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Revision Notes on Principles Related to Practical Chemistry

 

  • Qualitative Analysis of Organic Compounds: 

This is used to detect nitrogen, halogen and   sulphur present in organic compound. 

a) Sodium Extract: Aqueous solution containing soluble sodium salt of the elements i.e NaCl, Na2S and NaCNS formed by fusion of compound with sodium metal.

b) Formation of Sodium Extract: It is a two step process 

Step 1: Organic compounds are fused with dry sodium in a fusion-tube

Step 2: Fused mass after extraction with water is boiled and filtered.

c) Use of Sodium Extract: Sodium extract (S.E.) is used to detect elements (other than C and H) and the tests are given in the table.

Element

Sodium Extract (S.E.)

Confirmed Test

  Nitrogen

 

  Na + C + N +Δ→ NaCl

 S.E.+ FeSO4 +NaOH, boil and cool
                         +
FeCl3 + conc. HCl → Blue/ green colour

Reactions Involved:

   Sulphur

 

2Na + S → Na2S 

(i) S.E. + sodium nitroprusside → Violet Colour

(ii)  S.E + CH3CO2H + (CH3CO2)2Pb → black ppt.

Reactions Involved:

Halogen

Na +X + Δ→ NaX

(X = Cl, Br, I)

S.E. + HNO3+AgNO3

(i) White ppt soluble in aq NH3 confirms Cl.

(ii) Pale yellow ppt partially soluble in aq. NH3 confirms Br.

(iii) Yellow ppt insoluble in aq. NH3 confirms I.

Reactions Involved:

Nitrogen and sulphur together

 

Na+ C + N + S+ Δ→  NaCNS

with excess of Na the thiocyanate formed decomposes into cyanide and sulphide.

NaCNS  + 2Na → NaCN +Na2S

 

 

As in test for nitrogen; instead of green or blue colour, blood red colouration confirms presence of N and S both.

Reactions Involved:

 

  • Qualitative Analysis of Inorganic Salts:

1.  Physical Examination of Salts/Mixture

Observation

Inference

1.   Substance is coloured

 

i)     Blue

Copper salt

ii)    Dark green

Chromium salt

iii)   Green

Salts of Fe(II), Ni, Cu or Cr

iv)   Light yellow or brown

Salts of Fe(III)

v)    Dark brown

PbO2,Bi2S3

vi)   Light pink

Salts of Mn

vii)  Pink

Salts of Co

viii) Red

HgO, HgI2,Pb3O4

ix)   Orange red

Sb2S3

2.   Substance is deliquescent

CaCl2,ZnCl2,MgCl2, MnCl2, nitrites, nitrates

3.   Substance is heavy  

Salts of Pb, Hg and Ba

4.   Substance is light

Carbonates of Bi, Mg, Al, Zn, Ca, Sr

2. Effect of Heating:

Observation

Inference

1.    Substance melts

Salts of alkali metals and salts having water of crystallisation.

2.    Substance decripitates (crackling noise)

NaCl, KI, Pb(NO3)2 and Ba(NO3)2

3.    Substance swells (due to loss of water of crystallisation)

Alums, borates and phosphates

4.    The substance sublimes and the colour of sublimate is            

 

 

        i)     White

HgCl2, Hg2Cl2,NH4X, AlCl3, As2O3, Sb2O3

        ii)    Yellow

As2S3 and HgI2 (turns red when rubbed with glass rod).

        iii)   Blue black and violet vapours

Iodides

5.    A residue (generally oxide) is left and its colour is

 

        i)     Yellow (hot) and  white (cold)    

ZnO

        ii)    Reddish brown (hot); yellow (cold)

PbO

        iii)   Black (hot); Red (cold)

HgO, Pb3O4

        iv)   Black (hot); Red brown (cold)

Fe2O3

6.    Gas is evolved

 

        (A)  Colourless and odourless             

 

        i)     O2 - rekindles a glowing splinter

Alkali nitrates (2KNO3 → 2KNO2 + O2)

        ii)    CO2 - turns lime water milky

Carbonates and oxalates (CaCO3 → CaO + CO2)

        iii)   N2

Ammonium nitrite (NH4NO2 → N2 + 2H2O)

        (B)  Colourless gas with odour

 

        i)     NH3 - Turns red litmus blue and mercurous nitrate paper black

Ammonium salts (NH4)2SO4 → NH4HSO4 + NH3

        ii)    SO2 - Smell of burning sulphur, turns acidified  K2Cr2O7 paper green

Sulphites and thiosulphates

CaSO3 →CaO + SO2

        iii)   HCl - Pungent smell, white fumes with ammonia

Hydrated chlorides

CaCl2.6H2O → Ca(OH)2 + 4H2O + 2HCl

        iv)   H2S - smell of rotten eggs, turns lead acetate paper black

Sulphides

Na2S + 2H2O → 2NaOH + H2S

        (C)  Coloured gas

 

        i)     NO2 - Brown, turns starch iodide paper blue         

Nitrites and nitrates of heavy metals

2Cu(NO3)2 → 2CuO + 4NO2 + O2

        ii)    Br2 - Reddish brown       

Bromides

2CdBr2 + O2 → 2CdO + 2Br2

        (A) Turns starch paper yellow

 

        (B) turns starch iodide paper blue

 

        iii)   I2 - Violet, turns starch paper blue       

Iodides
2CdI2 + O2 → 2CdO + 2I2

        iv)   Cl2 - Greenish yellow

Chlorides

        (A)  bleaches moist litmus paper

CuCl2 + H2O → CuO + 2HCl

CuO + 2HCl → Cu + H2O + Cl2

        (B) bleaches indigo solution

 

        (C)  turns starch iodide paper blue

 

3.  Flame test:

 

Metals

Colour

Li

crimson red

Na

golden yellow

K

Violet

Ca

Brick red

Sr

Crimson

Ba

apple green

  • Test for Anions:

1. Carbonate (CO32-

 

i) Dilute HCl : gives effervescence, due to the evolution of carbon dioxide.

CO32-   +  2H+ →   CO2­  +  H2O

The gas gives turbidity with lime water and baryta water.

CO2 + Ca2+ + 2OH- → CaCO3 ¯ + H2O

CO2 + Ba2+ + 2OH-→ BaCO3 ¯ + H2O

On prolonged passage of carbon dioxide in lime water, the turbidity slowly disappears due to the formation of soluble hydrogen carbonate.

CaCO3  ¯→ + CO2 + H2O → Ca(HCO3)2

ii)    Barium chloride or Calcium chloride solution: White ppt of  barium  or calcium carbonate is obtained, which is soluble in mineral acid.

CO32- + Ba2+ → BaCO3 ¯

CO32- + Ca2+ → CaCO3 ¯

iii)   Silver nitrate solution: White ppt of silver carbonate is obtained.

CO32- + 2Ag+ → Ag2CO3¯

The ppt so obtained is soluble in nitric acid and in ammonia. The ppt becomes yellow or brown on addition of excess reagent and same may also happen if the mix is boiled, due to the formation of silver oxide

Ag2CO3¯ → Ag2O ¯ + CO2 ­

2.   Sulphites (SO32-)

i) Dilute HCl or Dilute H2SO4 : decomposes with the evolution of sulphur dioxide

SO32- + 2H+ → SO2 + H2O

The gas has a suffocating odour of burning sulphur.

ii) Acidified potassium dichromate solution:  Turns filter paper moistened with acidified potassium dichromate solution,  green due to the formation of  Cr3+ions.

iii) Lime  water : On passing the gas through lime water, a milky ppt is formed.

Precipitate dissolves on prolonged passage of the gas, due to the formation of hydrogen sulphite ions.

iv) Barium chloride or Strontium chloride solution: Gives white ppt. of barium or strontium sulphite.

3.   Sulphide (S-2)

i) Dil. HCl or Dil. H2SO4: A colourless gas smelling of rotten eggs (H2S) is evolved.

S2- + 2H+ →  H2

ii) The gas turns lead acetate paper black

iii) Gives yellow ppt. with CdCO3

Na2S + CdCO3 → CdS¯ + Na­2CO3

iv) Silver nitrate solution: black ppt. of silver sulphide insoluble in cold but soluble in hot dil nitric acid.

S2- + 2Ag+ → Ag2S ¯

v) Sodium nitroprusside solution : Turns sodium nitroprusside solution purple

Na2S + Na2[Fe(CN)5NO] → Na4[Fe(CN)5NOS]

4.   Nitrites (NO2-)

i)     Dil HCl and Dil. H2SO4 : Adding to solid nitrite in cold yields pale  blue liquid (due to the presence of free nitrous acid HNO2 or its anhydride N­2O3) & the evolution of brown fumes of nitrogen dioxide, the latter being largely produced by combination of nitric oxide with the oxygen of the air

ii)    Silver nitrate solution : White crystalline ppt. is obtained

NO2- + Ag+ → AgNO2¯

iii) Turns acidified KI - starch paper blue

2KI + 2NO2 → 2KNO2 + I2 ­

Starch + I2  → Blue colour

iv)   Brown ring test: When the nitrite solution is added carefully to a conc. solution of Iron(II) sulphate acidified with dil acetic acid or with dilute sulphuric acid, a brown ring is formed, due to the formation of [FeNO]SO4  at the junction of the two liquids.

5. Acetate (CH3COO-)

i) Dilute Sulphuric Acid : Smell of vinegar is observed.

CH3COO- + H+ → CH3COOH ­

The following test is performed with the aqueous salt solution.

ii) Iron (III) Chloride Solution: Gives  deep - red colouration

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

Brown colour

6. Thiosulphates 

i) Dil Hydrochloric acid: Gives sulphur & sulphur di oxide

ii) Silver nitrate solution: Gives white ppt. of silver thiosulphate.

S2O32-+ 2Ag+ → Ag2S2O3 ¯

The ppt. is unstable, turning dark on standing, when silver sulphide is formed.

Ag2S2O3¯ + H2O → Ag2S + H2SO4

iii)   Lead acetate or Lead nitrate solution: Gives white ppt.

S2O32-+ Pb2+ → PbS2O3 ¯

On boiling it turns black due to the formation of PbS.

PbS2O3 ¯ + H2O →PbS ¯ + 2H+ + SO42 

7. Chloride (Cl-)

i) Conc. H2SO4 : decomposes with the evolution of HCl.

Cl- + H2SO4 → HCl  + HSO

Gas so produced

(1) Turns blue litmus paper red

(2) Gives white fumes of NH4Cl when a glass rod moistened with ammonia solution is brought near the mouth of test tube.

 ii)  Silver nitrate solution: White, curdy ppt. of AgCl insoluble in water & in dil .nitric acid, but soluble in dilute ammonia solution.

v)  Chromyl chloride test: When a salt containing chloride ion is heated with K2Cr2O7 and conc. H2SO4 orange red fumes of chromyl chloride (CrO2Cl2) are formed.

K2Cr2O7 + 4NaCl +  6H2SO4 → 2KHSO4 + 4NaHSO4 + 2CrO2Cl2 ­ + 3H2O

                                                                               orange – red fumes

Chlorides of mercury, owing to their slight ionization, do not respond to this test and only partial conversion to CrO2Cl2 occurs with the chlorides of lead, silver, antimony and tin.

When chromyl chloride vapours are passed into sodium hydroxide a yellow solution of sodium chromate is formed which when treated with lead acetate gives yellow ppt. of lead chromate.

                CrO2Cl2 + 2NaOH → Na2CrO4 + 2HCl

                                               Yellow solution

                Na2CrO4 + (CH3COO)2 Pb → 2CH3COONa + PbCrO4 ¯   (yellow ppt.)

8.   Bromide (Br-)

i)  Conc. H2SO4 : Gives reddish brown vapours of bromine accompanying the hydrogen bromide.

ii)    Manganese dioxide and conc. sulphuric acid : When a mix of solid bromide, MnO2 and conc. H2SO4 is heated reddish brown vapours of bromine are evolved.

                2KBr + MnO2 + 2H2SO4 → Br2 ­ + K2SO4 + MnSO4 + 2H2O

The following tests are performed with the aqueous salt solution.

iii)   Silver nitrate solution: Pale yellow ppt. of silver bromide is obtained. This ppt. is sparingly soluble in dil but readily soluble in conc. ammonia solution and insoluble in dil. HNO3.

iv) Lead acetate solution: White crystalline ppt. of lead bromide which is soluble in
boiling water.                   

9.   Iodide (I-)

i)     Conc. H2SO4 : Gives violet vapours of iodine

ii)    Silver nitrate solution: Yellow  ppt. of silver iodide AgI, very slightly soluble in conc. ammonia solution and insoluble in dil nitric acid.

10.  Nitrate (NO3- )

i)     Conc H2SO4 : Gives reddish - brown vapours of nitrogen dioxide

                4NO3- + 2H2SO4 →  4NO2­ + 2SO42- + 2H2O + O2­

 ii)    Brown ring test: When freshly saturated solution of iron (II) sulphate is added to nitrate solution and conc. H2SO4 is poured slowly down the side of the test - tube, a brown ring is obtained.

2NO3- + 4H2SO4 + 6Fe2+ → 6Fe3+ +2NO + 4SO4–2 + 4H2O

Fe2+ + NO­ → [Fe(NO)]2+

On shaking and warming the mix, the brown colour disappears, nitric oxide is evolved and a yellow solution of Iron(III) ions remains.

11. Sulphate (SO42-)

 i)     Barium chloride solution: White ppt. of barium sulphate BaSO4 insoluble in warm dil. hydrochloric acid and in dilute nitric acid, but moderately soluble in boiling, conc. hydrochloric acid.

ii)    Mercury (II) nitrate solution: Gives yellow ppt. of basic mercury (II) sulphate

12. Chromate CrO42 -and Dichromate (Cr2O)

 i)     Barium chloride solution: Pale - yellow ppt. of barium chromate soluble in dilute mineral  acids but insoluble in water and acetic acid.

CrO + Ba2+    → BaCrO4 ¯

Dichromate ion also gives the same ppt. but due to the formation of strong acid precipitation is partial.

Cr2O+ 2Ba2+ + H2O  2 BaCrO4 ¯ + 2H+

If sodium hydroxide or sodium acetate is added, precipitation becomes quantitative.

 ii)    Silver Nitrate Solution: Brownish - red ppt. of silver chromate Ag2CrO4 which is soluble in dil. nitric acid & in ammonia solution, but insoluble in acetic acid.

A reddish brown ppt. of silver dichromate Ag2Cr2O is formed with a conc. solution of a dichromate.             

13. Permanganate MnO 

i) Hydrogen peroxide : It decolourises acidified potassium permanganate solution

                2MnO4- + 5H2O2 + 6H+ ¾¾® 5O2 ­ + 2Mn2+ + 8H2O.             

ii) Iron (II) sulphate, in the presence of sulphuric acid, reduces permanganate to manganese (II). The solution becomes yellow  because of the formation of iron (III) ions

                MnO4- + 5Fe2+ + 8H+ →  5Fe3+ + Mn2+  + 4H2O

  • Test For Cations:

Group

Group reagent

Ions

Colour & ppt.

Group I

dil HCl

                Pb2+, Hg+, Ag+

PbCl2, Hg2Cl2, AgCl - white

Group II

Group II A

 

Group II B

 

H2S in dil HCl

 

Hg2+, Cu2+, Bi3+, Cd2+

 

As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+

Yellow-CdS,As2S3,

               As2S5 , SnS2

Black - HgS, CuS, PbS

Orange - Sb2S3, Sb2S5

Brown - Bi2S3 ,SnS

Group III A

NH4OH in presence of NH4Cl

Fe3+, Al3+, Cr3+

Fe(OH)3, Al(OH)3,Cr(OH)3 Brown      White      Green

GroupIII B

H2S in presence of NH3 & NH4Cl or (NH4)2S.

Ni2+, Co+2, Mn+2, Zn2+

ZnS  - white or grey,  

Black - CoS, NiS

MnS - Buff (light pink)

Group IV

 

(NH4)2CO3 in presence of NH4Cl & NH4OH.

Ba+2, Sr2+, Ca+2

 

BaCO3, SrCO3, CaCO3   -  white                                  

Group V

No common group reagent.

Mg+2, Na+, K+, NH4+

¾

Group I (Pb2+, Ag+, Hg+)

(A)  PbCl2 gives a yellow ppt. with K2CrO4. The ppt. is insoluble in acetic acid but soluble in NaO

(B)  PbCl2 + 2KI → PbI2  + 2KCl

                            (Yellow)

PbCl2 + 2KI (excess) → ...

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