Qualitative analysis - KCSE Chemistry Paper 3 - Practicals

Process of identifying unknown compounds

Compounds may be:

1. Inorganic
2. organic

Inorganic analysis

This involve mainly identification of ionic compounds containing cations and anions.

Cations present in an ionic compounds are identified by adding a precipitating reagent that forms a precipitate unique to the cation/s in the compound.

The main precipitating reagents used are:

  2M NaOH and/or 2M NH_3(aq)

When using 2M sodium hydroxide:

1. No white precipitate formed if K + and Na + ions are present
2. No white precipitate formed if NH4+ ions are present but a colourless gas with pungent smell of urine is produced which may not be recognized in a school laboratory examination setting.
3. White precipitate that dissolves / soluble in excess if           Zn²⁺ Pb²⁺ Al³⁺ ions are present.
4. White precipitate that do not dissolves/insoluble in excess if Ba²⁺ Mg²⁺ Ca²⁺ ions are present.
5. Blue precipitate that do not dissolves /insoluble in excess if Cu²⁺ ions are present.
6. Green precipitate that do not dissolves/insoluble in excess if Fe²⁺ ions are present.
7. Brown precipitate that do not dissolves/insoluble in excess if Fe³⁺ ions are present.

When using 2M aqueous ammonia

1. No white precipitate is formed if K⁺ ,NH⁴⁺ Na⁺ ions are present
2. White precipitate that dissolves / soluble in excess if  Zn²⁺ ions are present.
3. White precipitate that do not dissolves/insoluble in excess if Ba²⁺ Mg²⁺ Ca²⁺ Pb²⁺ Al³⁺ ions are present.
4. Blue precipitate that dissolves /soluble in excess to form a deep/royal blue solution in excess if Cu²⁺ ions are present.
5. Green precipitate that do not dissolves/insoluble in excess if Fe²⁺ ions are present.
6. Brown precipitate that do not dissolves/insoluble in excess if Fe³⁺ ions are present.

Anions present in an ionic compounds are identified by adding a specific precipitating reagent that forms a precipitate unique to the specific anion/s in the compound.

Lead(II)nitrate(V) solution

Lead forms insoluble PbSO₄ ,PbSO₃ ,PbCO₃, PbS, PbI₂,PbCl₂

PbS is a black precipitate,

PbI₂ is a yellow precipitate.

All the others are white precipitates.

1. If a Lead(II)nitrate(V) solution is added to a substance/ solution/ compound :
   
   1. A yellow ppt shows presence of I^- ions
   2. A black ppt shows presence of S^2- ions
   3. A white ppt shows presence of SO₄^2- ,SO₃^2- ,CO₃^2- Cl^-
2. If  the white precipitate is added dilute nitric(V) acid

   1. It dissolves to show presence of SO₃^2- ,CO₃^2-

   2. It persist/remains to show presence of SO₄^2-, Cl^-
3. If the white precipitate in b(i) is added acidified potassium manganate(VII)/ dichromate(VI)

   1. acidified potassium manganate(VII) is decolorized /orange colour of acidified potassium dichromate(VI) turns to green to show presence of SO₃^2-

   2. acidified potassium manganate(VII) is not decolorized /orange colour of acidified potassium dichromate(VI) does not turn to green/remains orange to show absence of SO₃^2- /presence of CO₃^2-
4. If the white precipitate in b(ii) is boiled:

   1. It dissolves to show presence of Cl^-

   2. It persist/remains to show presence of SO₄^2-

Barium(II)nitrate(V)/Barium chloride solution

Barium(II)nitrate(V)/Barium chloride solution precipitates BaSO₄ ,BaSO₃ , BaCO₃,  from SO₄^2- ,SO₃^2- ,CO₃^2- ions.

Inorganic qualitative analysis require continous practice discussion

Organic analysis

This involve mainly identification of the functional group:

These functional groups can be identified by:

1. burning-a substance which “catches fire” must reduce in amount. Candidates should not confuse burning with flame coloration/test
2. Decolorization of bromine water/chlorine water/acidified KMnO 4 / to show presence of
               
   
   
3. Turning orange acidified K₂Cr₂O₇ to green to show presence as in above.
4. pH 1/2/3 for strongly acidic solutions. pH 4/5/6 for weakly acidic solutions
5. Turning blue litmus paper red. red litmus paper remaining red show presence of H⁺ ions