Orgo lab manual Distillation of a binary mixture 4.3, 4.4 p. 129-143

Chapter 4: Liquids, Distillation, and Boiling Points

Key Concepts

• Boiling Point of Aqueous Solutions

  • When mixed with non-volatile solutes like ethylene glycol, the boiling point of water increases above 100 °C due to lowered vapor pressure, according to Raoult’s law.

  • Non-volatile impurities reduce vapor pressure leading to higher boiling points.

General Distillation Queries

• Absence of Water Droplets in Oil Bath:

  • Water droplets can cause inaccurate temperature readings and may interfere with processes requiring precise boiling points.

• Micro Boiling-Point Technique Limitations:

  • Not suitable above 200 °C with mineral oil due to its lower boiling point and thermal stability compared to silicone oil.

• Rotary Evaporator Usage:

  • Solvent removal below normal boiling points due to vacuum conditions lowering boiling points, allowing for efficient solvent recovery.

  • Preferred for isolating reactions, yielding higher-purity products faster than traditional methods.

Simple Distillation Process

• Efficiency and Application:

  • Allows separation of volatile substances from non-volatile residues.

  • For mixtures with a boiling point difference of 40–50 °C, like diethyl ether (bp 35 °C) and toluene (bp 111 °C).

• Impurities Impact on Vapor Pressure:

  • An analysis on the effects of impurities like sugar in water shows that the boiling point of the resulting solution increases, demonstrating Raoult’s law.

  • The vapor pressure graph illustrates lower pressure and elevated boiling temperature with nonvolatile impurities.

Raoult’s Law and Dalton’s Law

• Raoult's Law:

  • States the vapor pressure of a component in a mixture equals the pure component pressure multiplied by its mole fraction in the mixture:

    • ( P_X = P^0_X N_X )

  • Valid for ideal solutions where interactions are similar.

• Dalton’s Law for Mixtures:

  • The total vapor pressure is the sum of the partial pressures of individual volatile components:

    • ( P_{total} = P_X + P_Y + P_Z )

Distillation Procedures

• Simple Distillation Setup:

  • Preparation: Ensure apparatus is intact and open to atmospheric pressure; careful assembly is crucial to prevent spillage.

  • Distillation Process: Adjust heat to ensure a controlled drip rate of distillate; record head temperature accurately throughout.

• Fractional Distillation:

  • More efficient in separating components with closer boiling points by allowing intermolecular vapor-liquid equilibria.

  • Packing materials like Raschig rings enhance interaction within the column, providing more theoretical plates for separation.

Temperature-Composition Curves in Distillation

• Understanding Distillate Composition:

  • Mixtures with defined boiling points exhibit specific behavior on heating, where the vapor phases will vary in composition from the liquid.

  • Lower boiling point compounds distill first, enriching the vapor with more volatile components, observable in the boiling point-composition diagrams.

Application of Azeotropes

• Behavior in Minimum-Boiling Azeotropes:

  • Azeotropic mixtures exhibit constant boiling points regardless of composition changes.

  • Cannot separate completely through normal fractional distillation, requiring additional methods.

Practical Considerations in Fractional Distillation

• Column Design:

  • The height and packing of the column significantly influence the separation quality by enhancing contact areas for vapor and liquid.

  • Proper thermal management ensures each component evaporates at the correct thermal gradients.

Safety Considerations

• Laboratory Safety:

  • Always use safety gear and ensure setup is checked to avoid mishaps during highly flammable procedures.

  • Monitor vapor exposure to maintain a safe working environment.

Conclusion

• Distillation Types and Efficiency:

  • Understanding the principles of distillation is essential for the effective separation of volatile compounds in organic chemistry. Choose between simple and fractional distillation based on component boiling point differences and desired purity.

Review Questions

1. Define and explain the significance of: simple distillation, Raoult’s law, Dalton’s law, and ideal solutions.

2. Discuss conditions where fractional distillation is necessary over simple distillation.

3. Describe the process and advantages of using a rotary evaporator in solvent recovery.