Thursday, 26 February 2015

8.2 Qualitative analysis of salts

Qualitative analysis of salts
■ Qualitative analysis

A series of tests conducted on a chemical substance to identify the unknown component or components in it.

Can determine the cation and anion present.
■ This video contains information on the qualitative analysis of salt .
■ Technique of qualitative analysis.

Colour of salt

Solubility of salt in water

The effect of heat on the salt

Flame test and gas test

Identifying the precipitate formed with a specific chemical reagent

Confirmatory tests for ions


Colour of salt
■ Colour of salt

All cations are not coloured except copper(II) ions, [blue], iron (II) ion [green], iron (III) ion [yellow or brown].

None of the anions are coloured.

Colour Solid state Aqueous state
Colourless None Salts for Ca2+, Mg2+, Al3+, Pb2+, NH+4
White Salts for Ca2+, Mg2+, Al3+, Pb2+, NH+4 None
Black CuO, CuS, PbS, FeS None
Brown Fe3+, PbO Salt for Fe3+
Green Fe2+, CuCO3, CuCl2 Fe2+, Cu2+
Blue Salt for Cu2+ Salt for Cu2+
Yellow PbO, PbI2, PbCrO4, BaCrO4 Fe3+, CrO2-4


Solubility of salt in water
■ Solubility of salt in water

Solubility in water enable inferences to be formed on the type of cation and anion that may be present in the salt.

Salt Solubility in water
Na+, K+, NH+4 All are soluble
Nitrates All are soluble
Chlorides All are soluble except AgCl, HgCl and PbCl2 (soluble in hot water)
Sulphates All are soluble except BaSO4, PbSO4 and CaSO4
Carbonates All are soluble except Na2CO3, K2CO3 and (NH4)2CO3
Oxides All are soluble except Na2O, K2O and CaO
Hydroxides All are soluble except NaOH, KOH, Ca(OH2) and Ba(OH2)


Effect of heat on salts
■ Effect of heat on salts

A salt may dissociate to release certain gases. (Give information regarding the identity of the anion)

A salt may change in colour. (Give information regarding the identity of the cation)
■ This video contains information on the effects of heat on salts.
■ Gas tests (to identify the type of gas) :

Observation of the colour

Tested with damp blue litmus paper and damp red litmus paper

Tested with limewater

Tested with glowing wooden splinter

Tested with acidic potassium manganate(VII) solution
■ Characteristics and confirmatory tests for several gases.

Gas Colour Smell Confirmatory Test
Hydrogen Colourless Odourless Give 'pop' sound with lighted wooden splinter
Oxygen Colourless Odourless Lights up a glowing wooden splinter
Water vapour Colourless Odourless White anhydrous copper (II) sulphate turns blue
Carbon dioxide Colourless Odourless Turns limewater chalky
Ammonia Colourless Pungent Turns damp red litmus blue.
Produces thick white fumes with hydrogen chloride gas
Chlorine Greenish yellow Pungent Bleaches damp red litmus paper.
Turns damp blue litmus paper to red and the bleaches it.
Nitrogen dioxide Brown Pungent Turns damp blue litmus paper red
Sulphur dioxide Colourless Pungent Bleaches the purple colour of acidic potassium manganate (VII) solution
Hydrogen chloride Colourless Pungent Fumes with glass rod dipped in concentrated ammonia solution
■ Effects of heat on salts:

Carbonate ions
All carbonate salts (except Na2CO3 and K2CO3) will decomposes when heated to release carbon dioxide gas, CO2.
Example:
MgCO3(s)ΔMgO(s)+CO2(g)
CuCO3(s)ΔCuO(s)+CO2(g)

Nitrate salts
Sodium nitrate and potassium nitrate decomposes when heated to produce oxygen gas, and nitrites
Other metal nitrates produces oxygen gas, nitrogen dioxide gas and metal oxides when heated.
Example:
2NaNO3(s)Δ2NaNO2(s)+O2(g)
Zn(NO3)2(s)Δ2ZnO(s)+4NO2(g)+O2(g)

Sulphate salts
Most sulphate salts are not decomposed by heat.
Only a few sulphates such as iron(II) sulphate, zinc sulphate and copper sulphate decompose to sulphur dioxide or sulphur trioxide when heated.
Example:
2FeSO4(s)ΔFe2O3(s)+SO2(g)+SO3(g)
ZnSO4(s)ΔZnO(s)+SO3(g)
CuSO4(s)ΔCuO(s)+SO3(g)

Chloride salts
All chlorides salts are stable when heated except ammonium chloride.
Example:
NH4Cl(s)ΔNH3(s)+HCl(g)
■ Gas produced in heat test enable inferences to be made on the types of salts.

Type of gas produced Types of ion present
CO2 CO2-3 ion present (except Na2CO3 and K2CO3)
O2 NO-3 ion present
NO2 NO-3 ion present (except NaNO3 and KNO3)
SO2 sO2-4 ion present
NH3 NH-4 ion present
■ Residues in heat test enable inferences to be made on the types of salts.

Colour of original salts Colour of residue Inference
White Yellow when hot, white when cooled ZnO, presence of ion Zn2+
White Brown when hot, yellow when cooled PbO, presence of ion Pb2+
Blue/Green Black CuO, presence of ion Cu2+
Green/Yellow Brown Fe2O3, presence of ion Fe3+ or Fe2+


Test for anions
■ Anions that are normally tested in qualitative analysis:

Test for carbonate ions, CO2-3
All carbonates except K2CO3 and Na2CO3 release carbon dioxide when heated.
MgCO3(s)ΔMgO(s)+CO2(g)
All carbonates produce carbon dioxide when reacts with dilute acid.
MgCO3(s) + 2HCl(aq) → MgCl2 + H2O(l) + CO2(g)
Na2CO3(aq) + 2HCl(aq) → 2NaCl2 + H2O(l) + CO2(g)
This video shows the test for carbonate ions.


Test for nitrate ions, NO-3
All nitrates except NaNO3 and KNO3 give out brown gas, nitrogen dioxide and oxygen when heated. NaNO3 and KNO3 release oxygen when heated.
2NaNO3(aq) → 2NaNO2(aq) + O2(g)
2Mg(NO3)2(aq) → 2MgO(aq) + 4NO2(g) + O2(g)
This video shows the effects of heat on nitrate ions.
Brown Ring Test
About 2cm3 nitrate solution is mixed with dilute sulphuric acid and iron(II) sulphate solution in test tube.
Then a few drops of concentrated sulphuric acid are added slowly and carefully down the side of the test tube.
A brown ring is formed between the layers of concentrated acid and the nitrate mixture.
The brown ring, composed of a complex ion, confirm the presence of nitrate.
This video shows the Brown Ring test for nitrate ions.

Test for sulphate ions, SO2−4
About 2cm3 sulphate solution is mixed with dilute hydrochloric acid followed by barium chloride solution. A white precipitate, barium sulphate forms, confirming the presence of a sulphate.
Ba2+(aq) + SO2−4(aq) → BaSO4(s)
This video shows the test for sulphate ions.

Test for chloride ions
About 2cm3 of a chloride solution is mixed with dilute nitric acid followed by the silver nitrate solution.
A white precipitate, silver chloride forms, confirming the presence of a chloride.
Ag+(aq) + Cl(aq) → AgCl(s)
This video shows the test for chloride ions.

Laboratory Activity 8.2.1 :Test for anions
Test for cations
■ The following tests are conducted to identify cations in solutions:

Test with aqueous sodium hydroxide, NaOh (aq)

Test with aqueous ammonia, NH3 (aq)

Confirmatory tests with specific reagents.
■ This video contains information on the test for cations
■ Test with aqueous sodium hydroxide, NaOH (aq)

Colourless cation aqueous
Observation Inferences
Ammonia gas NH3 is released when heated up. NH4+ ions may be present.
White precipitate not soluble in excess NaOH solution Mg2+ or Ca2+ ions may be present.
White precipitate soluble in excess NaOH solution Zn2+ or Al3+ or Pb2+ ions may be present.

Coloured cation aqueous
Observation Inferences
Blue precipitate not soluble in excess NaOH solution Cu2+ ions may be present.
Green precipitate not soluble in excess NaOH solution Fe2+ ions may be present.
Brown precipitate not soluble in excess NaOH solution Fe3+ ions may be present.

This video contains information on the test for cations with NaOH
■ Test with aqueous ammonia, NH3(aq)

Observation Inferences
No reaction Na4+ or Ca2+ ions may be present.
White precipitate not soluble in excess NH3 solution Mg2+ or Al2+ or Pb2+ ions may be present.
White precipitate soluble in excess NH3 solution Zn2+ ions may be present.

Coloured cation aqueous
Observation Inferences
Blue precipitate soluble in excess NH3 solution forming dark blue solution Cu2+ ions may be present.
Green precipitate not soluble in excess NH3 solution Fe2+ ions may be present.
Brown precipitate not soluble in excess NH3 solution Fe3+ ions may be present.

This video contains information on the test for cations with ammonia solution
■ Confirmatory test for :

iron(II) ion, Fe2+ and iron(III) ion, Fe3+
Cation Potassium hexacyanoferrate(II), K4Fe(CN)6 Potassium hexacyanoferrate(III), K3Fe(CN)6 Potassium thiocyanate, KSCN
Fe2+ Light blue precipitate Dark blue precipitate No red solution
Fe3+ Dark blue precipitate Greenish brown precipitate Blood red solution

lead (II) ion, Pb2+
Cation Potassium iodide, KI Potassium chloride, KCl Potassium sulphate, K2SO4
Pb2+ Yellow precipitate
Pb2+ + 2I → PbI2
Dissolves in hot water when heated
White precipitate
Pb2+ + 2Cl → PbCI2
Dissolves in hot water when heated
White precipitate
Pb2+ + SO2−4 → PbSO4

This video contains information on the confirmatory test for cations

ammonium ion, NH4+
Brown precipitate formed with Nessler reagent.
■ Analysis qualitative summary of cations:

Cation NaOH(aq) NH3(aq) Na2CO3(aq) H2SO4 or Na2SO4(aq) HCl or NaCl (aq) Confirmatory test
Na+ Flame test - yellow flame
Al3+ White precipitate soluble in excess NaOH White precipitate soluble in excess NH3 White precipitate soluble in excess HNO3
Pb2+ White precipitate soluble in excess NaOH White precipitate soluble in excess NH3 White precipitate soluble in excess HNO3 White precipitate White precipitate soluble in hot water KI - yellow precipitate
KCL - white precipitate
Both soluble in hot water
Zn2+ White precipitate soluble in excess NaOH White precipitate soluble in excess NH3 White precipitate soluble in excess HNO3 White precipitate with potassium hyxacyanoferrate(II), K4Fe(CN)6
Mg2+ White precipitate soluble in excess NaOH White precipitate soluble in excess NH3 White precipitate soluble in excess HNO3
Ca2+ White precipitate soluble in excess NaOH No precipitate White precipitate soluble in excess HNO3 White precipitate
Cu2+ Blue precipitate soluble in excess NaOH Blue precipitate soluble in excess NH3 Blue precipitate Blue solution Blue solution Blue precipitate with potassium hyxacyanoferrate(III), K3Fe(CN)6
Fe2+ Dirty green precipitate insoluble in excess NaOH Dirty green precipitate soluble in excess NH3 Green precipitate Green solution Green solution Dark blue precipitate with potassium hyxacyanoferrate(III),K3Fe(CN)6
Fe3+ Brown precipitate soluble in excess NaOH Brown precipitate soluble in excess NH3 Brown precipitate Brown solution Brown solution Blood red solution with potassium thiocyanate, KSCN
NH4+ NH3 released upon heating Brown precipitate with Nessler's reagent
Laboratory Activity 8.2.2 : Qualitative analysis of salts


Flow chart for qualitative analysis of anion and cation
■ Qualitative analysis of a salt

To confirm the presence of the cation and anion of a named salt.

To identify the cation and anion of one or more unknown salts.

The animation below summarized the analysis of anions in salts.

The animation below summarized the analysis of cations in salts.


⇲ For exercise(objective and subjective), download for free on Android OS.

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