Study Notes on Salts

Objective of Salt Preparation

• Identify an appropriate method of salt preparation based on solubility.

• General knowledge of the solubility of various salts: sulfates, nitrates, chlorides, carbonates, and bases.

Preparation Methods of Salts

7.7 - Salts Preparation Methods

• Learn the methods to prepare different types of salts:

  • Preparation of Insoluble Salts:

  • Method: Precipitation.

  • Preparation of Soluble Salts:

  • Direct Combination: Combination of two elements.

  • Replacing Hydrogen Ions:

    • Replacing hydrogen ions of an acid directly or indirectly by a metal or ammonium radical.

7.8 - Uses and Dangers of Salts

• Discuss the various uses and potential dangers associated with salts:

  • Everyday Uses:

  • Baking Powder: A common leavening agent in baking.

  • Calcium Carbonate: Used in the manufacture of cement.

  • Food Preservation: Sodium chloride, sodium nitrite, sodium nitrate, and sodium benzoate.

  • Medical Applications: Plaster of Paris (calcium sulfate), Epsom salts (magnesium sulfate).

  • Health Risks:

  • Sodium Nitrate: Linked to brain damage in infants; also suspected to be carcinogenic.

7.9 - Acid Salts vs. Normal Salts

• Normal Salts:

  • Definition: Formed when all hydrogen ions in the acid are replaced, yielding a neutral pH.

  • Examples: NaCl, Na2SO4, Na3PO4.

• Acid Salts:

  • Definition: Formed when not all hydrogen ions are replaced, retaining hydrogen ions in the formula which results in an acidic pH.

  • Examples: NaHSO4, Na2HPO4, NaH2PO4.

7.10 - Neutralization Reactions

• Investigate neutralization reactions using indicators and measuring temperature changes:

  • Indicators' Color Change: Used to determine the end point of neutralization.

  • Temperature Changes: Opposite reactions produce heat or absorb energy (

    • Example of toothpaste action: Fluoride ions replacing hydroxide ions in enamel.

    • Condition of soil affected by lime addition.

• Consider the impact: Adding lime and ammonium fertilizers simultaneously to soil.

7.11 - Volumetric Analysis Calculations

• Perform calculations based on volumetric analysis data:

  • Determine:

  • (a) Number of moles reacting.

  • (b) Mole ratio of reactants combining.

  • (c) Molar and mass concentration of reactants.

  • Refer to A6.6, A6.7.

Definition of a Salt

• Definition: A salt is formed when hydrogen ions from an acid are replaced by metal or ammonium ions. The anion of the salt derives from the acid and the cation from the reacting species.

• Examples of Reactions:

  • HCl + NaOH ⇒ NaCl + H2O

  • 2 NH4OH + H2SO4 ⇒ (NH4)2SO4 + H2O

Types of Salts

Normal Salts

• Characteristics: No hydrogen ions in formula, formed by complete replacement of hydrogen ions from the acid, neutral pH.

• Examples: NaCl, Na2SO4, Na3PO4.

Acid Salts

• Characteristics: Contains hydrogen ions in the formula; formed by incomplete replacement of hydrogen ions, acidic pH.

• Examples: NaHSO4, Na2HPO4, NaH2PO4.

Water of Crystallisation

• Definition: Some salts contain water integrated within their crystal structures, called hydrated salts.

• Examples of Hydrated Salts:

  • CuSO4•5H2O

  • FeSO4•7H2O

Removal of Water from Hydrated Salts

• Process: Heating causes a hydrated salt to decompose, producing an anhydrous salt and steam.

• Example Reaction:

  • CuSO4•5H2O(s) → CuSO4(s) + 5H2O(g)

  • Color Change: Blue (hydrated) to White (anhydrous).

Solubility of Salts and Salt Preparation Method

• Method Selection: The preparation of a specific salt is influenced by its solubility.

  • Method for Insoluble Salts: Ionic precipitation.

  • Method for Soluble Salts: Direct combination or acid-base reactions.

Ionic Precipitation Method

1. Select two soluble solutions.

2. Ensure first solution contains the cation of the desired salt; the second solution should contain the respective anion.

3. When mixed, ions switch, yielding solid precipitate and liquid (double displacement reaction).

4. Collect the solid salt by filtration.

Direct Combination Method

• Applicable for binary salts:

  • Heat the two constituent elements together.

  • Create chlorides by passing chlorine over heated metals.

  • For Soluble Salts Creation:

    • Select an acid with the anion of target salt, and a compound containing the desired salt's cation (metal, metal oxide, or carbonate).

    • Stir mixture until no more solid reacts, indicating completion.

    • Filter unreacted solid to obtain salt solution, then evaporate to collect the solid salt.

    • For crystal formation, utilize crystallization method.

Acid-Base (Titration) Method

1. Choose an acid containing the anion of the target salt.

2. Select a base containing the cation of the target salt.

3. Pipette 25 cm³ of the base into a flask and titrate with acid (using a suitable indicator).

4. Determine the amount of acid for neutralization, then repeat without the indicator to form the salt solution.

5. Evaporate to allow crystal formation, then filter, wash, and dry the resulting salt crystals.

Everyday Applications of Salts

• Overview of Common Salts and Their Uses:

  • Sodium Chloride (NaCl): Food flavoring, preservative, essential nutrient in small amounts.

  • Ammonium Nitrate (NH4NO3): Used as an inorganic fertilizer and in explosive production.

  • Calcium Sulphate (CaSO4): Component of Plaster of Paris.

  • Calcium Carbonate (CaCO3): Essential for cement manufacture, diapers, and paints.

  • Sodium Carbonate (Na2CO3): Also known as soda ash, utilized in glass manufacturing.

  • Sodium Nitrate (NaNO3): Food preservation agent.

  • Magnesium Sulphate (MgSO4): Known as Epsom salt, serves as a bath salt, fertilizer, antiseptic, and purgative.

Research Considerations on Salts

• Health and Environmental Considerations:

  1. Sodium chloride’s connection to hypertension.

  2. Ammonium nitrate (fertilizers) contributing to eutrophication.

  3. Sodium nitrate’s association with carcinogenic properties.