Thursday, 20 October 2011

Chemistry note: Qualitative method to identify chemical species

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Qualitative analysis on different compounds
Ionic compound: cation (Al3+, Ca2+, Cu2+, Fe2+, Fe3+, Pb2+, Mg2+, K+, Na+, Zn2+, Ag+, NH4+)
1)       Appearance, e.g. Cu2+ is blue-green, Fe3+ is brown.
2)       Solubility: Nitrates and hydrogencarbonate are all soluble
Halides are all soluble except silver halides AgX
Sulphates are all soluble except BaSO4, PbSO4, CaSO4 (slightly soluble)
Carbonates and sulphites are all insoluble except Na+, K+, (NH4)2CO3
3)       Flame test: use a clean Pt wire to dip conc. HCl followed by the sample solid, burn under non-luminous flame (luminous flame has its own colour) to see the characteristic colour. Principle: the compound forms metal chloride under conc. HCl (H2SO4 can’t be used as metal chlorides are more volatile than metal sulphates), and the excited metal cation emit photon at visible spectrum to give the characteristic colour.
Na+: [persistent] golden yellow, K+: lilac, Ca2+: brick red, Ba2+: green, Cu2+: bluish green
4)       Actions on dilute HCl/H2SO4 followed by solubility test, etc.
5)       Actions on NaOH: Metal ion [except Na+, K+] forms hydroxide ppt under NaOH, Al3+, Pb2+, Zn2+ re-dissolve in excess NaOH to give hydroxyl complex.
6)       Actions on NH3: Metal ion [except Na+, K+] forms hydroxide ppt under NH3, Cu2+, Ag+, Zn2+ re-dissolve in excess NH3 to form amine complex. Note:
I)                     [Cu(NH3)4]2+ is in deep blue colour, not the blue colour given by Cu2+.
II)                   Ag form Ag2O rather than AgOH by dehydration 2AgOH Ag2O + H2O, but it forms diamminesilver(I) ion under excess NH3, i.e., [Ag(NH3)2]+
III)                  Al(OH)3 and Pb(OH)2 can be distinguished either by actions in H2SO4 or heat until dryness (Al2O3 VS PbO), PbO change colour when heated.
7)       Test for ammonium: Warm with NaOH to give pungent small/turn wet red litmus paper blue.
Test for gases (NH3, CO2, SO2, HX)
1)       CO2: turning lime water milky and become clear when CO2 is in excess; note that SO2 has similar effect by Ca(OH)2 + SO2 CaSO3 + H2O but CO2 don’t have reducing power.
2)       SO2: It act as an reducing agent so it reduces Cr2O72- (orange) to Cr3+
3)       SO3: equivalent to SO42-
4)       Cl2: turn wet blue litmus paper red then bleached (white) since Cl2 + H2O and HCl + HOCl are in equilibrium. HCl, HOCl are both acidic while OCl- has bleaching effect.
5)       NH3: pungent smell, form dense white fumes (NH4Cl) with HCl(g), there are test each other.
6)       HX: HCl can be tested by NH3, HBr and HI are in equilibrium with H2 + X2 so they gives white fumes and brown liquid (Br2) or black solid (I2) as well. They can also be tested by actions on conc. H2SO4.

Ionic compound: anion (NO3-, CO32-, SO32-, SO42-, OCl-, S2O32-, X-)
1)       Actions of heat: Carbonates decompose upon Bunsen flame: Carbonate oxide + CO2
2)       Sulphates are resistant under lab. flames, but iron(II) sulphates decomposes under heating: 2FeSO4 Fe2O3 + SO2 + SO3.
3)       Actions on HCl: bleaching effect of OCl-: OCl- + H+ Cl2 + H2O
4)       Sulphites and thiosulphates gives SO2 under acid:
SO32- + 2H+ SO2 + H2O, S2O32- + 2H+ SO2 + H2O + S, so thiosulphates gives white ppt besides SO2.
5)       Barium test: sulphates, sulphites and carbonates forms ppt with Ba2+, common reagents includes BaCl2, Ba(NO3)2. BaSO3, BaCO3 dissolve in HCl but BaSO4 is resistant in acid. This serves as an unique test for sulphate ions.
6)       Silver nitrate test: Dissolve the sample in HNO3 followed by AgNO3, X- form ppt AgCl (white) / AgBr (pale yellow) / AgI (yellow). They can be further distinguished by:
(I)                   AgCl dissolve in NH3, AgBr only dissolve in conc. NH3, AgI is insoluble in NH3
(II)                 AgCl turns grey in sunlight, AgBr turns yellowish grey, AgI remains yellow
Tests for organic compounds
1)       Saturated hydrocarbons: blue/clear yellow flame under non-luminous flame
2)       Alkene/alkyne: smoky flame under non-luminous flame since higher carbon-hydrogen ratio leads to more CO produced.
Test for addition: add a few drops of Br2 (in organic solvents)/K2Cr2O7/H+, it decolorize rapidly.
3)       Haloalkanes: Boil with ethanolic KOH (KOH with EtOH), acidify with HNO3 followed by AgNO3, ppt of AgX is expected.
4)       Alcohol:
(I)                   Esterification: Add acyl chloride (RCOOX) to give colourless/white fumes (HCl) (The smell of ester is probably covered by acid)
(II)                 Esterification: Add carboxylic acid and heat with a few drops of conc. H2SO4, a sweet fruity smell is given (ester)
(III)                Lucas test: Add conc. HCl with ZnCl2, cloudiness appears (NOT ppt; the mixture of liquid in different phase) according to the degree of carbon: primary alcohol (1hr), secondary alcohol (5 min) and tertiary alcohol (1 min).
(IV)              Iodoform test for alcohol that the carbon connected to hydroxyl group also connected to a methyl group. Add iodine with NaOH, yellow ppt (CHI3) with characteristic smell is given. The mechanism is as follows:
(V)                Under K2Cr2O7/H+, primary and secondary alcohol oxidizes but tertiary alcohol is resistant to oxidation since CH3- is a poor leaving group. Note that KMnO4 can be used as the oxidizing power is too strong and it may oxidizing the tertiary alcohol.
5)       Carbonyl group (aldehydes/ketones):
(I)                   Test by 2,4-dinitrophenylhydrazine: Aldehydes and ketones react with this reagent to give red~yellow ppt (2,4-dinitrophenylhydrazone). Accurate species can be determined by crystallization of ppt and determine the melting point.

(II)                 Iodoform test (for ketone CH3COR): yellow ppt under I2/NaOH
(III)                Tollens’ reagent (silver mirror test): warm with [Ag(NH3)2]+, the reagent act as oxidizing agent that gives out silver, silver mirror is deposited in the inner surface of the tube
6)       Carboxylic acid
I)                     Esterification with ethanol under conc. H2SO4 as catalyst to give sweet, fruity smell
II)                   RCOOH is the only organic compound to react with NaHCO3. (Most acidic)

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