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5.3 Precipitation Reactions

• Definition: A precipitation reaction forms an insoluble compound (precipitate) when ions in a solution combine.

• Natural Occurrence: Found in undersea hydrothermal vents creating mineral sulphide chimneys, supporting diverse aquatic communities.

• Applications: Used for removing minerals from drinking water, heavy metals from wastewater, and in water purification.

Ions in Solution

• Soluble vs. Insoluble: Some ionic compounds dissolve in water (e.g., NaCl, CuSO4) while others do not.

• Dissolution Process:

  • Example Equation:NaCl(s) → Na+(aq) + Cl-(aq)CuSO₄(s) → Cu²+(aq) + SO₄²-(aq)

Reactions Between Ionic Compounds in Solution

• Cloudiness Indication: Mixing colorless solutions, like silver nitrate and sodium chloride, can produce a cloudy precipitate (silver chloride).

• Ionic Interaction:

  • AgNO₃(aq) → Ag+(aq) & NO₃-(aq)

  • NaCl(aq) → Na+(aq) & Cl-(aq)

• New Combinations: Possible new combinations from mixing:

  • Ag+(aq) + Cl-(aq) → AgCl(s) (precipitate)

  • Na+(aq) + NO₃-(aq) (remains soluble)

Solubility Tables

• Usage: Solubility tables indicate which compounds are soluble or insoluble in water.

  • Soluble: Nitrates, sodium compounds.

  • Insoluble: Silver chloride is insoluble while most chlorides are soluble.

Example of Precipitation Reaction

• Silver Nitrate and Sodium Chloride:

  • Reactants: AgNO₃(aq) + NaCl(aq)

  • Products: AgCl(s) + NaNO₃(aq)

  • Spectator Ions: Na+ and NO₃- remain in solution.

Case Study: Limescale Accumulation

• Limescale Source: Resulting from precipitation of calcium carbonate when water is boiled, with high dissolved ion levels in hard water.

• Impact: Thin limescale can reduce heating efficiency significantly.

Predicting Precipitation Reactions

• Process: Identify ions, create potential new combinations, and use solubility tables to predict precipitate formation.

  • Example Reaction: Mixing potassium hydroxide with lead(II) nitrate forms lead(II) hydroxide precipitate.

Equations for Precipitation Reactions

• Full Equation Example:

  • AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).

• Ionic Equation:

  • Ag+(aq) + Cl-(aq) → AgCl(s) (spectator ions are omitted).

Historical Context of Pigments in Art

• Art Pigments: Historically derived from precipitation reactions; e.g., Prussian blue from iron(III) hexacyanoferrate(II).

• Examples of Pigments:

  • Cadmium Yellow (CdS)

  • Chrome Yellow (PbCrO₄)

• Impact of Chemistry on Art: Advancements in chemistry have improved pigment variety and reliability.