Chemistry Lab Techniques Quiz

Acid-Base Extraction

• Acid-base extraction is a method to separate organic acidic or basic compounds from a mixture.

• Organic acids and neutral compounds (RCOOH and RH) or organic bases and neutral compounds (RNH₂ and RH) can be separated.

• All components are soluble in ether.

• The objective is to make organic compounds water soluble, enabling separation.

• Reactions to make organic acids water soluble:

  • RCOOH + NaOH → RCOO⁻Na⁺ + H₂O

• Reactions to make organic bases water soluble:

  • RNH₂ + HCl → RNH₃⁺Cl⁻

• The solute will distribute itself according to the partition coefficient K for these two solvents.

• A suitable solvent must be chosen to separate the components. This solvent must be immiscible with the first solvent and dissolve the solute better, be easy to evaporate, and not react with the solute.

• The denser liquid is always at the bottom of the separatory funnel.

• The procedure involves dissolving the solid mixture in ether followed by adding acid or base, then separating the layers in a separatory funnel.

Separatory Funnel

• The separatory funnel is used to separate immiscible liquids based on density differences.
• The stopcock allows controlled release of the bottom layer.
• Safety precautions are crucial; the funnel should never be more than 3/4 full and always placed above a receiving flask or beaker. The stopcock should be lubricated and closed.

Drying Agents

• Drying agents, anhydrous salts, are used to remove residual water from organic phases.
• Examples include anhydrous CaCl₂, anhyd. MgSO₄, anhyd. Na₂SO₄.
• The drying agent is combined with the layers, and then the mixture is decanted or filtered.

Procedure (Acid-Base Extraction)

• The procedure outlines dissolution of the solid mixture in ether, followed by addition of acid or base and separation in the separatory funnel.

Procedure (Caffeine Isolation)

• Take caffeine from tea leaves to hot water (solid-liquid extraction)
• Take caffeine from water to an organic solvent (DCM) (liquid-liquid extraction)
• Remove solvent and obtain impure caffeine.
• Purify caffeine with sublimation.
• Characterize caffeine (melting point, IR, NMR)
• For acid (RCOOH): add NaOH solution
• To form RCOO- Na+ : a salt of the RCOOH; neutralize and add HCL aq. to separate RH

Procedure (Soap Synthesis)

• Dissolve NaOH in 1:1 EtOH:H₂O.
• Reflux triglyceride and NaOH aqueous solution for 30 minutes.
• Prepare a saturated NaCl solution.
• Precipitate the formed soap by salting out.
• Add colorings and perfumes.
• Filter and rinse with cold water.
• Shape and dry.

Chromatography

• Chromatography is a selective distribution of chemicals (solute) between a stationary and a mobile phase.
• It depends on physical processes such as adsorption, solubility, charge, size, or specific binding.
• Types of chromatography include liquid chromatography (using liquid as mobile phase) and gas chromatography (using gas as mobile phase). Further sub-types within these include column and thin layer chromatography.

Column Chromatography

• Separation of compounds in a column filled with an adsorbent (e.g., silica gel).
• The adsorbent holds the compounds differently, allowing them to separate as they travel down the column with the solvent.
• The separation process is based on the differences in polarity. A flat silica layer vs. uneven silica layer.

Thin Layer Chromatography (TLC)

• It involves using a thin layer of adsorbent on a solid support like aluminum, glass, or plastic.
• The mobile phase moves by capillary action through the stationary phase, carrying the components of the mixture at different speeds.
• This difference in speed (depending on compounds interaction with adsorbent) allows separation.
• The Rf value (retention factor) measures the relative distance moved by a particular compound compared to the solvent.

Simple and Fractional Distillation

• Distillation is used to separate miscible liquids based on their different boiling points.
• Simple distillation is used for liquids with significantly different boiling points.
• Fractional distillation is used for liquids with close boiling points for more effective separation.
• This involves a fractionating column filled with glass beads or theoretical plates.

Recrystallization

• Recrystallization is a method to purify solid organic compounds by dissolving the impure sample in a hot solvent, filtering out solid impurities, cooling the solution to allow the desired product to crystallize, and filtering out the crystals.
• Crucial for the method is a solvent that dissolves the compound well at high temperature, but not at low temperature.

Melting Point (mp)

• The melting point is used to identify and confirm the purity of a compound.
• Pure compounds have sharp melting points, while impure compounds show a broader melting range.

Vacuum filtration

• A method for rapid filtration of a solid from a liquid using reduced pressure.

This quiz covers essential laboratory techniques in chemistry, including recrystallization, filtration methods, and the use of boiling chips. Test your understanding of the purification processes and the proper use of equipment in a lab setting. Perfect for chemistry students looking to reinforce their knowledge of experimental methods.