Chemistry - Qualitative Analysis

qualitative analysis

the instance in which the composition of a substance is determined by conducting tests that produce results associated with its constituents

What are the two methods used to identify cations?

Flame tests and reactions with diagnostic reagents

Describe the flame test and its methodology (1/3)

The flame test is the instance in which a sample of the unknown substance is heated with a colorless flame and a flame is produced a color that correlates with a particular element

1. Clean a platinum or Nichrome wire by dipping it repeatedly in concentrated HCl.

2. Place or dip a sample of the unknown substance on the end of the wire

3. Heat the sample onto a non-luminous Bunsen burner flame and notice any change in color of the flame

What are the relevant results of a flame test? (5)

Potassium - Lilac
Sodium - Yellow
Calcium - Brick-red
Barium - Yellow-green
Copper - Blue-green

Which diagnostic reagents are relevantly used to identify the presence of cations? (3)

Sodium carbonate, sodium hydroxide and ammonia (all aqueous)

Describe the expected results from a test with sodium carbonate and denote an ionic equation for the dropwise reaction

The cations form metal carbonates

Dropwise: nMⁿ⁺ (aq) + CO₃²⁻ (aq) +Mₙ(CO₃)ₙ

Describe the expected formation from dropwise reactions and excess reactions with a cationic solution and sodium hydroxide, stating which precipitates dissolve in excess. Create ionic equations for both (2/4/2)

The cations form metal hydroxides

Aluminium, lead (II), chromium (III) and zinc hydroxides dissolve in excess sodium hydroxide to form tetrahydroxo-compelexes

Dropwise equation: nMⁿ⁺ (aq) + OH⁻ (aq) → Mₙ(OH)ₙ (s)
Excess: M(OH)ₙ (s) + nOH⁻ (aq) → [M(OH)₄]ⁿ⁻ (aq)

Describe the expected formation from dropwise reactions and excess reactions of cationic solutions with ammonia, stating which hydroxides dissolve in excess and any exceptions. Create ionic equations for both (3/2/2)

The cations form metal hydroxides. Calcium and barium do not react with ammonia.

Copper (II) and zinc hydroxides dissolve in excess ammonia to form ammine complexes.

Dropwise equation: nMⁿ⁺ (aq) + OH⁻ (aq) → Mₙ(OH)ₙ (s)
Excess equation: Mₙ(OH)ₙ (s) + 4NH₃ (aq) → [Mₙ(NH₃)₄]ⁿ⁺ (aq) + nOH⁻ (aq)

State the colors of the precipitates of the metal hydroxides/carbonates and why ammonia does not form a precipitate in carbonate or hydroxide solution (7/1)

All non-transitional metals form white precipitates.

The transitional metals will form the following coloured precipitates: chromium (II) — grey-green; manganese (II) — white → brown (in air); iron (II) — green; iron (III) — brown; copper (II) → blue

Ammonia does not form a precipitate because it forms an unstable hydroxide/carbonate that easily decomposes upon heating to form ammonia gas which turns moist red litmus paper blue

What are the two main tests used to identify carbonates? (2)

Acid test and metal-cation test

Describe the acid test for carbonates and state an ionic equation for the expected reaction

Dilute HCl or concentrated sulfuric acid is added to the substance and carbon dioxide is liberated, which is testable as it turns moist blue litmus paper red

CO₃²⁻(s) + 2H⁺(aq) → H₂O (l) + CO₂ (g)

Describe the metal cation test for carbonates and state ionic equations for the expected reactions (1/2)

Aqueous barium or calcium is added to the solution to form a metal carbonate which dissolves in acid to form carbon dioxide which turns blue moist litmus paper red

• Before acid: nMⁿ⁺ (aq) + nCO₃²⁻(aq) → Mₙ(CO₃)ₙ

• After acid: CO₃²⁻(s) + 2H⁺(aq) → H₂O (l) + CO₂ (g)

What are the two main tests used to identify sulphates? (2)

Acidified barium test and lead test

Describe the acidified barium test and state an ionic equation for its expected reaction

Acidified barium ions are introduced to the solution with the intention of forming the insoluble barium sulphate

Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)

Describe the lead test to identify sulphates and state an ionic equation for its expected reaction

Lead (II) ions are introduced to the solution with the intention of forming the water-insoluble lead (II) sulphate, which dissolves in hot concentrated sulfuric acid

Pb²⁺ (aq) + SO₄²⁻ (aq) → PbSO₄ (s)

What are the four main tests used to determine the presence of nitrates in a substance? (4)

Decomposition test, copper test, powdered reducing agent test and brown ring test

Describe the decomposition test for nitrates and state ionic equations for its expected reactions

Concentrated sulfuric acid is added to the substance and nitric acid gas and nitrogen dioxide are given off. Nitric acid gas formed decomposes into nitrogen dioxide and oxygen. Confirmation is found in the evolution of a brown nitrogen dioxide.

Before decomposition: NO₃⁻(aq) + H₂SO₄ (aq) → HNO₃ (g) + HSO₄⁻ (aq)
Decomposition: 4HNO₃ (g) → 2H₂O (l) + 4NO₂ (g) + O₂ (g)

Describe the copper test to identify nitrates and state ionic equations for its expected reactions

Concentrated sulfuric acid is added to the substance in the presence of copper and nitric acid is liberated which reacts with the copper to form nitrogen dioxide and a blue-green solution

Before copper: NO₃⁻(aq) + H₂SO₄ (aq) → HNO₃ (g) + HSO₄⁻ (aq)
Copper: 4HNO₃ (g) + Cu (s) → Cu(NO₃)₂ (aq) + 2NO₂ (g) + 2H₂O (l)

Describe the powdered reducing agent test for identifying nitrates and state an ionic equation for the expected reaction

Powdered aluminum or zinc and aqueous NaOH are added to the substance and upon heating ammonia gas is given off which turns moist red litmus paper blue.

NO₃⁻ (s) + nM(s) + nOH⁻ (aq) + 6H₂O (l) → n[M(OH)₄]ⁿ⁻ (aq) + NH₃ (g)

Describe the brown ring test and state an ionic equation for its expected reaction

Iron (II) sulphate is mixed with the solution followed by a gentle addition of sulfuric acid to form a layer above the iron solution from which iron (II) nitrosyl sulphate evolves forming a brown ring

NO₃⁻ (aq) + 3Fe²⁺ (s) + 4H⁺ (aq) → NO (g) + 3Fe³⁺ (aq) + 2H₂O (l)

NO (g) + FeSO₄ (s) + Fe(NO)SO₄ (s)

What are the three main tests used to identify sulphites?

Acid test, barium test and silver nitrate test

Describe the acid test to identify sulfites and state an ionic equation for the expected reaction

Dilute HCl or sulfuric acid is added to the solution and sulfur dioxide is evolved when the solution is warmed, which turns moist blue litmus paper red

SO₃²⁻ (aq) +2H⁺ (aq) → SO₂ (g) + H₂O (l)

Describe the barium test to identify sulfites and state ionic equations for its expected reactions

Barium is introduced to the solution to form barium sulfite which readily dissolves in HCl and sulfur dioxide gas is liberated which turns moist blue litmus paper red

Before acid: Ba²⁺ (aq) + SO₃²⁻ (aq) → BaSO₃ (s)

After acid: SO₃²⁻ (aq) +2H⁺ (aq) → SO₂ (g) + H₂O (l)

Describe the silver nitrate test to identify sulfites and state an ionic equation for its expected reaction

Silver nitrate is added to the solution and silver sulfite is formed which turns from grey to black on warming as it decomposes to silver

2Ag⁺ (aq) + SO₃²⁻ (aq) → Ag₂SO₃ (s)

What are three main tests to identify halides?

Silver nitrate test, sulfuric acid test and lead test

Describe the silver nitrate test to identify halides, state an ionic equation for its expected reaction and how you would differentiate between the halides (1/1/3)

Silver nitrate is added to the solution followed by aqueous ammonia to form a silver halide

Ag⁺ (aq) + [Halide](aq) → Ag[Halide]

Silver chloride is white and dissolves in ammonia. Silver bromide is cream and partially dissolves in ammonia. Silver iodide is yellow and is insoluble in ammonia

Describe the sulfuric acid test to identify halides, state ionic equations for its expected reactions and how you would differentiate between the halides (1/3/1)

Concentrated sulfuric acid is added to the solution and a hydrogen halide gas evolves.

HCl gas is colourless; HBr is brown-red and HI is colourless but decomposes into a violet iodine precipitate when warmed

H₂SO₄ (aq) + [Halide]⁻ (s) → H[Halide] (g) + HSO₄⁻ (aq)
Decomposition of hydrogen iodide gas: 2HI (g) + [O] → I₂ (s) + H₂O (l)

Describe the lead test to identify halides, state ionic equations for its expected reactions and how you’d differentiate between the lead halides (1/3/1)

Lead ions are introduced to the solution and a lead halide precipitate forms.

Lead chloride and lead bromide are both white but can be differentiated through water solubility as the bromide is more soluble. Lead iodide is a yellow precipitate that dissolves in excess iodide solution.

Formation of lead halide: Pb²⁺ (aq) + 2[Halide]⁻ (aq) → Pb[Halide]₂ (s)
Dissolution of lead iodide: PbI₂ (s) + 2I⁻ (aq) → [PbI₄]²⁻ (aq)

What are the three main tests used to identify chromates (VI)?

Silver nitrate test, barium test and lead test

Describe the silver nitrate test for chromates (VI) and state an ionic equation for its expected reaction

Silver nitrated solution and the red-brown silver chromate (VI) precipitate forms which is soluble in ammonia

2Ag⁺ (aq) + CrO₄²⁻ (aq) → AgCrO₄ (s)

Describe the barium test to identify chromates (VI) and state an ionic equation for its expected reaction

Barium is introduced to the solution and a pale yellow barium chromate precipitate forms which is soluble in strong acids

Ba²⁺ (aq) + CrO₄²⁻ (aq) → BaCrO₄ (s)

Describe the lead test for chromates (VI) and state an ionic equation for its expected reaction

Lead is introduced to the solution and a yellow precipitate of lead chromate (VI) forms

Pb²⁺ aq) + CrO₄²⁻ (aq) → PbCrO₄ (s)