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Reaction of alkalis
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Reaction of alkalis

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📚 Detailed Review Report

Key Concepts to Remember:

  • Amphoteric Hydroxides: Zn²⁺, Pb²⁺, Al³⁺ ppts dissolve in excess NaOH.
  • Ammine Complexes: Zn²⁺, Cu²⁺, Ag⁺ ppts dissolve in excess NH₃.
  • Gas Collection: Upward (Mr < 28.8), Downward (Mr > 28.8). Collection over water is ONLY for insoluble gases.
  • Indicators: Litmus (Red→Blue), Methyl Orange (Red→Yellow), Phenolphthalein (Colourless→Pink/Red).
📚 Study Guide

Reaction with dilute NaOH

Ion Ppt? Colour Soluble in excess?
Al³⁺YesWhiteYes (Colourless)
Ca²⁺YesWhiteNo
Pb²⁺YesWhiteYes (Colourless)
Mg²⁺YesWhiteNo
Fe²⁺YesDirty greenNo
Fe³⁺YesRed brownNo
Cu²⁺YesPale blueNo
Zn²⁺YesWhiteYes (Colourless)

Reaction with aqueous NH₃

Ion Ppt? Colour Soluble in excess?
Al³⁺YesWhiteNo
Ca²⁺No——No
Pb²⁺YesWhiteNo
Mg²⁺YesWhiteNo
Fe²⁺YesDirty greenNo
Fe³⁺YesRed brownNo
Cu²⁺YesPale blueYes (Deep blue)
Zn²⁺YesWhiteYes (Colourless)

Redissolving Ppt (Complex Ions)

Zinc (Zn²⁺)

In excess NaOH:
Zn(OH)₂(s) + 2OH⁻(aq) → [Zn(OH)₄]²⁻(aq)

In excess NH₃:
Zn(OH)₂(s) + 4NH₃(aq) → [Zn(NH₃)₄]²⁺(aq) + 2OH⁻(aq)

Aluminium (Al³⁺)

In excess NaOH:
Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq)

In excess NH₃: Insoluble

Lead(II) (Pb²⁺)

In excess NaOH:
Pb(OH)₂(s) + 2OH⁻(aq) → [Pb(OH)₄]²⁻(aq)

In excess NH₃: Insoluble

Copper(II) (Cu²⁺)

In excess NaOH: Insoluble

In excess NH₃:
Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2OH⁻(aq)

Silver (Ag⁺)

In excess NaOH: Forms brown Ag₂O(s). Insoluble.

In excess NH₃:
Ag₂O(s) + 4NH₃(aq) + H₂O(l) → 2[Ag(NH₃)₂]⁺(aq) + 2OH⁻(aq)

Part A: Indicator Colours

IndicatorAcidAlkali
LitmusRedBlue
Methyl OrangeRedYellow
PhenolphthaleinColourlessPink/Red

Conclusion: Complex Ions

Amphoteric Hydroxides: Zn²⁺, Pb²⁺, and Al³⁺ precipitates redissolve in excess NaOH to form soluble complex ions.

Ammine Complexes: Zn²⁺, Cu²⁺, and Ag⁺ precipitates redissolve in excess NH₃ to form soluble ammine complexes.

Gas Collection Rules

  • Upward delivery: For gases less dense than air (Mr < 28.8).
  • Downward delivery: For gases denser than air (Mr > 28.8).
  • Over water: For insoluble/slightly soluble gases ONLY.
Part A: Indicators

Instruction: Select the correct colour. Cells light up instantly when correct.

Indicator HCl CH₃COOH NaOH NH₃

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