RP4 : Testing for Cations : Group 2 , NH4+

Question 1

what’s the method

Answer 1

1) place about 10 drops of 0.1moldm-3 metal ion solution in a test tube
2) Add about 10 drops of 0.6moldm-3 sodium hydroxide solution , mixing well
3) Continue to add sodium hydroxide solution , dropwise with gentle shaking until in excess

Question 2

what can this test be used on

Answer 2

group 2 metal ions and transition metal ions

Question 3

what are the results for group 2

Answer 3

Magnesium hydroxide :
- insoluble in water so white ppt
- slightly alkaline (pH 9) so some OH- ions must have been produced by a slight dissolving
Calcium hydroxide :
- Partially soluble in water so white ppt
- More alkaline (pH 11) as it’s more soluble so there will be more OH- present in solution
Strontium and Barium salts :
- Not form a hydroxide ppt due to their high solubility .
Highly alkaline

Question 4

What are the results for Group 3

Answer 4

• Aluminium salts :
  white ppt of aluminium hydroxide which will dissolve in excess NaOH to form a colourless solution
  [Al(H2O)6]3+ + 3OH- => Al(H2O)3(OH)3 (s) + 3H2O
  Al(H2O)3(OH)3(s) + OH- => [Al(OH)4]- + 3H2O

Question 5

What are the results for transition metals

Answer 5

Copper :
Forms a blue ppt
[Cu(H2O)6]2+ + 2OH- => Cu(H2O)4(OH)2 (s) + 2H2O
Iron (II) :
forms a green ppt
[Fe(H2O)6]2+ + 2OH- => Fe(H2O)4(OH)2 (s) + 2H2O
Iron (III) :
forms a brown ppt
[Fe(H2O)6]3+ + 3OH- => Fe(H2O)3(OH)3 (s) + 3H2O

Question 6

what’s the method for testing for ammonium ions ( NH4+)

Answer 6

1) place about 10 drops of 0.1moldm-3. ammonium chloride in a test tube
2) Add about 10 drops of 0.4moldm-3 sodium hydroxide solution . Shake the mixture
3) Warm the mixture in the test tube gently using a water bath
4) Test the fumes released from the mixed by holding a piece of damp red litmus paper in the mouth of the test tube

Question 7

what are the results of testing for ammonium ions

Answer 7

Alkaline ammonia gas is released which turns red litmus blue

Question 8

What’s the method for adding sulfate ions

Answer 8

1) place about 10 drops of 0.1moldm-3 metal ion solution in a test tube
2) Add about 10 drops of 0.1moldm-3 sulfuric acid
3) Continue to add sulfuric acid solution , dropwise with gentle shaking , until in excess

Question 9

what are the results of adding sulfate ions

Answer 9

Are an application of the trend that group 2 sulfates become less soluble down the group
Magnesium and calcium salts :
- Will not form a sulfate precipitate in addition of sulfate ions due to their high solubility
Strontium and barium :
- will form white ppt with addition of sulfate ions
Full equation : SrCl2 (aq) + Na2SO4(aq) => 2NaCl(aq) + SrSO4(s)
Ionic equation : Sr2+ + SO42- (aq) => SrSO4 (s)
Ionic equation : Ba2+ (aq) + SO42- (aq) => BaSO4(s)