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Qualitative Analysis Flashcards

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1
Q

6 gases to identify

A

Hydrogen, Oxygen, Carbon Dioxide, Ammonia, Chlorine, Sulfur Dioxide

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2
Q

A gas is liberated if e__________ or f__________ are observed

A

Effervescence or Fumes

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3
Q

3 preliminary tests for identifying gases

A

Colour, smell and reaction with moist litmus paper

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4
Q

2 considerations when using moist litmus paper to test

A
  1. When testing a gas with the litmus paper, the litmus paper must be moistened with distilled water.
  2. Hold the litmus paper above the mouth of the test tube. Ensure the litmus paper does not touch the sides of the test tube.
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5
Q

Colour, smell, acid-base nature and action on moist litmus paper of hydrogen.

A

Colourless, odourless, neutral, no effect

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6
Q

Specific test and observations when testing for hydrogen (assume positive)

A

Place a lighted splint at the mouth of the test tube. Gas evolved extinguishes a lighted splint with a ‘pop’ sound.

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7
Q

Colour, smell, acid-base nature and action on moist litmus paper of oxygen

A

Colourless, odourless, neutral, no effect

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8
Q

Specific test and observations when testing for oxygen (assume positive)

A

Insert a glowing splint into the test tube. Gas evolved relights/rekindles a glowing splint.

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9
Q

Colour, smell, acid-base nature and action on moist litmus paper of carbon dioxide

A

Colourless, odourless, acidic, mist blue litmus turns red.

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10
Q

Specific test and observations when testing for carbon dioxide (assume positive)

A

Bubble gas into limewater. Gas evolved forms a white precipitate when bubbled through limewater/calcium hydroxide.

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11
Q

Colour, smell, acid-base nature and action on moist litmus paper of chlorine.

A

Greenish-yellow, choking, irritating (swimming pool-like), acidic, moist blue litmus paper turns red then bleaches it (turns white)

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12
Q

Specific tests and observations when testing for chlorine (assume positive)

A

Place a moist blue litmus paper at the mouth of the test tube. Gas evolved turns moist blue litmus paper red.

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13
Q

Colour, smell, acid-base nature and action on moist litmus paper of sulfur dioxide.

A

Colourless, choking smell, acidic, moist blue litmus paper turns red

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14
Q

Specific tests and observations when testing for sulfur dioxide (assume positive)

A

Bubble the gas through an aqueous solution of acidified potassium manganate (VII) KMnO4 is an oxidising agent. Gas evolved turns acidified potassium manganate (VII) from purple to colourless.

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15
Q

Colour, smell, acid-base nature and action on moist litmus paper of ammonia

A

Colourless, pungent smell, basic, moist red litmus paper turns blue

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16
Q

Specific tests and observations when testing for ammonia

A

Place a moist red litmus paper at the mouth of the test tube. Pungent gas evolved turns moist red litmus paper blue.

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17
Q

3 ways to identify cations

A
  1. Using aqueous sodium hydroxide (NaOH)
  2. Using ammonia (NH3)
  3. Using flame tests
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18
Q

7 cations to identify

A
  1. Ammonium ion (NH4+)
  2. Calcium ion (Ca2+)
  3. Copper (II) ion (Cu2+)
  4. Iron (II) ion (Fe2+)
  5. Iron (III) ion (Fe3+)
  6. Zinc ion (Zn2+)
  7. Aluminum ion (Al3+)
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19
Q

The cation in a compound can be identified using ______________________ or _________________?

A

Aqueous sodium hydroxide and aqueous ammonia

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20
Q

Both aqueous sodium hydroxide and aqueous ammonia forms _________________ in solution

A

Hydroxide ions, OH-

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21
Q

Reaction of NaOH in solution

A

NaOH (aq) –> Na+ (aq) + OH- (aq)

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22
Q

Reaction of NH3 in solution

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A

NH3 (aq) + H2O (l) –> NH4+ (aq) + OH- (aq)

23
Q

Most metal ions react with the hydroxide ions to form insoluble metal hydroxides. These insoluble hydroxides appear as __________ in a solution.

24
Q

The possible identity of the cation can be deduced from?

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A

The colour of the precipitate and the solubility of the precipitate in excess NaOH or excess NH3+

25
How to form a solution of the cation
1. Since the tests for cations involve observing the precipitates formed in solutions, the unknown salt must first be dissolved in distilled water to form a salt solution. 2. The mixture is warmed is the substance does not dissolve in cold water 3. If the substance still does not dissolve in water, dilute nitric acid is added to dissolve it.
26
Process of testing with aqueous NaOH
1. Add sodium hydroxide dropwise to an unknown solution 2. Observe the precipitate formed and note its colour. If no precipitate is formed, warm the mixture and test for ammonia gas. 3. Add more sodium hydroxide dropwise till in excess. Shake the mixture and examine whether the precipitate dissolves in excess NaOH.
27
Possible results of testing with sodium hydroxide solution
1. Precipitate is soluble in excess NaOH 2. Precipitate is insoluble in excess NaOH
28
Ca2+ reaction with aqueous NaOH and ionic equation
A white precipitate is formed. White precpitate is insoluble in excess NaOH. Ca(OH)2 is formed, Ca2+ is present. Ca2+ (aq) + 2OH- (aq) --> Ca(OH)2 (s)
29
Zn2+ reaction with aqueous NaOH and ionic equation
A white precipitate is formed. White precipitate is soluble in excess NaOH to form a colourless solution. Zn(OH)2 is formed, Zn2+ is present. Colourless complex of [Zn(OH)4]2- is formed. Zn2+ (aq) + 2(OH)- (aq) --> Zn(OH)2 (s) Zn(OH)2 (s) + 2OH- (aq) --> [Zn(OH4)]2- (aq) (soluble complex)
30
Al3+ reaction with aqueous NaOH and ionic equation.
A white precipitate is formed. White precipitate is soluble in excess NaOH to form a colourless solution. Al(OH)3 is formed, Al3+ is present. Colourless complex of [Al(OH)4]- is formed. Al3+ (aq) + 3OH- (aq) --> Al(OH)3(s) Al(OH)3 (s) + OH- (aq) --> [Al(OH)4]- (aq)
31
Cu2+ reaction with aqueous NaOH and ionic equation
A light blue precipitate is formed. Light blue precipitate is insoluble in excess NaOH. Cu(OH)2 is formed, Cu2+ is present. Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)
32
Fe2+ reaction with aqueous NaOH and ionic equation
A green precipitate is formed. Green precipitate is insoluble in excess NaOH. Green precipitate turns into brown precipitate on contact with air. Fe (OH)2 is formed, Fe2+ is present. Green Fe(OH)2 is oxidised to brown Fe(OH)3, Fe2+ is present Fe2+ + 2OH- (aq) --> Fe(OH)2 (s) Fe(OH)2 (s) --> Fe(OH)3 (s)
33
Fe3+ reaction with aqueous NaOH and ionic equation
A red brown precipitate is formed. Red brown precipitate is insoluble in excess NaOH. Fe(OH)3 is formed, Fe3+ is present. Fe3+ (aq) + 3OH- (aq) --> Fe(OH)3 (s)
33
NH4+ reaction with aqueous NaOH and ionic equation
No precipitate is formed. On heating, pungent gas evolved turns moist red litmus paper blue. Gas is ammonia, NH3. NH4+ is present. NH4+ (aq) + OH- (aq) --> NH3 (g) + H2O (l)
34
How to identify a cation using aqueous NH3?
1. Add aqueous ammonia dropwise to an unknown solution 2. Observe the precipitate formed and note its colour 3. Add more aqueous ammonia solution dropwise till in excess. Shake the mixture and examine whether the precipitate dissolves in excess aqueous NH3
35
Ca2+ reaction with aqueous NH3
No precipitate is formed, Ca2+ is present
36
Zn2+ reaction with aqueous NH3 and ionic equation
A white precipitate is formed. White precipitate is soluble in excess NH3 (aq) to form a colourless solution. Zn(OH)2 is formed,Zn2+ is present. Colourless complex of [Zn(NH3)4]2+ is formed. Zn2+ (aq) + 2OH- (aq) --> Zn(OH)2 (s) Zn(OH)2 (s) + 2OH- (aq) --> [Zn(NH3)4]2+ (aq)
37
Al3+ reaction with aqueous NH3 and ionic equaton
A white precipitate is formed. White precipitate is insoluble in excess NH3 (aq). Al(OH)3 is formed, Al3+ is present. Al3+ (aq) + 3OH- (aq) --> Al(OH)3 (s)
38
Cu2+ reaction with aqueous NH3 and ionic equation.
A blue precipitate is formed. Blue precipitate is soluble in excess NH3 (aq) to form a dark blue solution. Cu(OH)2 is formed, Cu2+ is present. Dark blue complex of [Cu(NH3)4]2+ is formed. Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s) Cu(OH)2 (s) + 2OH- (aq) --> [Cu(NH3)4]2+
39
Fe2+ reaction with aqueous NH3 and ionic equation
A green precipitate is formed. Green precipitate is insoluble in excess NH3 (aq). Green precipitate turns into brown precipitate on contact with air. Fe(OH)2 is formed, Fe2+ is present. Green Fe(OH)2 is oxidised to brown Fe(OH)3, Fe2+ is present. Fe2+ (aq) + 2OH- (aq) --> Fe(OH)2 (s) Fe(OH)2 (s) --> Fe(OH)3 (s)
40
Fe3+ reaction with aqueous NH3 and ionic equation
A red brown precipitate is formed. Red brown precipitate is insoluble in excess NH3 (aq). Fe(OH)3 is formed, Fe3+ is present. Fe3+ (aq) + 3OH- (aq) --> Fe(OH)3 (s)
41
Which 3 ions give similar observations with both aqueous NaOH and aqueous NH3?
Fe2+, Fe3+, Zn2+
42
What type of substance can the flame tests be performed on?
Solid and aqueous solution
43
What cation is present when the flame is brick red?
Ca2+
44
What cation is present when the flame is bluish-green?
Cu2+
45
What cation is present when the flame is lilac?
K+
46
What cation is present when the flame is bright yellow?
Na+
47
What cation is present when the flame is apple-green?
Ba2+
48
5 anions that we need to identify
Nitrate, carbonate, chloride, iodide, sulfate
49
How to test for carbonate(assume positive)
Add dilute HNO3 to the unknown. Test the gas evolved by bubbling the gas through Ca(OH)2. Effervescence is observed. The gas evolved forms a white precipitate with limewater. Gas evolved is CO2, CO3 2- is present. CO3 2- + 2HNO3 --> 2NO3- + H2O + CO2 CO2 + Ca(OH)2 --> CaCO3 + H2O
50
How to test for nitrate (assume positive)
Add aqueous NaOH, aluminum powder or Al foil (Devarda's alloy) and then warm. Test the gas evolved using moist red litmus paper. Effervescence is observed and the gas evolved turns moist red litmus blue. Gas evolved is NH3, NO3- is present. NO3- is reduced to NH3+.
51
How to test for chloride
Acidify the unknown solution with dilute nitric acid. Add lead(II) nitrate, Pb(NO3)2 or silver nitrate, AgNO3 solution. White precipitate is formed. If Pb(NO3)2 is used, the white precipitate is PbCl2. Cl- is present. If AgNO3 is used, the white precipitate is AgCl, Cl- is present. Pb2+ + 2Cl- --> PbCl2 Ag+ + Cl- --> AgCl
52
How to test for iodide
Acidify the unknown soltution with dilute nitric acid. Add lead (II) nitrate, Pb(NO3)@ or silver nitrate, AgNO3 solution. Yellos precipitate formed. If Pb(NO3)2 used, the yellow precipitate is PbI2, I- is present. If AgNO3 is used, the yellow precipitate is AgI, I- is present. Pb2+ + 2I- --> PbI2 Ag+ + I- --> AgI
53
How to test for sulfate
Acidify the unknown solution with dilute nitric acid. Add barium chloride, BaCl2 or barium nitrate, Ba(NO3)2 solution. White precipitate formed. White precipitate is BaSo4, SO4 2- is present. Ba2+ + SO4 2- --> BaSO4

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