Chapter 11 - Qualitative Analysis Flashcards
What does qualitative analysis refer to?
Salts are ionic compounds which contains cations and anions. You can identify the cations and anions by adding certain reagents to a portion of the salt solution. This process of identifying an unknown substance (cations and anions) is aka QA
What are the 3 things that there are chemical tests to identify?
1) Cations
2) Anions
3) Gases
What are the reagents used to identify cations?
1) Aqueous sodium hydroxide solution, NaOH (aq)
2) Aqueous ammonia, NH3 (aq)
All cations give precipitates with the alkalis except for ____, ____, ____
Na+, K+, NH4+
What factors are used to identify a cation?
1) Colour of precipitate produced (if any), when reagent is added
2) Whether the precipitate is soluble or insoluble in excess of the reagent
3) Whether ammonia gas is evolved on the addition of aqueous sodium hydroxide solution
What are the 2 stages for adding NaOH (aq) or NH3 (aq) to a solution containing cations?
1) Add a few drops
2) Add excess
* Look out for the colour of precipitate, and whether it dissolves in excess
What are the cations to be identified?
1) Aluminium ion, Al^3+
2) Zinc ion, Zn^2+
3) Iron (II) ion, Fe^2+
4) Iron (III) ion, Fe^3+
5) Ammonium ion, NH4+
6) Calcium ion, Ca^2+
7) Copper (II) ion, Cu^2+
8) Lead (II) ion, PB^2+
What can you observe when you add a few drops of sodium hydroxide, NaOH, to calcium, Ca^2+ ?
White precipitate of calcium hydroxide forms, Ca(OH)2
Ca^2+ (aq) + 2OH^- (aq) –> Ca(OH)2 (s)
What can you observe when you add in excess sodium hydroxide, NaOH, to calcium, Ca^2+?
White precipitate remained, so its insoluble in excess of NaOH (aq)
What can you observe when you add a few drops of sodium hydroxide, NaOH, to zinc ion, Zn^2+?
White precipitate of zinc hydroxide, Zn(OH)2, is formed
What can you observe when you add in excess sodium hydroxide, NaOH, to zinc ion, Zn^2+??
White precipitate dissolves to form a colourless solution
What can you observe when you add a few drops of sodium hydroxide, NaOH, to aluminium ion, Al^3+?
White precipitate of aluminium hydroxide, Al(OH)3, is formed
Al^3+ (aq) + 3OH^- (aq) –> Al(OH)3 (s)
What can you observe when you add in excess sodium hydroxide, NaOH, to aluminium ion, Al^3+?
White precipitate dissolves to form a colourless solution
What can you observe when you add a few drops of sodium hydroxide, NaOH, to iron (II) ion, Fe^2+?
Green precipitate of, iron (II) hydroxide, Fe(OH)2 is formed
Fe^2+ (aq) + 2OH^- (aq) –> Fe(OH)2 (s)
What can you observe when you add in excess sodium hydroxide, NaOH, to iron (II) ion, Fe^2+?
Green precipitate remains as the precipitate is insoluble in excess. the precipitate turns brown on standing
What can you observe when you add in a few drops of sodium hydroxide, NaOH, to iron (III) ion, Fe^3+?
Reddish brown precipitate of, iron (III) hydroxide, Fe(OH)3 formed
Fe^3+ (aq) + 3OH^- (aq) –> Fe(OH)3 (s)
What can you observe when you add in excess sodium hydroxide, NaOH, to iron (III) ion, Fe^3+?
Reddish brown precipitate remained, as it is insoluble in excess
What can you observe when you add in a few drops of sodium hydroxide, NaOH, to copper (II) ion, Cu^2+?
Blue precipitate of, Cu(OH)2 formed
Cu^2+ (aq) + 2OH^- (aq) –> Cu(OH)2 (s)
What can you observe when you add in excess sodium hydroxide, NaOH, to copper (II) ion, Cu^2+?
Blue precipitate remains as precipitate is insoluble in excess
What can you observe when you add in a few drops of sodium hydroxide, NaOH, to ammonium ion, NH4+?
No precipitate formed
Ammonia gas (NH3) released when heated
What can you observe when you add in a few drops of ammonia, NH3, to calcium ion, Ca^2+?
No precipitate
What can you observe when you add in excess ammonia, NH3, to calcium ion, Ca^2+?
No change is observed
What can you observe when you add in a few drops of ammonia, NH3, to zinc ion, Zn^2+?
White precipitate of zinc hydroxide, Zn(OH)2 is formed
Zn^2+ (aq) + 2OH^- (aq) –> Zn(OH)2 (s)
What can you observe when you add in excess ammonia, NH3, to zinc ion, Zn^2+?
White precipitate dissolves in excess to form a colourless solution
What can you observe when you add in a few drops of ammonia, NH3, to aluminium ion, Al^3+?
White precipitate of aluminium hydroxide, Al(OH)3 is formed
Al^3+ (aq) + 2OH^- (aq) –> Zn(OH)2 (s)
What can you observe when you add in excess ammonia, NH3, to aluminium ion, Al^3+?
White precipitate remained as it is insoluble in excess
What can you observe when you add in a few drops of ammonia, NH3, to iron (II) ion, Fe^2+?
Green precipitate of iron (II) hydroxide, Fe(OH)2 is formed
Fe^2+ (aq) + 2OH^- (aq) –> Fe(OH)2 (s)
What can you observe when you add in excess ammonia, NH3, to iron (II) ion, Fe^2+?
Green precipitate remained as it is insoluble in excess. The precipitate turns brown on standing
What can you observe when you add in a few drops of ammonia, NH3, to iron (III) ion, Fe^3+?
Reddish brown precipitate of iron(III) hydroxide, Fe(OH)3 formed
Fe^3+ (aq) + 3OH^- (aq) –> Fe(OH)3 (s)
What can you observe when you add in excess ammonia, NH3, to iron (III) ion, Fe^3+?
Reddish brown precipitate remained as it is insoluble in excess
What can you observe when you add in a few drops of ammonia, NH3, to copper (II) ion, Cu^2+?
A light blue precipitate of copper (II) hydroxide, Cu(OH)2 is formed
Fe^3+ (aq) + 3OH^- (aq) –> Cu(OH)2 (s)
What can you observe when you add in excess ammonia, NH3, to copper (II) ion, Cu^2+?
Light blue precipitate dissolves in excess to form a dark blue solution
The precipitate formed in each reaction is the ____ of the metal ion
Hydroxide
Are hydroxides soluble in water?
Most hydroxides are insoluble in water except for group 1 hydroxides and aqueous ammonia
Some precipitates dissolve in excess aqueous sodium hydroxide or aqueous ammonia. What reacts with the reagents and what do they form?
When reagents are added in excess, the precipitate reacts with the reagents to form soluble compounds which can dissolve in water
How is identifying anions different from identifying cations?
Different anions are identified by adding different, specific reagents unlike testing cations where only 2 reagents are added
What are the 5 anions to be identified?
1) Nitrate ion, NO3-
2) Carbonate ion, CO3^2-
3) Chloride ion, Cl-
4) Iodide ion, I-
5) Sulfate ion, SO4^2-
What do you look out for after adding the reagents when testing for anions?
1) For NO3-, CO3^2-: Test for gas released
2) For Cl-, I-, SO4^2-: Observe the colour of the precipitation
What reagent is used to test for anions in nitrate ion, NO3-?
1) Add aqueous sodium hydroxide, (NaOH), and aluminium powder/foil
2) Warm carefully, use a damp red litmus paper to test for gas released
What can be observed when testing for anions in nitrate ion, NO3-?
1) Effervescence is observed due to the release of NH3 gas
2) Damp red litmus paper turns blue (NH3 is a weak alkali)
What reagent is used to test for anions in carbonate ion, CO3^2-?
1) Add any acid
2) Bubble the gas into limewater to test for gas released
What can be observed when testing for anions in carbonate ion, CO3^2-?
1) Effervescence is observed due to the release of CO2 gas
2) White precipitate formed in lime water when CO2 is bubbled into it
What reagent is used to test for anions in chloride ion, Cl-?
Add dilute nitric acid and then aqueous silver nitrate solution
What can be observed when testing for anions in chloride ion, Cl-?
A white precipitate of silver chloride is formed
Ag+ (aq) + Cl- (aq) –> AgCl (s)
What reagent is used to test for anions in iodide ion, I-?
Add dilute nitric acid and then aqueous silver nitrate solution
What can be observed when testing for anions in iodide ion, I-?
A yellow precipitate of silver iodide is formed
Ag+ (aq) + I- (aq) –> AgI (s)
What reagent is used to test for anions in sulfate ion, SO4^2-?
Add dilute nitric acid and then aqueous barium nitrate solution
What can be observed when testing for anions in sulfate ion, SO4^2-?
A white precipitate of barium sulfate is formed
Ba^2+ (aq) + SO4^2- (aq) –> BaSO4 (s)
Why is dilute nitric acid added to identify chloride, iodide, and sulfate ions?
Nitric acid is added to remove the presence of any carbonate or hydroxide ions, ensuring there is no error in the analysis obtained
What are the gases to be identified?
1) Ammonia, NH3
2) Carbon dioxide, Co2
3) Chlorine, Cl2
4) Hydrogen, H2
5) Oxygen, O2
6) Sulfur dioxide, SO2
Which 3 gases are odourless?
CO2, H2, O2
Which 3 gases are pungent?
NH3, Cl2, SO2
What is the testing reagent and observation for carbon dioxide?
Bubble the gas through limewater. If carbon dioxide is present, a white precipitate is formed and dissolves in excess CO2
What is the testing reagent and observation for hydrogen?
Place a burning splint at the mouth of a test tube. If hydrogen is present, the burning splint extinguishes with a ‘pop’ sound
What is the testing reagent and observation for oxygen?
Insert a glowing splint into test tube. If oxygen is present, the glowing splint relights
What is the testing reagent and observation for ammonia?
Place a piece of damp red litmus paper at the mouth of the test tube. If ammonia is present, the damp red litmus paper, turns blue
What is the testing reagent and observation for chlorine?
Place a piece of damp blue litmus paper at the mouth of a test tube. Chlorine is present if damp blue litmus paper turns red, and is then bleached
What is the testing reagent and observation for sulfur dioxide?
Place a piece of filter paper soaked with acidified potassium manganate (VII) at the mouth of a test tube. If sulfur dioxide is present, the purple acidified potassium manganate (VII) turns colourless
