Unit 3 prac tests

Unit 3 tests

How to test for Ammonium ion ? Observations

COLD Sodium hydroxide to ‘ammonium salt’ and gas above with red litmus paper

• Smelly ammonia released (room temp)

• Red litmus → blue (gentle warming)

How to test for acid (not co3)? Observation

Litmus paper or universal pH indicator

• Litmus paper turns red

• Red - strong pH → Orange - middle → Yellow - Weak

How to test acid? Observation

CO₃ powder

• Effervesence usually CO₂

Flame test colours

Lithium - Red

Sodium - Yellow

Potassium - Lilac

Strontium - Crimson red

Calcium - Brick red

Barium - Apple green

Copper - Blue

Method of flame test

The metal salt is mixed with few drops of concentrated hydrochloric acid

Strongly heated in hottest part of bunsen burner flame

Chloride most volatile give stronger so → use of HCl

Why colours shown in flame test

Electrostatic excitation to higher energy level

Light emitted as electron fall down to lower energy level

More accurate method than flame test

Spectroscope - Emission spectroscopy

Specific emission lines of characteristic frequency

Magnesium and OH^- and NH₃

NaOH

Mg²⁺ + 2OH^- → Mg(OH)₂ White ppt → excess insoluble

NH₃ → white ppt → excess insoluble

• No change in excess → not amphoteric

Calcium and OH^- and NH₃

NaOH

Ca²⁺ + 2OH^- → Ca(OH)₂ white ppt → insoluble in excess

NH₃ no ppt

Copper and OH^- and NH₃

NaOH Cu²⁺ + 2OH^- → Cu(OH)₂ blue ppt → Insoluble in

NH_{3 -} Cu(OH)₂ light blue ppt

[Cu(H₂O)₆] ²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂] ²⁺ + 4H₂O DEEP BLUE COMPLEX

Iron and OH^- and NH₃

NaOH

Fe^2/3+ + 2/3 OH^- → Fe(OH)_2/3 GREEN / BROWN ppt

NH₃ Same

Distinguishing Iron oxidation states

Potassium hexacyanoferrate solution

Fe²⁺ → Deep blue ppt → Turnball’s blue

Fe²⁺ → Deep blue ppt → Prussian blue

Aluminium OH^-

NaOH Al³⁺ + 3OH^- → Al(OH)₃ WHITE PPT

Excess - DISSOLVES Al(OH)₃ + OH^- → [Al(OH)₄]^-

Aluminium and NH₃

Al³⁺ + 3OH^- → Al(OH)₃ WHITE PPT

Does NOT dissolve in excess

Zinc and OH^- and NH₃

NaOH Zn²⁺ + OH^- → Zn(OH)₂ WHITE PPT

Excess - DISSOLVES Zn(OH)₂ + 2OH^- → [Zn(OH)₄]^2-

NH₃ White ppt

Excess [Zn(H₂O)₆​]²⁺ + 4NH₃ ⇌ [Zn(NH₃​)₄ (H₂O)₂​]²⁺ SOLUBLE

Chromium and OH^- and NH₃

NaOH Cr³⁺ + 3OH^- → Cr(OH)₃ GREY GREEN PPT

Excess → Cr(OH)₃ + 3OH^- → [Cr(OH)₆]^3- DARK GREEN

NH₃ → Cr(OH)₃ Grey green ppt

Insoluble in excess

Manganese and OH^- and NH₃

NaOH Mn²⁺ + 2OH^- → Mn(OH)₂ OFF WHITE PPT → Oxidise to brown MnO₂

Excess → Insoluble

NH₃ same behaviour

Lead and OH^- and NH₃

NaOH Pb²⁺ + 2OH^- → Pb(OH)₂ WHITE PPT

Excess → Pb(OH)₂ + 2OH^- → [Pb(OH)₄]^2-

NH₃ → Pb(OH)₂ does not dissolve in excess

Barium and OH^- and NH₃

No ppt formed → too soluble

Lead and potassium iodide test

Pb²⁺ + 2I^- → PbI₂ Yellow ppt

Thermal decomposition of carbonates

Carbonate → metal oxide and Co2

Copper (2) carbonate

CuCO₃ → CuO + Co₂

GREEN solid → BLACK solid + colourless gas

• Co2 limewater test → Milky

Zinc carbonate

ZnCO₃ → ZnO + CO₂

WHITE solid → YELLOW (HOT) WHITE (COOL) and colourless gass

• Limewater test → Milkt