4 +NaCl  NaH2SbO4 +KCl

white ppt

• Uranyl Mg. acetate solution :

NaCl + 3UO2(CH3COO)2 + Mg(CH3COO)2 + CH3COOH
HCl + NaMg(UO2)3 (CH3COO)9 yellow ppt

• Uranyl zinc acetate reagent :

Yellow crystalline Sod. Zinc acetate NaZn(UO2)3 (CH3COO)9 9H2O

POTASSIUM (K+)

• Chloroplatinic acid : Golden yellow crystalline ppt. in neutral or acid medium.
• Sod. Cobaltinitrite :

Na3[Co(NO2)6] + 3KCl K3[Co(NO2)6] + 3NaCl

      K.cobaltinitrite yellow ppt. (fischer’s salt)

• Sod. hydrogen tartrate : In conc. solution the reagent in the presence of alcohol gives a white ppt on scratching the sides with a glass rod

2KCl+H2PtCl6 K2PtCl6+ 2HCl

ACTION OF HEAT ON SOME INORGANIC COMPOUNDS

CARBONATES

Except  all alkali metal carbonates are stable.

  +

+ H

smelling salt

2Ag2CO3 4Ag + 2CO2+ O2

2Hg2CO3  4 Hg + 2 +

Rest – give metal oxide and

NITRATES

Except  all alkali metal nitrates decompose to nitrites and .

Na, K, Rb, Cs +

4 2 + 4 +

2Ag 2 Ag + 2 +

Hg2 2 Hg + 2 +

NH4NO3 2+ (laughing gas)

Rest  metal oxide +  +

SULPHATES

Alkali metal sulphates are stable to heat. On heating they lose water of crystallisation.

.10 + 10

Glauber’s salt salt cake

 MgSO4. MgSO4                       

CaO +

                                                                          Anhydride

.CuO ++

.7 +7  +  +

Green vitriol             white                        Red brown      

+ 3

3Hg  + Hg + 2 SO2 + 2

.H +   +  + SO2 +

2 Ag +  +

. 7 .    ZnO +

BICARBONATES

On heating give  +

2   + +

Ca   + +

CHLORIDES

+ HCl  (It sublimes)

Ammonium chloride

.2  + 2

2 +

Zn.2 Zn(OH)Cl + HCl +

2Zn(OH)Cl Zn2O +

Mg.6 Mg(OH)Cl   + HCl + 5

Mg(OH)Cl MgO + HCl

Mg.6  Mg + 6

2 2 +

 Hg  +  Hg

Calomel            Corrosive sublimate

IODIDES

2Cu   +

PHOSPHATES

     Na    +

Sod. pr. phosphate    sod. meta phosphate

2Mg  +   2 +

                         Mg. Pyrophosphate

Na(NH4)HPO4.4H2O  Na(NH4)HPO4 + 4H2O Na +  +

Microcosmic salt                                                         Sod. meta phosphate (Glassy Bead)

[CuO + Na CuNa PO4 ]

                      Blue bead

2               +   2H +

Orthophosphorus acid Pyrophosphoric acid         Metaphosphoric acid

2   +

Hypophosphorous acid

4 3 +

Orthophosphorus acid

OXIDES

2PbO +

 6PbO +

Minimum Red lead, litharge

2HgO  2 Hg +

 2 BaO +

ZnO (White)        ZnO (Yellow)

CHROMATE / DICHROMATE
+  + 4

4  4+ 2+ 3

   Green

PERCHLORATE

4KCl +  2 + 2KCl

KCl + 2

BORATE

. 10  + 10 2Na +

Borax Swelled Borax        Glassy Residue

           2 +

Orthoboric acid          Metaboric acid Tetraboric acid         Boric anhydride (glassy residue)

CYANATE

MANGANATE

2      + Mn +

Pot. permanganate     Pot. Manganate

NITRITE

COMMON ALUM

 +  + 4

NITRIC ACID

4 4

Chemistry Study Notes

• Some Basic Concepts of Chemistry
• Structure of Atom
• Classification of Elements and Periodicity in Properties
• Chemical Bonding and Molecular Structure
• States of Matter
• Thermodynamics
• Chemical Equilibrium
• Ionic Equilibrium
• Redox Reactions
• Hydrogen
• s-block Elements – Alkali Metals
• s-block Elements – Alkaline Earth Metals
• p-Block Elements – Boron Family
• p-Block Elements – Carbon Family
• Purification, Qualitative and Quantitative Analysis of Organic Compounds
• Classification and Nomenclature of Organic Compounds
• Hybridisation & Shapes of Organic Molecules
• General Organic (Basic Concepts)
• Isomerism
• Hydrocarbons
• Environmental Chemistry
• Solid State
• Solutions
• Electrochemistry
• Chemical Kinetics
• Surface Chemistry
• General Principles & Processes of Isolation of Elements
• p-Block Elements - Nitrogen Family
• p-Block Elements - Oxygen Family
• p-Block Elements - Halogens
• p-Block Elements – Noble Gases
• d & f-Block Elements
• Coordination Compounds
• Haloalkanes and Haloarenes
• Alcohols, Phenols and Ethers
• Aldehydes & Ketones
• Carboxylic Acids and their Derivatives
• Amines
• Biomolecules
• Polymers
• Chemistry in Everyday Life
• Volumetric Analysis
• Principles related to Practical Chemistry (Part-1)
• Principles related to Practical Chemistry (Part-2)
• Principles related to Practical Chemistry (Part-3)

Study Notes for JEE Main/JEE Advanced

Study Notes for NEET/AIIMS