In the captivating world of chemistry, the pursuit of knowledge often begins with a keen appreciation for the art and science of experimentation. As we embark on the journey into the realm of the first-year chemistry curriculum, Chapter 2 beckons us with its focus on “Experimental Techniques in Chemistry.” This chapter serves as a fundamental cornerstone, providing aspiring chemists with the essential skills and methodologies required to conduct experiments, gather data, and analyze results effectively.
From mastering laboratory safety protocols to understanding the intricacies of precise measurements and instrumental analysis, this chapter equips students with the indispensable tools needed to explore the fascinating intricacies of matter and its transformations. Join us as we delve into the intriguing world of experimental techniques, where observation and discovery merge to unlock the secrets of the molecular universe.
Chemistry 1st Year Chapter 2 Experimental Techniques In Chemistry Short Question Notes
What is analytical chemistry, and what does a complete chemical characterization of a compound involve?
Analytical chemistry is the science of chemical characterization. A complete chemical characterization of a compound includes both qualitative and quantitative analyses. Qualitative analysis involves detecting or identifying the elements present in a compound, while quantitative analysis determines the relative amounts of these elements.
What are the four major steps in a complete quantitative determination?
The four major steps in a complete quantitative determination are:
(i) Obtaining a sample for analysis
(ii) Separation of the desired constituent
(iii) Measurement and calculation of results
(iv) Drawing conclusions from the analysis
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What is the purpose of filtration in analytical chemistry, and what are the two common methods of filtration mentioned?
Filtration in analytical chemistry is used to separate insoluble particles from liquids. Two common methods of filtration mentioned are filtration through filter paper and filtration through a filter crucible.
Why is the choice of filter paper pore size important in filtration, and what considerations should be made when setting up a filtration assembly?
The choice of filter paper pore size depends on the size of particles in the precipitate. The filter paper should be large enough to be one-fourth to one-half full of precipitate at the end of filtration. The funnel should extend 1 to 2 cm above the top circumference of the paper. To ensure a smooth filtration process, the funnel’s stem should remain continuously full of liquid, and the stem should extend into the receiving beaker while touching its side.
Describe the proper way to fold filter paper for filtration.
Filter paper should be folded twice. The first fold should be along the diameter of the paper, and the second fold should be in a way that the edges do not quite match. The paper should be opened on the slightly larger section to create a cone with three-fold thickness halfway around and one thickness the other halfway around, with an apex angle slightly greater than 60 degrees. This folded paper can then be inserted into a 60-degree funnel, moistened with water, and firmly pressed down to aid the filtering process.
What is the purpose of using a Fluted Filter Paper in filtration?
The purpose of using a Fluted Filter Paper in filtration is to considerably increase the filtration rate through a conical funnel. It is created by folding ordinary filter paper in a way that forms a fan-like arrangement with alternating elevations and depressions, which enhances the efficiency of filtration.
What are the two types of crucibles commonly used for suction filtration, and how are they different?
The two types of crucibles commonly used for suction filtration are Gooch Crucible and Sintered glass crucible.
The Gooch Crucible is made of porcelain with a perforated bottom, which is covered with paper pulp or filter paper. It is suitable for quick filtration, especially for precipitates that need high-temperature ignition. If its perforations are covered with asbestos mat, it can also filter solutions that react with paper.
The Sintered glass crucible is a glass crucible with a porous glass disc sealed into the bottom. It is convenient to use and does not require any additional preparation like the Gooch crucible.
What is crystallization, and what are the key steps involved in the crystallization process?
Crystallization is the process of removing a solid from a solution by increasing its concentration above the saturation point in such a way that the excess solid precipitates out in the form of crystals. The key steps involved in the crystallization process are:
- Choosing a suitable solvent.
- Dissolving the solute in the chosen solvent at its boiling point.
- Allowing the solution to cool, leading to the formation of well-formed crystals of the pure compound.
What are the desirable characteristics of an ideal solvent for crystallization?
An ideal solvent for crystallization should possess the following characteristics:
i. It should dissolve a large amount of the substance at its boiling point and only a small amount at room temperature.
ii. It should not react chemically with the solute.
iii. It should either not dissolve the impurities or prevent impurities from crystallizing along with the solute.
iv. When cooled, it should allow the pure compound to form well-defined crystals.
v. It should be cost-effective.
vi. It should be safe to use and easy to remove, especially if it’s flammable.
What is the first step in the preparation of a saturated solution?
After selecting a suitable solvent, the substance is dissolved in a minimum amount of solvent and heated with constant stirring.
What should be done if all the solute doesn’t dissolve during heating in the preparation of a saturated solution?
If necessary, more solvent should be added to the boiling solution until all the solute has dissolved.
How are insoluble impurities removed from a saturated solution?
Insoluble impurities are removed by filtering the hot saturated solution through a normal or a luted filter paper.
Why is slow cooling preferred during the crystallization process?
Slow cooling yields medium-sized crystals and reduces the likelihood of including a considerable amount of solvent carrying impurities.
How are crystals collected after crystallization is complete?
The mixture of crystals and mother liquor is filtered through a Gooch crucible using a vacuum pump, and then the crystals are washed with a cold solvent.
What is the purpose of drying crystals in a vacuum desiccator?
Drying crystals in a vacuum desiccator removes moisture without crushing the crystals and uses drying agents like CaCl2, silica gel, or phosphorus pentoxide.
How can undesirable colors be removed during the preparation of a substance?
Undesirable colors can be removed by boiling the substance in the solvent with finely powdered animal charcoal and then filtering the hot solution.
What is sublimation?
Sublimation is a process in which a solid, when heated, vaporizes directly without passing through the liquid phase, and these vapors can be condensed to form the solid again.
Why is sublimation frequently used in chemistry?
Sublimation is frequently used in chemistry to purify solids, especially those that can undergo sublimation, such as ammonium chloride, iodine, naphthalene, benzoic acid, and others.
Describe the apparatus used for sublimation.
To carry out sublimation, the substance is taken in a watchglass covered with an inverted funnel. The substance is then heated slowly over a sand-bath, and the funnel is cooled with wet cotton. The pure solid deposits on the inner side of the funnel.
What is solvent extraction?
Solvent extraction is a technique in chemical analysis used to separate a solute from a solution by shaking the solution with a solvent in which the solute is more soluble. The added solvent does not mix with the solution during this process.
Provide an example of a common laboratory application of solvent extraction.
A common laboratory application of solvent extraction is ether extraction, which is used to separate organic products from water. This is often employed in organic synthesis.
What is the distribution law or partition law in solvent extraction?
The distribution law or partition law in solvent extraction states that a solute distributes itself between two immiscible liquids in a constant ratio of concentrations, irrespective of the amount of solute added.
How does the distribution coefficient (K) relate to solvent extraction?
The distribution coefficient (K) is a constant that represents the ratio of concentrations of a solute in two immiscible liquids in a solvent extraction process. It quantifies the equilibrium distribution of the solute between the two phases.
What is chromatography, and where does the word “chromatography” originate from?
Chromatography is a method used for the separation of a sample mixture based on the distribution of a solute between a stationary phase and a mobile phase. The word “chromatography” originates from the Greek word “Khromatos,” which means “color writing.”
What is the primary purpose of chromatography?
Chromatography is primarily used for the separation of components in a sample mixture.
What are the two phases involved in chromatography, and what forms can they take?
The two phases in chromatography are the stationary phase and the mobile phase. The stationary phase can be either a solid or a liquid supported as a thin film on the surface of an inert solid, while the mobile phase can be a gas or a liquid.
How are substances separated in chromatography, and what governs their distribution between the phases?
Substances are separated in chromatography based on their relative affinities for the stationary and mobile phases. The distribution of components is governed by the distribution coefficient (K).
What does the distribution coefficient (K) represent in chromatography?
The distribution coefficient (K) is defined as the ratio of the concentration of a component in the mobile phase to its concentration in the stationary phase.
What type of chromatography involves a solid stationary phase, and what is it called?
Chromatography with a solid stationary phase is called adsorption chromatography.
In partition chromatography, what forms the stationary phase, and how are substances distributed?
In partition chromatography, the stationary phase is a liquid, and substances being separated are distributed throughout both the stationary and mobile phases.
What are the three common methods of carrying out paper chromatography?
The three common methods of paper chromatography are ascending, descending, and radial/circular. The ascending method is discussed here.
What is the stationary phase in paper chromatography, and what is the typical mobile phase?
In paper chromatography, the stationary phase is a liquid (e.g., water) adsorbed on paper, and the typical mobile phase is usually an organic liquid.
How is paper chromatography carried out using the ascending technique?
The solvent mixture is poured into a chromatographic tank, a strip of chromatographic paper is prepared with a sample spot, and the paper is suspended in the solvent mixture, allowing the solutes to move upward through capillary action.
What is a chromatogram, and how is it used to identify components in a mixture?
A chromatogram is the pattern obtained on the chromatographic paper after separation. Components of the mixture, if colored, can be visually identified from the chromatogram. If colorless, additional chemical or physical techniques are used to identify the spots.
What is the retardation factor (Rf) in chromatography, and how is it related to the distribution coefficient (K)?
The retardation factor (Rf) is a specific value for each component in chromatography and is related to its distribution coefficient (K). It is calculated as the distance traveled by the component divided by the distance traveled by the solvent front.
What is the primary purpose of chromatography in organic synthesis?
The primary purpose of chromatography in organic synthesis is for the separation, isolation, and purification of products.
Why are chromatography techniques important in qualitative and quantitative analyses?
Chromatography techniques are important in qualitative and quantitative analyses because they can separate and identify different components in a mixture while also quantifying their amounts accurately.
How does chromatography help determine the purity of a substance?
Chromatography helps determine the purity of a substance by separating it from impurities or contaminants, allowing for the measurement of the substance’s purity based on the separation results.
Chemistry 1st Year Chapter 2 Experimental Techniques In Chemistry Long Question Notes
Question: Why is there a need to crystallize the crude product?
Answer: Crystallization of a crude product is a crucial step in many chemical processes for several important reasons:
Purity Enhancement: The crude product obtained from a chemical reaction often contains impurities such as unreacted starting materials, by-products, or other contaminants. Crystallization allows for the selective separation of the desired compound from these impurities, resulting in a purer final product.
Separation of Isomers: In some cases, chemical reactions may produce multiple isomers or stereoisomers of a compound. Crystallization can be used to separate and isolate specific isomers based on their differing solubilities in the chosen solvent system.
Removal of Solvents and Reagents: Many chemical reactions involve the use of solvents or reagents that are not desired in the final product. Crystallization provides a means to remove these unwanted components by dissolving the crude product in a suitable solvent and then allowing it to re-crystallize in a purer form.
Particle Size Control: Crystallization allows for control over the size and morphology of the crystals formed. This can be important for various applications, such as in the pharmaceutical industry, where the physical characteristics of a drug substance can impact its bioavailability and efficacy.
Improved Yield: In some cases, the crude product may not be obtained in high yields due to incomplete reactions or side reactions. By carefully controlling the crystallization conditions, it is possible to improve the overall yield of the desired compound.
Analysis and Characterization: Crystallization produces a well-defined, pure sample of the compound, making it easier to perform accurate analytical techniques like spectroscopy, chromatography, and elemental analysis to confirm the identity and purity of the product.
Consistency and Reproducibility: Crystallization provides a reproducible and consistent method for isolating a compound, which is important in research, development, and manufacturing processes. It allows scientists to obtain the same compound with high purity repeatedly.
Overall, crystallization is a versatile and powerful technique in chemistry, essential for obtaining pure and well-defined products, which is crucial for various industries such as pharmaceuticals, petrochemicals, and materials science, among others.
Question: A water insoluble organic compound aspirin is prepared by the reaction of salicylic acid with a mixture of acetic acid and acetic anhydride. How will you separate the product from the reaction mixture?
Answer: To separate the water-insoluble organic compound aspirin from the reaction mixture, you can employ a process called “recrystallization.” Here’s a step-by-step guide on how to do this:
Materials Needed
- Reaction mixture containing aspirin, unreacted starting materials, and impurities
- Solvent (usually hot water)
- Heat source
- Filtration apparatus (e.g., Buchner funnel)
- Glassware (flasks, beakers, etc.)
- Cooling apparatus (e.g., refrigerator or ice bath)
- Drying apparatus (e.g., oven)
Procedure
Dissolution: Begin by dissolving the reaction mixture, which includes aspirin and impurities, in a suitable hot solvent. In this case, hot water is commonly used because aspirin is sparingly soluble in cold water but more soluble in hot water. Use the minimum amount of solvent necessary to dissolve the aspirin.
Filtration: While the solution is still hot, filter it to remove any insoluble impurities, unreacted starting materials, or other solid contaminants. Use a Buchner funnel or any suitable filtration apparatus.
Cooling: Allow the hot filtrate to cool slowly at room temperature or, preferably, in a refrigerator or ice bath. As the solution cools, the solubility of aspirin in water decreases, leading to the formation of aspirin crystals.
Crystallization: Crystals of aspirin should start to form in the cooled solution. You can encourage crystal growth by scratching the sides of the container gently with a glass stirring rod or by seeding the solution with a small amount of previously obtained aspirin crystals.
Isolation of Crystals: Once the crystallization is complete, separate the solid aspirin crystals from the remaining liquid using filtration again. This will yield a wet cake of crude aspirin.
Washing: Rinse the collected aspirin crystals with a small amount of ice-cold water to remove any remaining soluble impurities. Be careful not to use too much water, as it could redissolve the aspirin.
Drying: Transfer the wet aspirin crystals onto a filter paper or a watch glass and allow them to air-dry or, for faster results, place them in an oven at a low temperature to evaporate any remaining water.
Final Product: Once completely dry, you will obtain pure aspirin crystals. Calculate the yield and compare it to the theoretical yield based on the starting materials to assess the purity of your product.
Recrystallization is a standard technique in chemistry for purifying solid compounds and is particularly effective for separating a desired organic compound from a reaction mixture.
Question: A solid organic compound is soluble in water as well as in chloroform. During its preparation, it remains in aqueous layer. Describe a method to obtain from this layer.
Answer: To obtain a solid organic compound that is initially present in the aqueous layer but is soluble in both water and chloroform, you can use a technique called “partitioning” or “liquid-liquid extraction.” This method takes advantage of the differential solubilities of the compound in two immiscible solvents, in this case, water and chloroform. Here’s a step-by-step guide on how to perform this extraction:
Materials Needed
- Aqueous solution containing the organic compound
- Chloroform (organic solvent)
- Separatory funnel
- Glassware (beakers, flasks, etc.)
- Distilled water (for washing)
- A suitable container for collecting the organic phase
- Drying apparatus (if necessary)
Procedure
Prepare the Separatory Funnel: Set up a separatory funnel and ensure that it is clean and dry. It’s important to have proper ventilation because chloroform can release harmful vapors.
Transfer the Aqueous Solution: Pour the aqueous solution containing the organic compound into the separatory funnel.
Add Chloroform: Add an appropriate volume of chloroform to the separatory funnel. The volume of chloroform should be chosen based on the expected solubility of the organic compound in chloroform. Typically, you would add enough chloroform to form a distinct organic layer when the mixture is shaken.
Shake and Ventilate: Close the separatory funnel and gently shake it vigorously in an up-and-down motion. Release any pressure built up during shaking by slightly opening the stopcock or cap of the funnel to allow gases to escape. Repeat this shaking process several times.
Allow Phase Separation: After shaking, let the mixture sit for a while, allowing the two phases (organic and aqueous) to separate. The less dense organic phase will rise to the top, and the denser aqueous phase will settle at the bottom.
Drain the Organic Phase: Carefully open the stopcock at the bottom of the separatory funnel and allow the organic phase (chloroform layer) to drain into a separate container. Be cautious not to transfer any of the aqueous phase during this process.
Repeat if Necessary: If a significant amount of the organic compound is still in the aqueous phase, you can repeat the extraction process by adding fresh chloroform to the aqueous phase and shaking again. This will increase the efficiency of the extraction.
Wash the Organic Phase: To remove any traces of water that may have been carried over with the organic phase, you can wash the organic phase with a small amount of distilled water. Shake gently, allow separation, and drain the water layer.
Dry the Organic Phase (if necessary): If the organic compound is sensitive to water, you may need to use drying agents like anhydrous sodium sulfate or magnesium sulfate to remove any remaining water traces from the organic phase.
Evaporate Chloroform: If your final product is in chloroform and you need the solid organic compound, you can evaporate the chloroform under controlled conditions to obtain the solid.
Question: In solvent extraction technique, why repeated extraction using small portions of solvent are
more efficient than using a single extraction but larger volume of solvent?
Answer: Repeated extraction using small portions of solvent is more efficient than a single extraction with a larger volume of solvent due to several key factors:
Increased Surface Area: When you use small portions of solvent for repeated extractions, you create a larger interface between the solvent and the sample in each extraction. This increased surface area enhances the contact between the two phases and facilitates the transfer of the target compound from the sample into the solvent. In contrast, a single large volume of solvent may not efficiently penetrate and interact with the sample.
Equilibrium Considerations: Solvent extraction is a dynamic process that reaches equilibrium, meaning that as the extraction proceeds, the rate of transfer of the target compound from the sample to the solvent gradually decreases. By performing multiple small extractions, you reset the equilibrium with each addition of fresh solvent, allowing for a more complete extraction of the target compound.
Reduced Loss of Target Compound: When using a single, large volume of solvent, it may become saturated with the target compound before all of it is extracted. This can lead to incomplete recovery of the compound. In contrast, using small portions of solvent ensures that the solvent remains relatively unsaturated, minimizing the risk of target compound loss.
Efficient Mixing: Smaller volumes of solvent are easier to mix thoroughly with the sample, ensuring better contact and more effective extraction. In a larger volume, mixing can be less efficient, leading to uneven distribution of the target compound between the two phases.
Reduced Emulsion Formation: In some cases, using a large volume of solvent can lead to the formation of emulsions, especially when dealing with complex mixtures. Emulsions can be challenging to break and can trap the target compound. By using smaller solvent portions, emulsion formation is less likely, and any emulsions that do form are easier to manage.
Easier Handling: Working with smaller volumes of solvent is often more practical and easier to handle in the laboratory. It reduces the risk of spills and minimizes the amount of solvent needed for the process.
Efficiency and Control: Repeated extractions allow for better control over the process. If, after a few extractions, you find that the target compound is no longer being extracted, you can stop the process and avoid wasting additional solvent and time.
In summary, using repeated extractions with small portions of solvent offers several advantages, including increased surface area, better equilibrium control, reduced compound loss, improved mixing, and more efficient handling of the process. These factors collectively contribute to the overall efficiency and effectiveness of solvent extraction techniques.
Question: Write down the main characteristics of a solvent selected for crystallization of a compound.
Answer: Selecting an appropriate solvent for the crystallization of a compound is a critical step in the purification process. The choice of solvent greatly influences the yield, purity, and crystalline characteristics of the final product. Here are the main characteristics to consider when selecting a solvent for crystallization:
Solubility of the Compound
The solvent should dissolve the compound at elevated temperatures but have limited solubility at lower temperatures to allow for crystallization upon cooling.
High Purity
The solvent itself should be of high purity to avoid introducing impurities into the crystallized product.
Non-reactive with the Compound
The solvent should not chemically react with the compound being crystallized to prevent unwanted side reactions or degradation.
High Boiling Point
A solvent with a relatively high boiling point is preferred to allow for controlled evaporation during the crystallization process.
Low Toxicity
The solvent should have low toxicity to ensure safety during handling and disposal.
Non-flammable
Non-flammable solvents are safer to work with in a laboratory setting.
Easily Removed After Crystallization
The solvent should be easy to remove after crystallization, either by evaporation or filtration.
Good Thermal Stability
The solvent should be thermally stable to withstand the heating and cooling cycles involved in the crystallization process.
Wide Liquid Range
A solvent with a broad liquid range (the temperature range between its freezing and boiling points) is more forgiving during the crystallization process.
Compatibility with Glassware
The solvent should not react with or damage common laboratory glassware.
Availability and Cost
The solvent should be readily available and cost-effective for the specific application.
Selective for the Desired Compound:
Ideally, the solvent should selectively dissolve the desired compound while leaving impurities behind.
Question: You have been provided with a mixture containing three inks with different colours. Write down the procedure to separate the mixture with the help of paper chromatography.
Answer: To separate a mixture of three inks with different colors using paper chromatography, you can follow these steps:
Materials Needed
- Mixture of three inks
- Filter paper or chromatography paper
- Pencil or pen
- Ruler
- Glass or plastic container with a lid (to act as a developing chamber)
- Solvent (e.g., water, rubbing alcohol, or acetone)
- Capillary tubes or toothpicks
- Tape
- Paper towels
- Watch or timer
Procedure
Prepare the Paper
Cut a strip of chromatography paper. The size and shape of the paper strip will depend on the size of your container but usually, a strip about 2-3 cm wide and 10-15 cm long is suitable.
Use a pencil or pen to draw a horizontal line near the bottom of the strip, about 1-2 cm from the edge. This is where you will spot the ink mixture.
Prepare the Mixture
Take the mixture of three inks and carefully place a small spot of it on the pencil line you drew in step 1. Make sure not to overload the paper with too much ink, as it may lead to poor separation.
Prepare the Developing Chamber
Take the glass or plastic container and add a small amount of solvent to the bottom (about 1-2 cm). The solvent should not touch the ink spot.
Place the Paper in the Developing Chamber
Hang the paper strip vertically in the container so that the bottom of the strip is in the solvent but not submerged in it. You can use tape to attach the strip to a pencil or the container’s lid to keep it suspended.
Cover and Wait
Seal the container with a lid to create a closed environment. This helps the solvent vapor to saturate the chamber, promoting a better separation.
Allow the chromatography process to proceed for some time (usually 20-30 minutes). Keep an eye on the solvent level to make sure it doesn’t reach the ink spot.
Observation
As the solvent travels up the paper, it will carry the ink components with it. Different ink components will move at different rates up the paper due to differences in their solubility and interaction with the paper.
Record Results
Once the solvent has traveled close to the top of the paper (or as far as you desire), remove the paper strip from the container.
Analyze the Separation
Observe the separation of the ink components on the paper. You should see distinct spots or bands of different colors on the paper.
Calculating Rf Values (optional)
If desired, you can calculate the Rf (retention factor) values for each separated component. Rf values can help identify the individual ink components and compare them to known standards.