11. Salts Preparation (Lecture Notes - Student)

Lesson Overview

  • Subject: Salts Preparation

  • Course: A0001F Applied Science

  • Institution: Centre for Foundational Studies (CFS), Republic Polytechnic

  • Focus: Techniques for preparation, separation, and purification of salts.

Learning Outcomes

  • Ability to:

    • Describe techniques used for salt preparation, including:

      • Precipitation

      • Titration

      • Reactions of acids with metals, insoluble bases, and insoluble carbonates

    • Prepare a given salt from suitable starting materials using appropriate methods.

Types of Salts

Definition of a Salt

  • Formation: A salt forms when hydrogen ions of an acid are replaced by metal ions or ammonium ions.

    • E.g. MgO + 2 HCl → MgCl2 + 2 H2O

  • Soluble Salts:

    • Salts that dissolve in water to form an aqueous solution (e.g., NaCl).

  • Insoluble Salts:

    • Salts that precipitate out of solution and do not dissolve in water (e.g., PbCl2).

Solubility of Salts in Water

Soluble Salts:

  • All nitrate compounds

  • All ammonium compounds

  • All Group I metal compounds

  • All chlorides except AgCl and PbCl2

  • All bromides except AgBr and PbBr2

  • All iodides except AgI and PbI2

  • All sulfates except BaSO4, CaSO4, and PbSO4

Insoluble Salts:

  • Most carbonates (except ammonium carbonate and Group I metals)

  • Most metal hydroxides (except ammonium hydroxide and Group I metals)

Chemical Reactions Involved in Salt Preparation

Types of Reactions:

  1. Combination Reactions:

    • Two or more reactants combine to form one product.

    • E.g. A + B → AB

  2. Decomposition Reactions:

    • One reactant breaks down into multiple products.

    • E.g. Water → Hydrogen + Oxygen

  3. Combustion Reactions:

    • Release heat, typically with oxygen, producing CO2 and H2O.

  4. Single Replacement Reactions:

    • One element replaces another in a compound.

    • E.g. A + BC → AC + B

  5. Double Replacement Reactions:

    • Exchange of ions between two compounds.

    • Subtypes include:

      • Acid-Base Reactions: Acid + Base → Salt + Water

      • Precipitation Reactions: Formation of an insoluble ionic compound.

Preparation of Insoluble Salts (Precipitation)

  • Definition: Reaction of two soluble reactants to form an insoluble precipitate.

  • Example: Sequential preparation of silver chloride (AgCl).

    • Reactants: Silver nitrate (AgNO3) and sodium chloride (NaCl)

    • Reaction: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

  • Procedure: Dry precipitate between filter paper.

Preparation of Soluble Salts

Methods:

  1. For Ammonium/Group I Salts: Using titration.

    • Requires two soluble reactants (acid + ammonium hydroxide/base).

  2. For Non-Ammonium/Non-Group I Salts: Using an excess of an insoluble reactant.

    • React acid with moderately reactive metals or insoluble bases.

    • E.g., Acid + Zn(OH)2 producing soluble ZnCl2 in reaction with HCl.

Summary of Salt Preparation Techniques

  • Flowchart: Determines the correct method based on the solubility of the salt.

    • Titration for soluble salts.

    • Precipitation for insoluble salts.