Qualitative Analysis : Acid radicals-Analysis of Category-2

 

Analysis of Category 2: Acid Radicals

Group reagent of this group is conc. H2SO4. In a dry test tube substance/mixture is treated with conc. H2SO4. first in cold and then by heating. Evolution of coloured/pungent smelling gases indicates the possibility of radicals of this group.

 

Experiment/Observation Inference
1. Chloride:  
(i) Colourless, pungent smelling gas is evolved Chloride (CI) may be.
(ii) White fumes are formed when a rod of NH4OH is brought in contact of gas. White fumes may be of NH4Cl and Cl is confirm.
(iii) On passing gas in AgNO3 solution, white ppt, is obtained which is soulbe in excess of NH4OH. If acidified with HNO3, again white ppt, is obtained. White ppt, is of AgCl and Cl is confirmed.
(iv) Salt/mixture is heated with MnO2 and conc. H2SO4 and yellow-green gas is evolved Yellow-green gas is Cl2 and Cl is confirmed.
(v) Salt is dissolved in HNO3 and is filtered. Now addition of AgNO3 gives white ppt. which is soluble in excess of NH4OH. The test may also be performed with acidified soda extract. Cl is confirmed.
(vi) Chromyl-chloride test:

(a) In a dry test tube, mixture and K2Cr2O7 are taken ratio 1 : 3. Now conc. H2SO4 is added in it and mixture obtained is heated. A dark red-brown gas is evolved.

Gas may be chromyl chloride (CrO2Cl2)
(b) Dark coloured gas is passed in NaOH solution which turns yellow. This yellow solution is divided into two parts. Yellow colour may be due to formation of K2CrO4,
Part-I: Yellow solution is acidified with CH3COOH and Pb(CH3COO)2 is added in it. A curdy yellow ppt. is obtained. Cl is confirmed.
Part-II: Yellow NaOH solution is acidified with dil. H2SO4 and now amyl alcohol or diethyl ether is added followed by addition of H2O2. The organic layer turns blue. Cl is confirmed. 

 

 

 

Reactions:
(i) NaCl + H2SO4 → NaHSO4 + HCl­ ↑ Colourless, pungent smelling gas is HCl.
(ii) HCl + NH4OH → NH4Cl + H2O NH4­Cl gives white fumes.
(iii) HCl + AgNO3 → AgCl + HNO3

AgCl + 2NH4OH → [Ag(NH3)2]Cl + 2H2O

[Ag(NH3)2]Cl + 2HNO3 → AgCl + 2NH4NO3

White ppt. of AgCl is soluble in excess of NH4OH due to formation of soluble amino complex,
(iv) NaCl + H2SO4 ↓ NaHSO4 + HCl MnO2 + 4HCl → MnCl2 + 2H2O + Cl2­ ↑ Cl2 is yellow-green gas.
(v) NaCl + AgNO3 → AgCl ↓ + NaNO3
(vi) (a) K2Cr2O7 + 2H2SO4 → 2KHSO4 + 2CrO3 + H2O

CrO3 + 2HCl → CrO2Cl2­ ↑ + H2O

(b) CrO2Cl2 + 4NaOH → Na2VrO4 + 2NaCl + 2H2O

Na2CrO4 + Pb(CH3COO)2 → PbCrO4 ↓ 2CH3COONa

Na2CrO4 + H2SO4 + 2H2O2  \underrightarrow { \quad alc.\quad }  Na2SO4 + CrO5 + 3H2O

CrO2Cl2 (chromyl chloride) is of deep red-brown color.

Na2CrO4 (sodium chromate) provides yellow colour to NaOH solution.

Lead chromate (PbCrO4) gives yellow precipitate.

Chromium diperoxide (CrO5) is of blue colour.

2. Bromide (Br)  
(i) Dark brown-red vapour are obtained which turns water tallow and solution in test tube also turns red-brown Brown gas may be bromine and salt may contain bromide (Br)
(ii) On adding MnO2, the evolution of brown fumes intensifies. Br is confirmed.
(iii) Slat/mixture or acidified soda extract is treated with HNO3 and in solution AgNO3 is added. Yellowish-white ppt. is obtained which is partially soluble in NH4OH. Br is confirmed ppt. is of AgBr
(iv) Water extract of salt/mixture is acidified with dilute HNO3 and CCl4 or CHCl3 is added in it. Organic layer turns brown. Cl2 water may also be used after addition of organic solvent Br is confirm and brown colour is due to Br2 dissolved in CCl4­.
Reactions:
(i) NaBr + H2SO4 → NaHSO4 + HBr

    2HBr + H2SO4 → Br2 ↑­ + 2H2O + SO2

Brown coloured gas is bromine (Br2)
(ii) 2NaBr + MnO2 + 3H2SO4 → 2NaHSO4 + MnSO4 + 2H2O + Br2­↑ MnO2 favours the evolution of Br2.
(iii) NaBr + AgNO3 → AgBr ↓ + NaNO3

AgBr +2NH4OH → [Ag(NH3)2]Br + 2H2O

Yellowish AgBr forms [Ag (NH3)2] Br which is partly soluble in water.
(iv) 2NaBr + Cl2 → 2aCl + Br2

      Br2 + CCl4 → Brown layer of CCl4

Br2 is very soluble in CCl4 and so layer turns brown.
3. Iodide (I)  
(i) Violet coloured fumes are evolved and some violet coloured particles are condensed in the sides of test tube, Violet fumes may be I2 and I (iodide) may be present.
(ii) On adding MnO2 in reaction mixture evolution of violet fumes intensifies. Confirm
(iii) On bringing acidified KI-starch paper in contact of violet vapours, paper turns blue. Blue colour is due to I2-starch complex and I is confirmed.
(iv) On heating salt-mixture with NaNO2­ and dil. H2SO4 fumes are obtained. Is confirmed.
(v) Water extract of salt/mixture is acidified with dil. HNO3 and AgNO3 is added in it. a yellow ppt. is obtained which is insoluble in NH4OH. Test may also be performed with acidified soda extract. Yellow ppt. is of AgI and I is confirmed.
(vi) Water extract/soda extract is acidified with dil. H2SO4 and CHCl3 or CCl4 is added followed by addition of Cl2 water. I2 turns CCl4 layer violet and I is confirmed.
Reactions:
(i) NaI + H2SO4 → NaHSO4 + HI

    2HI + H2SO4 → I2 ↑­ SO2 + 2H2O

Violet vapour are of I2
(ii) 2NaI + MnO2 + 3H2SO4 → 2NaHSO4 + MnSO4 + 2H2O + I2­ ↑ MnO2 facilitates the evolution of I2
(iii) I2 + Starch → Deep blue complex
(iv) 2NaI + 2NaNO2 + 2H2SO4 → 2Na2SO4 + 2NO + 2H2O + I2­ ↑ Violet colour with NaNO2 is due to evolution of I2
(v) NaI + AgNO3 → AgI ↓ + NaNO3

     AgI + NH4OH → No reaction

AgI does not form soluble complex with NH4OH.
(vi) 2NaI + Cl2 → 2NaCl + I2 I2 dissolves in CCl4 ad gives violet colour.
4. Nitrate (NO3):  
(i) A light brown gas having pungent smell in evolved and evolution intensifies on addition of Cu-turnings. Nitrate (NO3) may be and evolved gas may be NO2.
(ii) Ring test: In fresh aqueous or acidified soda extract of salt/mixture, fresh aqueous solution or FeSO4 is added. Then a drop of conc. H2SO4 is added through sides of test tube. A dark brown coloured ring is formed at the junction of two liquids. Brown ring is of ferrous nitroso sulphate and (NO3) is confirmed 
Reactions:
(i) NaNO3 + H2SO4 → NaHSO4 + HNO3

    4HNO3 → 4NO2­ ↑ + O2­ ↑ + 2H2O

   Cu + 4HNO3 → Cu(NO3)2 + 2H2­O + 2NO2­ ↑

Red-brown gas is nitrogen dioxide (NO2) and Cu reacts with HNO3 to give NO2.
(ii) Ring test:

NaNO3 + H2SO4 → NaHSO4 + HNO3

6FeSO4 + 2HNO3 + 3H2SO4 → 3Fe2(SO4)3 + 2NO +  4H2OFeSO4 + NO →    [Fe(NO)]SO4

Ferrous nitroso-sulphate (Brown ring)

Ferrous nitroso-sulphate is of dark brown colour.
5. Oxalate (C2O42–) :  
(i) Colourless, pungent smelling gas is evolved which burns with blue edged flame. Gas may be carbon monoxide and oxalate (C2O42–).
(ii) On adding CaCl2 solution, a white ppt, is formed. If KMnO4 is added in solution by H2SO4, colour is discharged. White ppt. is CaC2O4 and oxalate is confirmed.
(iii) With Fe2+, (C2O42–) ions gives yellow complex of [Fe(C2O4)2]2-
Reactions:
(i) Na2C2O4 + H2SO4 → Na2SO4 + H2C2O4

   H2C2O4  \underrightarrow { \quad \Delta \quad } CO + CO2 + H2O

CO burns with blue flame.
(ii) Na2C2O4 + CaCl2 → CaC2O4 + 2NaCl

CaC2O4 + H2SO­4 → CaSO4 + H2C2O4

 2KMnO4 + 3H2SO4 + 5H2C2O4 → K2SO4 + 2MnSO4 + 8H2O + 10CO2

(iii) (C2O42–) + Fe2+ → [Fe(C2O4)2]2–

       Yellow complex

 

   

NOTE:

(i)     Chromyl chloride test is not given by chlorides of Hg, Sn, Ag, Pb and Sb. In such cases chromyl chloride test can be performed with residue obtained after evaporation of sodium carbonate extract.

(ii)    Chromyl chloride test must be taken as positive only when all the three steps, i.e., evolution of red fumes, yellow NaOH solution and yellow ppt, are positive.

(iii)   The test tube must be dry otherwise chromyl chloride may hydrolyse.

                             CrO2Cl2 + 2H2O → H2CrO4 + 2HCl

(iv)   Bromides and iodides do not give chromyl chloride test and in this test they evolve Br­2 and I2.

(v)    If NaOH solution is incompletely acidified, than white ppt, lead hydroxide may be obtained.

(vi)   IN the organic layer test of Br, chlorine water must not be used in excess, otherwise layer attains pale yellow colour due to formation of bromine monochloride or it may be colourless due to formation of hypobromous acid.

                             Br2 + Cl2 → 2BrCl      (Bromine monochloride) 

                             Br2 + 2H2O + Cl2 → 2HBrO + 2HCl

                                                                    Colourless        

(vii)  In the organic layer test of I, excess of chloride water must be avoided otherwise the layer colourless due to formation of iodic acid.

                             I2 + 5Cl2 + 6H2O → 2HIO3 + 10HCl

(viii) Ring test in not reliable in presence of NO2, Br and I

(ix)   Smell of NH3 also indicates the presence of NO3, if nitrate salt is boiled with Zn/NaOH or Al/NaOH.

                             Zn + 2NaOH → Na2ZnO2 + 2[H]

                             Al + NaOH + H2O → NaAlO2 + 3[H]

                             NaNO3 + 8[H] → NaOH + 2H2O + NH3­ ↑

(x)    HF is corrosive and causes blisters on skin, so it should not be smelled.

(xi)   If on heating mixture with conc. H2SO4, a red oily substances is condensed on colder parts of test tube, then Cl and CrO4 (or Cr2O72–) both may be present in the mixture.

(xii)  During testing with conc. H2SO4 will get decomposed and evolution of boiling, otherwise H2SO4 will get decomposed and evolution of SO2 will make identification of other gases difficult.

(xiii) In the organic layer test of Br and I HNO3, CaOCl2, KMnO4, NaOCl or chloramine may also be used in place of chlorine water.

 

Qualitative Analysis : Acid radicals-Analysis of Category-3