CHEM182 McGill University Chromatography Experiment Discussion Questions

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3. Write a short paragraph discussing the relative movement of the individual halide ions in the 80% acetone solvent. Which ions moved the most/least and why? consider what you know about size and attractive forces between ions, the paper and the solvent to explain the relative movement.

4.The lower the intemroleculalr forces, the more easily a compound leaves the liquid phase, the faster the rate of evaporation, and thus the higher the vapor pressure. RAnk the non-polar alkanes - pentane, hexane, heptane, and octane - in terms of increasing vapor pressure. explain your ranking in terms of the molar mass and intermolecular forces present.

5. Rank the polar molecules ehtanol, acetone and ethyl acetate in terms of increasing vapor pressure. explain your ranking inter of the intermolecular forces present, and the effects (if any) of molar mass.

6. How would you rank the relative strengths of the iintermolecular forces present in the non-polar and polar molecules discussed in questions 4 and 5?

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CHEMISTRY 182 Experiment #5 INTERMOLECULAR FORCES INTRODUCTION: Many physical properties are determined by the intermolecular forces of attraction existing between the molecules of a given compound. These intermolecular forces are weaker than the bonding forces that exist between the atoms in a compound and are primarily due to an uneven distribution of the electron cloud existing in a molecule. This uneven charge distribution can be permanent, as in the dipole moment caused by polar bonds and 3-D geometry (dipole-dipole interactions), or temporary, caused by the temporary distortion of the electron cloud in non-polar molecules (London dispersion forces). The very strong dipole-dipole interaction that occurs only between O, N, F and the H atoms bonded to them is called hydrogen bonding. Hydrogen bonding is extremely important in many systems and primarily responsible for the unique properties of water. In addition, there can be interactions between ions and polar molecules. In discussions of physical properties, ion-ion interactions may also be considered. In general, intermolecular forces can be ranked in order of increasing strength as follows: London dispersion forces, dipole-induced dipole, dipole-dipole, hydrogen bonding, ion-dipole, ion-ion. There are many exceptions to this trend, depending on the strength of the dipole and the size of the molecule. When trying to rank compounds in terms of a physical property such as boiling point, the stronger the intermolecular forces between the molecules of the compound under consideration, the more energy is required to overcome those attractions. As a result, boiling point increases with increasing intermolecular forces. The same situation is true for melting point: the higher the intermolecular forces the higher the melting point. In both cases, increasing the temperature gives the molecules more kinetic energy and overcomes the forces attracting the molecules to each other. When considering vapor pressure, the opposite is true. This may seem counter-intuitive at first, but consider that the stronger the forces between molecules in the liquid phase the harder it is for some of those molecule to leave the liquid phase and enter the vapor phase. Thus, stronger intermolecular forces mean lower vapor pressure. Since vapor pressure increases with increasing temperature, a higher temperature is required to make the vapor pressure equal atmospheric pressure and a higher boiling point is the result. Chromatography: The first part of this experiment looks at a very common application of intermolecular forces, chromatography. Chromatography is one of the most popular laboratory techniques in use today. The name comes from the Greek words for “color” and “writing”, referring to the bands of color that were produced when using this technique to separate plant pigments, one of the earliest applications.. The basic principle of chromatography is that the substance of interest, the analyte, will separate into its component parts when dissolved in the solvent, more normally called the mobile phase, and passed across the supporting substance, the stationary phase. The components present in the analyte are carried across the stationary phase at different rates depending on many factors. Two of the factors include the composition and polarity of the mobile phase, the stationary phase and the analyte. Individual components are characterized by the Rf value, the retention factor. The Rf value compares the distance traveled by the component to the distance traveled by the solvent and always has a value less than 1.0. -1- CHEMISTRY 182 Intermolecular Forces Rf = Distance spot travels from the original spot point Distance solvent travels from the original spot point There are many different types of chromatography, classified by the nature of the mobile and stationary phases, as well as by the type of detection device used. Examples are HPLC, high performance liquid chromatography, often used in biological applications where both mobile and stationary phases are liquid and the detection device is a UV spectrometer; HPLC-FTIR, a liquid chromatography application with an infra-red spectrometer as a detector; GC, gas chromatography, where the mobile phase is a gas and the stationary phase is a liquid and GC-MS, gas chromatography with a mass spectrometer as a detection device. In addition to the instrumental techniques mentioned above, there are also many types of column and sheet chromatography, where the stationary phase is paper or a finely-divided solid and the mobile phase is a liquid. Detection techniques used range from visual inspection to various types of stains to collecting the analyte components and analyzing them in spectrophotometers. The mobile phase can travel across the stationary phase in various ways. Some techniques let the mobile phase move down the stationary phase (descending), some let the mobile phase travel up the stationary phase (ascending) and some travel across or through the stationary phase. This experiment will examine ascending paper chromatography where filter paper is the stationary phase and various acetone-water solutions are the mobile phase and travel up the paper. One part of this experiment studies the inks/dyes from various overhead transparency pens. The ink is spotted near the bottom of the paper, placing them in solvents of different composition and seeing how the ink components separate and move up the paper. One solvent is a solution of 20% acetone in water, while the other is a solution of 80% acetone in water. Since the ink contains colored components, detection and identification of the components is done by simply looking at them. Another part of the experiment the movement of the halide ions (F-, Cl-, Br-, I-) in 80% acetone. Since the halide ions are colorless, the filter paper will be sprayed with a solution of silver nitrate. Silver nitrate reacts with the halide ions to form silver halides which are light sensitive. Over a period of time, different colored spots will appear for each of the halide ions spotted. An unknown contain two of the halide ions will also be analyzed and identified, using color and Rf values. Physical Properties: The second part of the experiment will look at the effect of intermolecular forces on vapor pressure. The effect of intermolecular forces on vapor pressure will be studied in two different ways. The effect of increasing molecular size, of increasing London forces, will be studied using pentane, hexane, heptane and octane. These molecules are simple alkanes, containing only carbon and hydrogen, and thus have only London dispersion forces. -2- CHEMISTRY 182 Intermolecular Forces H2 C H3C CH3 C H2 H2 C H3C C H2 C H2 H2 C C H2 C H2 CH3 Hexane Pentane H3C H2 C H2 C C H2 CH3 H2 C H3C C H2 C H2 H2 C C H2 H2 C C H2 CH3 Heptane Octane The effect of dipole-dipole interactions on vapor pressure will be studied using ethanol, ethyl acetate and acetone. Structures for the compounds are given below. O O H3C H2 C OH C H2 Ethanol C H3C CH3 Acetone H3C C O CH3 Ethyl Acetate PROCEDURE: Part 1 – Overhead Pen Inks/Dyes 1. Use your 400-mL beaker and a second 400-mL beaker at your station and label one “20% Acetone” and the other “80% Acetone”. Pour 15 mL of each solvent into the corresponding beaker and cover with a piece of aluminum foil to let the air inside saturate with solvent vapors. 2. Obtain two pieces of the chromatography paper that are folded into 5 columns. For each piece of paper, at the very top of each column use pencil to write, “Red”, “Orange”, “Black”, “Green”, and “Blue”. In each column, exactly 1 cm from the bottom edge, use the “EXPO Vis-à-vis” pens to put a small spot of ink in the correct column. 3. When the ink is dry (important), fold the paper back to make a 5-sided vertical column with the spots at the bottom. Make sure the ends of the paper do not overlap. Remove the aluminum foil from the first beaker and put the paper column gently in the beaker. Make sure the spots are above the solvent level. Put the aluminum foil back over the beaker. Repeat with the second beaker and the second piece of paper. Record the time the papers are placed in the beakers. -3- CHEMISTRY 182 Intermolecular Forces 4. Observe the papers as you do the next part, but do not move the beakers once the process has begun otherwise you cause false movement of solvent due to splashing. 5. Remove the papers when the solvent has risen to within 1 cm of the top of the paper, recording the time when they are removed. Label the papers with the solvent used and let them dry on the counter. Staple the papers to the left hand page closest to the data in the lab notebook. Part II - Halide Determination 1. Use your 600-mL beaker. Place 15 mL of 80% acetone in the beaker and cover with a piece of aluminum foil. Let the beaker stand covered while you prepare the filter paper so that the air inside becomes saturated with solvent vapors. 2. Take a rectangular pieces of filter paper from the box and draw a pencil line parallel to the bottom of the long edge, about 1.5 cm from the edge. Mark with a pencil the location of the spots to be applied. Touch the paper only on the sides or top. 3. Locate the halide standards and use the attached capillary tubes to apply the halide standards, applying the spot just above the line as shown in the movie. Keep the tube in a vertical position and keep your finger on the top of the tube in order to make a small spot on the paper. Obtain a halide unknown from your instructor and spot on the same paper. 4. With clean, dry hands staple the right side of the paper to the left to form a cylinder. Be sure the two edges do not touch so that the solvent will not overlap and ruin the separation. 5. Remove the foil and place the paper cylinder inside, being careful that the solvent does not touch any of the spots. Do not splash or slosh the solvent around so that it travels up the paper by any means other than capillary action. Record the time the paper is placed in the beaker. 6. Remove the paper when the solvent is about1.5 cm from the top of the filter paper. Do not let the solvent run to the top of the paper. It takes around 20 – 30 minutes for the solvent to rise to the top. Record the time the paper is removed. 7. Open the cylinder and allow to air dry. Put the paper in the spray box and apply silver nitrate solution from the spray bottle. Be sure the fan is on and the paper thoroughly saturated with solution. Remember that silver nitrate will blacken your skin, so avoid any contact! Use your forceps to transfer the paper to a paper towel 8. Make note of the colors as the spots appear. All silver halides darken when exposed to light, but I- gives up its electrons most easily and therefore appears first. It may require several hours or even a couple of days for the F- spots to appear. Taking the paper outside, even if it is cloudy, will speed up the process. 9. When the spots have all appeared, calculate the Rf values for all the spots and identify the unknown by comparing both color and Rf. Staple the dried chromatograph to a left hand page in your notebook. -4- CHEMISTRY 182 Intermolecular Forces Part III: Vapor Pressure (Do this part in groups and keep vapors under the hoods!) 1. Locate the 10 cm filter paper strips, the Scotch tape, the multimeter with temperature probe, a stopwatch and the test tube rack containing the reagents in stoppered test tubes. 2. Fold three filter paper strips over the end of the temperature probe and tape in place. 3. Make sure the multimeter is in the temperature mode and turned on (red button in upper right). 4. Wait until the temperature reading has cooled to about 24°C (your fingers will warm it up), then record the initial temperature reading. 5. Remove the stopper from one of the test tubes, insert the filter paper covered probe into the test tube and remove it, starting the stopwatch. Make sure to record which regent you use. 6. Record the temperature after 45 seconds have passed. 7. Repeat this process with each of the reagents. 8. Make sure the Scotch tape is removed from the probe since acetone and ethyl acetate will start to dissolve the tape, making it sticky. A typical data table for this part would include the name of the compound, initial temperature, final temperature and time -5- CALCULATIONS: Part II: Halide Chromatography 1. Calculate the Rf factor for each halide ion. 2. Calculate the Rf factor for the spots in your unknown. Use color and R f to identify the ions present in the unknown. Part III: Vapor Pressure 1. Calculate the change in temperature for each of the compounds. 2. Calculate the rate of evaporation by dividing the change in temperature by the time. Faster evaporation rates should occur in compounds with lower intermolecular forces and higher vapor pressures. REPORT TABLE: Your report table should include the unknown number and identity of the ions present in the halide unknown for Part II, as well as the rate of evaporation for each compound in Part III. Other results will be given as part of the Discussion section. DISCUSSION QUESTIONS: 1. The more polar mobile phase is the 20% acetone solution (since it is 80% water). Paper is made from cellulose which contains a lot of oxygen atoms and OH groups. Write a short paragraph discussing which solvent best separated the ink components from the Vis-a-Vis pens. Consider in your discussion the relative attraction of the ink components for the solvent and for the paper. Does your discussion support the statement that these pens are water soluble? 2. Did any colors in the ink pens move differently using the two different mobile phases? Which is more polar, the blue dye molecule or the yellow dye molecule? 3. Write a short paragraph discussing the relative movement of the individual halide ions in the 80% acetone solvent. Which ions moved the most/least and why? Consider what you know about size and attractive forces between the ions, the paper and the solvent to explain the relative movement. 4. The lower the intermolecular forces, the more easily a compound leaves the liquid phase, the faster the rate of evaporation, and thus the higher the vapor pressure. Rank the non-polar alkanes – pentane, hexane, heptane and octane - in terms of increasing vapor pressure. Explain your ranking in terms of the molar mass and intermolecular forces present. -6- CHEMISTRY 182 Intermolecular Forces 5. Rank the polar molecules ethanol, acetone and ethyl acetate in terms of increasing vapor pressure. Explain your ranking in terms in terms of the intermolecular forces present, and the effects (if any) of molar mass. 6. How would you rank the relative strengths of the intermolecular forces present in the non-polar and polar molecules discussed in questions #4 and #5? Briefly explain if your data supports this ranking. -7- ...
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