8b Qualitative Analysis of Salts

September 21, 2017 | Author: Ishen Perumal | Category: Precipitation (Chemistry), Salt (Chemistry), Hydroxide, Solubility, Sodium Hydroxide
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QUALITATIVE ANALYSIS OF SALTS 1.

Qualitative analysis – is a chemical technique used to determine the identities of chemical substances present in a mixture but not their quantities. 2. Qualitative analysis of salt – scheme of tests carried out to identify the CATION AND ANION present in the salt. 3. The technique of qualitative analysis includes : a. Observe the colour of the SALT or the colour of the SALT SOLUTION b. Observe the EFFECT OF HEAT on the salt c. Identify the GAS FORMED when a test is performed on the salt d. Observe the SOLUBILITY in water e. Identify the PRECIPITATE formed, when a specific chemical reagent is added to the aqueous salt solution f. Carry out CONFIRMATORY TESTS (specific chemical test), to confirm the identity of cation or anion present in the salt

CONFIRMATORY TESTS 4. THE COLOUR OF SALT OR AQUEOUS SALT SOLUTION a. Most salts are white, and when dissolved in water, will form colourless aqueous salt solution COLOUR SOLID SOLUTION White or colourless Salts of Na+, K+, NH4+, Mg2+, Solution of salts of Na+, K+, Ca2+, Ba2+, Al3+, Pb2+, Zn2+ NH4+, Mg2+, Ca2+, Ba2+, Al3+, (only if all the anions are Pb2+, Zn2+ colourless too) Yellow PbO, PbI2, PbCrO4, BaCrO4 Solution of salts of Fe3+, CrO42-, chromate 2+ Blue Hydrated Cu salt Cu2+ CuSO4, Cu(NO3)2 , Cu(OH)2 CuSO4, Cu(NO3)2 Green CuCO3, CuCl2 Fe2+ FeSO4 , Fe(NO3)2 , FeCl2 , FeSO4 , Fe(NO3)2 , FeCl2 , Fe(OH)2 , Black CuO, CuS, FeO, FeS Brown Fe2(SO4)3 , Fe(NO3)3 , Fe3+, Cr2O72- dichromate FeCl3 , Fe(OH)3 , (orange) 5. SOLUBILITY OF SALTS IN WATER Type of salts Salts of Na+, K+, NH4+ Nitrate Sulphate Chloride Carbonate Oxide Hydroxide

Solubility n water All are soluble All are soluble All are soluble EXCEPT BaSO4, PbSO4, CaSO4 All are soluble EXCEPT AgCl, HgCl, PbCl2 All are INSOLUBLE EXCEPT Na2CO3, K2CO3 , (NH4)2CO3 All are INSOLUBLE EXCEPT Na2O, K2O , CaO (slightly soluble) All are INSOLUBLE EXCEPT KOH, NaOH, Ca(OH)2,

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Ba(OH)2

6. Test of gases Name of gas Colour of gas Oxygen, O2 Colourless

Smell of gas No smell

Effect on damp litmus No effect

Hydrogen, H2

Colourless

No smell

No effect

Carbon dioxide, CO2

Colourless

No smell

Ammonia, NH3

Colourless

Pungent smells like urine

Hydrogen Chloride, HCl

Colourless

Pungent

Sulphur dioxide, SO2

Colourless

Pungent

Moist blue litmus change to red (ACID) Moist red litmus change to blue (ALKALI) Moist blue litmus change to red (ACID) Moist blue litmus change to red (ACID)

Name of gas

Colour of gas Brown

Smell of gas Pungent

Nitrogen dioxide, NO2 Chlorine, Cl2

Yellowish green

Choking

Effect on damp litmus Moist blue litmus change to red (ACID) Decolourises moist red or blue litmus

Confirmatory test - Test the gas with a glowing wooden splinter. - The gas Will RElight the glowing wooden splinter Test the gas with a lighted wooden splinter. The gas will extinguish THE lighted wooden splinter with a ‘pop’ sound Will turn limewater chalky

When a glass rod dipped into concentrated HCl is placed near the mouth of the test tube with ammonia, white fumes are formed (NH4Cl) When a glass rod dipped into concentrated ammonia is placed near the mouth of the test tube with HCl, white fumes are formed (NH4Cl) Will decolourises the purple colour of acidified potassium manganate (VII) Or Will change the colour of acidified potassium dichromate (VI) solution from orange to green Confirmatory test

NO, ALREADY CONFIRMED BY ITS COLOUR

NO, ALREADY CONFIRMED BY ITS COLOUR

Bleaching effect (ACIDIC)

7. HEATING TESTS ON SALTS CARBONATE SALTS Effect of heat Potassium carbonate Will not decomposes (nothing happen) Sodium carbonate Decompose to metal oxide & CO2 Calcium carbonate CaCO3 → CaO + CO2

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Magnesium carbonate Zinc carbonate Iron (III) carbonate Lead (II) carbonate Copper (II) carbonate

Ammonium carbonate

NITRATE SALTS

MgCO3 (s) → MgO (s) + CO2 ZnCO3 (s) → ZnO (s) + CO2 Fe2(CO3)3 (s) → Fe2O3 (s) + 3CO2 PbCO3 (s) → PbO (s) + CO2 CuCO3 (s) → CuO (s) + CO2 Decomposes to CO2, NH3 & water vapour without any residue (NH4)2CO3(s) → 2NH3(g) + H2O(g) + CO2(g) H2O can change dry blue cobalt chloride paper to pink

Ammonium nitrate

EFFECT OF HEAT Decomposes to metal nitrite & oxygen gas 2KNO3 (s) → 2KNO2 (s) + O2 (g) 2NaNO3 (s) → 2NaNO2 (s) + O2 (g) Decomposes to metal oxide, oxygen gas & nitrogen dioxide gas 2Ca(NO3)2 (s) → 2CaO(s) + O2 (g) + 4NO2 (g) 2Mg(NO3)2 (s) → 2MgO(s) + O2 (g) + 4NO2 (g) 4Al(NO3)3 (s) → 2Al2O3(s) + 3O2 (g) + 12NO2 (g) 2Zn(NO3)2 (s) → 2ZnO(s) + O2 (g) + 4NO2 (g) 4Fe(NO3)3 (s) → 2Fe2O3 (s) + 3O2 (g) + 12NO2 (g) 2Pb(NO3)2 (s) → 2PbO(s) + O2 (g) + 4NO2 (g) 2Cu(NO3)2 (s) → 2CuO(s) + O2 (g) + 4NO2 (g) Decomposes to water vapour & nitrous oxide gas without any residue NH4NO3 (s) → 2H2O (g) + N2O (g)

AMMONIUM SALTS All ammonium salts

EFFECT ON HEAT Will produce ammonia gas (NH3) except for ammonium nitrate (N2O)

SULPHATE SALTS Most sulphate salts Iron (II) sulphate Zinc sulphate Copper (II) sulphate

EFFECT ON HEAT Stable & do not decomposes when heated EXCEPT: Will decomposes to sulphur dioxide OR sulphur trioxide 2FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g) ZnSO4 (s) → ZnO (s) + SO3 (g) CuSO4 (s) → CuO (s) + SO3 (g)

CHLORIDE SALTS Most chloride salts Ammonium chloride

EFFECT ON HEAT Stable & do not decomposes when heated EXCEPT : NH4Cl (s) → NH3 (g) + HCl(g)

Potassium nitrate Sodium nitrate Calcium nitrate Magnesium nitrate Aluminium nitrate Zinc nitrate Iron (III) nitrate Lead (II) nitrate Copper (II) nitrate

8. So, when a salt is heated a. The types of gas evolved has to be identified –-- This will give the information on the ANION (or CATION , NH4+) present b. The colour change of the solid in the test tube must be recorded. ---This will give information regarding the type of CATION present. Type of gas produced CO2 O2

Type of ion Carbonate ion, CO32- ( except Na2CO3 & K2CO3) Only (NaNO3 & KNO3)

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NO2 & O2 SO2 NH3

Nitrate ion, NO3- (except NaNO3, NH4NO3 & KNO3) Sulphate ion, SO42- (only zinc, iron, copper) Ammonium ion, NH4+

9. Most salts that decomposes produced METAL OXIDES as residue. The change of colour during heating gives a good indication towards the types of metal oxide formed. Original colour Colour of residue after heating Metal oxide Cations present in of salt produced salt White Yellow when hot, white when cold ZnO Zn2+ White Brown when hot, yellow when cold PbO Pb2+ Blue / green Black CuO Cu2+ Green / brown Black Fe2O3 Fe3+ / Fe2+

10. TEST FOR THE PRESENTS OF ANIONS IN AQUEOUS SOLUTIONS Reagent / Anion

CO32-

HCl (aq) or HNO3 (aq)

Gas evolved turns lime water milky

HNO3 (aq) followed by AgNO3 (aq)

HNO3 (aq) & Ba(NO3)2 or HCl (aq) & BaCl2 (aq)

-

-

NO3-

-

-

SO42-

-

-

Pb(NO3) (aq)

White precipitate soluble in acid

-

White precipitate

Other test

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Salt solution + dilute H2SO4 + freshly made FeSO4 + concentrated H2SO4 : Brown ring is formed

White precipitate soluble in acid

(i) Cl-

-

White precipitate

-

White precipitate soluble in acid or when heated

(ii)

Heat solid salt + concentrated H2SO4 : HCl gas is produced Heat solid salt + MnO2 + concentrated H2SO4 : Cl2 gas is produced

11. TEST FOR CATIONS a. The cations usually tested : Al3+, Pb2+, Zn2+, Mg2+, Ca2+, Fe2+, Fe3+, Cu2+, & NH4+ ions b. An aqueous solution of the cation is prepared by i. Dissolving the salt in water (if the salt soluble in water) ii. Dissolving the salt in dilute acid & then filter (if the salt is insoluble in water) – the filtrate contains the cation 2+ 2- + PbSO4 + HNO3 = H2SO4 + Pb(NO3)2

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c.

The aqueous solution is then tested with i. NaOH ii. NH3 (aq) iii. A specific reagent as a confirmatory test iv. NaOH & NH3 will supply OH- to produce metal hydroxides

12. COLOURS OF METAL HYDROXIDES Observation Blue precipitate Dirty green precipitate Brown precipitate White precipitate No precipitate

Formula of metal hydroxide Cu(OH)2 Fe(OH)2

Cation present Cu2+ Fe2+

Fe(OH)3 Al(OH)3, Pb(OH)2, Zn(OH)2, Mg(OH)2, Ca(OH)2, -

Fe3+ Al3+, Pb2+, Zn2+, Mg2+, Ca2+ Na+, K+, NH4+

13. HYDROXIDE TEST FOR CATIONS Cation NH4+

Excess NaOH (aq) No precipitate formed, NH3 gas evolved when heated White precipitate soluble in excess NaOH

A little NH3(aq) No precipitate formed

Excess NH3(aq) No precipitate formed

Pb2+

A little NaOH (aq) No precipitate formed, NH3 gas evolved when heated White precipitate

White precipitate

Zn2+

White precipitate

White precipitate soluble in excess NaOH

White precipitate

Al3+

White precipitate

White precipitate soluble in excess NaOH

White precipitate

Cation Mg2+

A little NaOH (aq) White precipitate

Excess NaOH (aq) White precipitate insoluble in excess NaOH

A little NH3(aq) White precipitate

Ca2+

White precipitate

No precipitate formed

Cation Cu2+

A little NaOH (aq) Blue precipitate

White precipitate insoluble in excess NaOH Excess NaOH (aq) Blue precipitate insoluble in excess NaOH

White precipitate insoluble in excess NH3 White precipitate soluble in excess NH3 White precipitate insoluble in excess NH3 Excess NH3(aq) White precipitate insoluble in excess NH3 No precipitate formed

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A little NH3(aq) Blue precipitate

Excess NH3(aq) Blue precipitate soluble in excess NH3 to form

Fe2+

Dirty green precipitate

Dirty green precipitate insoluble in excess NaOH

Dirty green precipitate

Fe3+

Brown precipitate

Brown precipitate insoluble in excess NaOH

Brown precipitate

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DARK BLUE solution Dirty green precipitate insoluble in excess NH3 Brown precipitate insoluble in excess NH3

14. CONFIRMATORY TESTS FOR Pb2+, NH4+, Fe2+, Fe3+ IONS CATIONS

SPECIFIC REAGENT KI, NaI,

KCl, NaCl, HCl Pb2+ Al

NH4+

Fe2+

Fe3+

OBSERVATION Pb – Yellow precipitate, soluble in hot water & recrystallises when cooled Al – white precipitate Pb – White precipitate, soluble in hot water & recrystallises when cooled Al – white precipitate – soluble in water

K2SO4, Na2SO4, H2SO4,

Pb – White precipitate, insoluble in hot water Al – white precipitate soluble in water

Nesslar reagent Potassium hexacyanoferrate (II), K4Fe(CN)6 Potassium hexacyanoferrate (III), K3Fe(CN)6 Acidified KMnO4 Potassium thiocyanate, KSCN Potassium hexacyanoferrate (II), K4Fe(CN)6 Potassium hexacyanoferrate (III), K3Fe(CN)6

Brown precipitate Light blue precipitate

Prussian blue (dark blue) precipitate Purple colour decolourises Blood red colour Turnbull’s blue (dark blue) precipitate Greenish-brown solution

Fe(NO3)3 Iron (III) nitrate

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-

TO DESCRIBE A TEST u must write i. name of the test (if any) ii. “mini” procedure of the test iii. Observable result brown ring test Add dilute sulphuric acid to salt P solution Add freshly made Iron (II) sulphate solution Slowly add concentrated Sulphuric acid to the solution

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A brown ring can be seen

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