Chemistry Lab Techniques Quiz

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Questions and Answers

What is the primary purpose of using boiling chips during heating?

To increase the boiling point of the liquid.

To prevent superheating and improve boiling stability. (correct)

To enhance the flavor of the liquid when heated.

To reduce the time required for the solution to boil.

Which step is NOT part of the recrystallization process?

Dissolve by heating.

Rinse with cold solvent.

Decantation. (correct)

Cold filter.

Why is rapid filtration important during the purification process?

To preserve high temperature and prevent crystallization. (correct)

To prevent the sample from becoming contaminated.

To remove colored impurities effectively.

To minimize the use of solvents.

How does activated charcoal assist in the decolorization process?

By adsorbing larger colored molecules due to its surface area. Signup and view all the answers

What happens to boiling chips after cooling?

Their pores become filled with liquid and cannot be reused. Signup and view all the answers

What is the effect of using greased joints in the reflux setup?

To improve the sealing and ease of detachment. Signup and view all the answers

In the context of recrystallization, why is hot filtration performed?

To collect the solid impurities without dissolving them. Signup and view all the answers

What is the function of a vacuum in vacuum filtration?

To increase the filtration rate compared to gravity filtration. Signup and view all the answers

What should be done first when using a separatory funnel?

Lubricate and close the valve Signup and view all the answers

What is the role of drying agents in the liquid-liquid extraction process?

To absorb residual water in the organic phase Signup and view all the answers

Which of the following compounds is NOT typically used as a drying agent?

Ethanol Signup and view all the answers

When performing a liquid-liquid extraction, how should pressure build-up be managed?

Gently shake and depressurize the funnel Signup and view all the answers

After completing a liquid-liquid extraction, what should be done with the layers?

Collect the bottom layer then let the top layer out Signup and view all the answers

What should NOT be done during the shaking of the separatory funnel?

Remove the stopper before shaking Signup and view all the answers

Which of these is an important consideration in the selection of solvents for liquid-liquid extraction?

Their miscibility with the aqueous phase Signup and view all the answers

In the process of liquid-liquid extraction, what is the purpose of shaking the mixture in the separatory funnel?

To allow the organic and aqueous phases to mix thoroughly Signup and view all the answers

How does molecular weight affect boiling point in similar molecules?

It increases the boiling point. Signup and view all the answers

What is the primary factor in boiling point increase related to molecular structure?

Hydrogen bonding strength. Signup and view all the answers

What is the primary goal of recrystallization?

To purify organic solids. Signup and view all the answers

What must be true about the solvent used in recrystallization?

It must not react with the sample. Signup and view all the answers

Why are soluble impurities removed only after cooling in recrystallization?

They can precipitate and contaminate the sample. Signup and view all the answers

Which property of acetanilide is crucial for its recrystallization?

It is a white crystalline solid. Signup and view all the answers

What kind of impurities does recrystallization effectively remove?

Both soluble and insoluble impurities. Signup and view all the answers

What is the critical solubility of acetanilide in hot water at 100 °C?

5.5 g/100 mL Signup and view all the answers

Study Notes

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.

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Description

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.