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physical properties of organic matter...

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Physical Properties 1

Physical Properties of Organic Molecules September 9, 2020 CHM2210L U03 Fall 2020

Table of Contents

Physical Properties 2 Purpose…………………………………………………………………………………………….3 Part 1: Melting Point Introduction………………………………………………………………………………..3 Procedures…………………………………………………………………....……………4 Data/Observations…………………………………………………………………….…...5 Conclusions...……………………………………………………………………………...5 Questions…………....……………………………………………………………..5 Part 2: Boiling Point Introduction………………………………………………………………………………..6 Procedures…………………………………………………………………....……………6 Data/Observations…………………………………………………………………….…...7 Conclusions...……………………………………………………………………………...7 Questions…………....……………………………………………………………..7 Part 3: Solubility Properties of Organic Solids Introduction………………………………………………………………………………..8 Procedures…………………………………………………………………....……………8 Data/Observations…………………………………………………………………….…...9 Conclusions...…………………………………………………………………………….10 Questions…………....……………………………………………………………10 References………………………………………………………………………………………..11

Physical Properties of Organic Molecules Experiment #1, “Organic Chemistry I & II Laboratory Manual” Fourth Edition, Keller, L., 2014, pages 9-18.

Physical Properties 3 Purpose The goal of this experiment is to experimentally observe the physical properties of organic chemicals. We will do this by learning how to measure and manipulate melting point, boiling point, solubility, and absorptivity. Part 1: Melting Point Introduction The first physical property we will learn about is melting point. This is the point, or temperature, where a substance changes state from solid to liquid, or the states are in equilibrium. It is an important characteristic of any solid to understand. However, since many substances have similar melting points, other physical properties must also be observed. Organic molecules, as will be seen, usually have narrow temperatures in melting points and melt very quickly. Impure organic molecules, such as compound substances, will melt less slowly over a wider range. Because it is difficult to find a specific melting point temperature in a laboratory setting, a melting point range is usually used. While a pure substance, like cinnamic acid, will melt at higher temperature like 135℃ , mixtures will melt at temperatures as low as 100℃ . The reason for this is because of melting point depression, which also applies to the substance's freezing point, which states that as purity decreases, melting point range increases.

Procedure 1. Pick one of the compounds between benzoic acid and urea. 2. Add a small amount of compound to

The compound must be completely dry for it to truly melt instead of gradually transfer to liquid.

Physical Properties 4

weighing paper. 3. Fold paper over the substance and crush it evenly.

The solid must be crushed into a fine powder so there are no clumps and the heat transfer is uniform.

4. Obtain a melting point tube and pinch a small amount of solid in the open end of the tube. 5. Plug in Mel-Temp apparatus and place the melting point tube and thermometers in their respective slits. 6. Turn the apparatus on and gradually raise the temperature. 7. For benzoic acid, raise the temperature to 110℃. 8. Keep raising the temperature and observe the state of the reagent through the apparatus. 9. Record two temperatures: one when liquid starts appearing in the tube, and the second when all the reagent is made liquid and there is no solid. 10. When trial is done, turn off the apparatus and carefully remove the melting point tube and thermometer. 11. Repeat steps 2-10 with the other solid, urea, but raise the temperature to around 130℃. Table of Physical Constants Compound

Formula

Formula Wt.

Melting point

Boiling point

Density

Solubility

Urea

CH4N2O

60.06 g

134-136℃

decomposes

1.6 g/cm^3

Highly soluble

Benzoic acid

C7H6O2

122.12 g

121-123℃

250℃

1.27 g/cm^3

Insoluble

Data/Observations When the sample was first being heated at 25℃ , there were no changes. It persisted as a solid until around 115℃. At 119℃, the substance in the melting tube began heating to a liquid, marking the first temperature of the melting point range. Being careful to gradually heat the sample without overheating, the solid continued melting at higher temperatures. It was entirely melted and liquid at 123℃ , which was identified as the end of the melting point range, as this

Physical Properties 5 was a visual cue that the energy was completely transferred to a liquid form. Conclusions According to this lab, it is easy to see that each substance has a melting point range, or a range of temperatures where said material changes from solid to liquid until completion. Using a powdered form rather than crystalline, the sample has a more uniform transfer of energy, which is needed to accurately determine a temperature range. With this information, the identity of an unknown sample can be found by comparing ranges. This experiment was successful because the theoretical and experimental melting points of the solids were the same. Questions 1. In the determination of a melting point, why is it necessary to: a. Use a powdered rather than a crystalline sample? Since the powdered material has smaller particles and higher surface area with no clumps, the heat transfer will be even, aiding in finding an accurate melting point range. b. Use a new capillary tube for each determination Reused capillaries may have impurities that would contaminate the sample and change the temperatures at which it would melt. c. Not heat the sample too rapidly near the melting point Overheating the powder may prevent us from finding the correct minimum temperature at which the material would be completely liquified. 2. The melting point of a pure unknown compound was found to be 95.4℃. The unknown was thought to be one of three unknown compounds, samples of which were all available. Describe a procedure for identifying the unknown as one of the three unknown compounds. You must conduct a trial with each of the three substances and one trial with the unknown substance. The substance that the unknown sample has the most similar melting point range with will be the identity of the unknown. 3. Which has a lower freezing point: ice or ice cream? Why? Ice cream has a lower freezing point because the molecules are more complex, making it more difficult for the bonds to break and attract water molecules in order to become solid ice.

Part 2: Boiling Point Introduction We will be studying a similar physical property to further examine the identity of a material. The next characteristic, boiling point, describes the point where a liquid heats up to the point where it transfers to a gas state. When this state change occurs, the molecules of the liquid of our experiment will vibrate more quickly and give the bonds enough energy to break. This will make the material become a lighter, gaseous state. The vapor pressure of a liquid will

Physical Properties 6 increase as the temperature increases, causing the molecules to continuously hit the sides of the container it’s surrounded by. Procedures 1. Take a capillary and fill it halfway with ethanol. 2. Use a clamp to fasten the capillary to the thermometer, and then to the stand. 3. Insert melting point tube upside down in the capillary. 4. To use the bunsen burner, open the gas and use a striker to light the flame. 5. Hover the burner under the capillary, not directly, up to around 78 degrees. 6. Observe the bubbles to confirm heating. 7. When it is constantly bubbling at around 85℃ , take away the burner. 8. Wait a few minutes for the bubbles to slow down. 9. Record the temperature where the ethanol stops bubbling. 10. Record the second temperature when the liquid starts bubbling up the melting tube. Table of Physical Constants Compound

Formula

Formula Wt.

Boiling point

Density

Solubility

Ethanol

C2H5OH

46.07 g

78-80℃

0.8 g/cm^3

miscible

Data/Observations After following proper ethanol heating methods with the bunsen burner, the sample started bubbling at 78℃ . This marks the first part of the boiling point range. The sample was then heated to just over 85℃ when it was constantly bubbling. We then had to wait a few minutes to record the temperature at which the bubbling slowed down and stopped. This temperature was 80℃ , which is the optimal temperature at which liquid ethanol will evaporate as gas molecules. Conclusions As seen in this lab experiment, once the temperature was about 78 degrees, the oxygen in

Physical Properties 7 the melting tube evaporated, and the pressure of the liquid ethanol was equal to the pressure of the atmosphere. This is an important concept to understand as it explains why the ethanol stopped bubbling as the temperature increased. Questions 1. Why is it necessary to position the sample tube right next to the thermometer bulb in the Thiele tube? The Thiele tube is designed to create convection currents and heat the entire tool evenly. The tube is placed right next the thermometer to accurately record the temperature at which bubbles start forming. 2. The normal boiling point of benzene is 80℃ . What is the vapor pressure of benzene at 80℃? The vapor pressure of a compound at its boiling point is equal to the atmospheric pressure. Therefore, the vapor pressure of benzene at 80℃ is 1 ATM. 3. Define boiling point. How does this definition describe what is occurring in microboiling point determination method; i.e., that the boiling point is the temperature at which the liquid levels inside and outside of the capillary tube are the same? Boiling point is defined as the temperature at which the vapor pressure of a liquid becomes equal to the atmospheric pressure. This is accompanied by a transfer of energy that causes bubbling or evaporating, and a change of the liquid to a gaseous state.

Part 3: Solubility Properties of Organic Solids Introduction Solubility is an important characteristic of any substance, especially when it is required to solve for an identity. It is defined as the ability for a given substance or solute to dissolve in a solvent, like water, to make a solution. If the solute dissolves, the solid changes physical state. Energy is needed for this change, so if a solid is only partly soluble, the temperature can be increased to continue dissolution to an extent. The heat energy is what breaks the bonds in the solid molecules. Solubility can be predicted in a few ways, but one that will be important in this experiment includes knowing the polarity of substances. Polar molecules tend to be soluble with other soluble molecules, like sodium chloride and distilled water. The same goes for nonpolar

Physical Properties 8 molecules. We can predict the least polar solute and solvent in this lab, naphthalene and hexane, will dissolve with each other. Procedures 1. Add a small portion of your solid in weighing paper and fold it to crush the powder. 2. Add fine powder into a new tube. 3. Use a full pipet to transfer each liquid into the test tube. 4. Manually swirl the test tube to determine the solubility of the powder in each liquid. 5. If the solute seems slightly or partially soluble, hold the test tube in a hot water bath for a few minutes. 6. Take the tube out and reanalyze the solubility of the solid. 7. Repeat steps 1-6 for all four solids and each of the six solvents.

One full pipet equals to about 25 drops of solvent. Swirling the tube will increase the surface area of the solute, allowing for more dissolution to occur.

Table of Physical Constants Compound

Formula

Formula Wt.

Boiling point

Melting point

Density

Solubility

Sodium chloride

NaCl

58.44 g

1465℃

801

2.2 g/cm^3

soluble

Urea

CH4N2O

60.06 g

134-136℃

decomposes

1.6 g/cm^3

Soluble in water

Benzoic acid

C7H6O2

122.12 g

121-123℃

250℃

1.3 g/cm^3

Insoluble in water

Naphthalene

C10H8

128.17 g

218℃

80.26℃

1.1 g/cm^3

insoluble

Distilled Water

H2O

18.02 g

100℃

0℃

1 g/cm^3

x

Ethanol

C2H5OH

46.07 g

78-80℃

-114℃

0.8 g/cm^3

miscible

Ethyl Acetate

C4H8O2

88.11 g

77℃

-83.6℃

0.9 g/cm^3

soluble

THF

C4H8O

72.11 g

66℃

-108.4℃

0.9 g/cm^3

miscible

Toluene

C7H8

92.14 g

110.6℃

-95℃

0.9 g/cm^3

insoluble

Hexane

C6H14

86.18 g

68.7℃

-95℃

0.7 g/cm^3

insoluble

Physical Properties 9

Data/Observations The following table shows that solutes dissolved in solvents of similar polarities--polar solutes dissolved in polar solvents, while nonpolar solutes dissolved in nonpolar solvents. In solvents of different polarities, solutes only dissolve with heat energy from the hot water bath. Even then, the solutes were partially soluble, and some would even recrystallize a few minutes after being taken out of the water bath. If kept in conditions of higher temperatures, we can assume the solute would continue being part of the solution. Solvent → Solute ↓

Distilled water

Ethanol

Ethyl Acetate

THF

Toluene

Hexane

NaCl

Soluble

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Insoluble

Urea

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Benzoic acid

Partially soluble (crystallize d)

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Partially soluble (crystallize d)

Naphthale ne

Insoluble

Partially soluble

Partially soluble

Partially soluble

Partially soluble

Soluble

Conclusions Using knowledge from general chemistry courses and basic organic chemistry, we knew that most organic molecules are usually nonpolar and soluble in organic solvents like hexanes, but not in water. This is because nonpolar substances dissolve in other nonpolar substances, and water is a polar molecule. Our experimental design supports this trend by showing that materials of opposite polarities will not dissolve with each other. If a solute and solute are slightly soluble, like THF and naphthalene, you can further dissolve the solute slightly by swirling and heating the test tube they are found in. The swirling not only manually breaks apart the solute, but heating it in a hot water bath gives the bonds enough energy to break apart. However, once the solvent cools again in room temperature air, the solute may reform those bonds and recrystallize into solids, as is their trend without the aid of heat energy. This is an important concept of organic molecules to understand and be able to manipulate. Questions 1. According to your results, list the solids tested in order of increasing polarity- starting

Physical Properties 10 with the least polar and finishing with the most polar. Explain how you arrived at your conclusions. Most polar, NaCl > urea > benzoic acid > naphthalene, least polar. The most polar solute was the easiest to dissolve in water, while the least plar was the hardest to dissolve. 2. Did any of the solutes recrystallize when the solvent was cooled? List the solute(s) and the solvent(s) where this occurred. Yes this happened with benzoic acid and solvents water and hexane. 3. What criteria make a solvent satisfactory for recrystallization? The polarity and the substances and their temperatures. If a solute needs to be heated to dissolve in a solvent, it will likely recrystallize after being cooled down again.

References Keller, Leonard, et. al. “Organic Chemistry I & II Laboratory Manual” 4th edition, Hayden-McNeil Publishing, 2014. Pp 9-18....