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Boiling and Melting Points

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Submitted By micelhuang
Words 3944
Pages 16
Premiere
2nd Quarter / SY 2012-2013

Engr. Josephine A. Ng
Instructor

CHM142L / B21
Novhanda, Michelle
Group No. 6

Experiment No. 1
MELTING POINT AND BOILING POINT OF ORGANIC COMPOUNDS

ABSTRACT One of the methods to identify an organic compound is by determining its physical properties. In this experiment, we will only looking for their melting point and boiling point on the effects of the chemical structures. Melting point is defined as the temperature at a specific pressure which the solid and liquid phases are in equilibrium with each other. The boiling point is used to characterize a new organic liquid and knowledge of the boiling point helps to compare one organic compound to another, as in identifying unknown organic compound. The purpose of this experiment is to determine the effects of: intermolecular forces of attraction and geometric isomerism in the melting point of compounds; purity on the melting point range of organic compounds; intermolecular forces of attraction and branching on the boiling point of organic compounds. To do the experiment, Thomas Hoover apparatus is used for determine the melting point and micro test for the boiling point. Salicylic acid has the highest melting point between the rest of the sample with 152.5-157.1°C range and naphthalene has melted fastest as it melted at 75.8-80.6°C for the first part of the melting point experiment. The melting point of fumaric acid is between 278-286°C which is higher than Maleic acid with 122.3-131.8°C. Meanwhile, the pure urea has 127-132.2°C melting point which is higher than impure urea with 98.5-128.5°C. Meanwhile, Propanoic acid has the highest boiling point of 138-142°C and 2-Butanone act as the lowest boiling point with 80-86°C. The tert-butanol (most branched molecule) has the lowest boiling point of 102-108°C and n-butyl which has straight chains molecule has the highest with 120-136°C boiling point. In other words, pure substance has higher melting point than the impure. The trans-isomers (fumaric acid) is also have higher melting point due to its greater stability compared to cis-isomers (maleic acid). Branching of organic compound will decrease boiling point as branched molecules have weaker dispersion forces, unlike the straight chains. Therefore, straight chains have higher boiling point since more energy is needed to break the intermolecular attraction.

II. INTRODUCTION
Physical properties can be observed or measured without changing the composition of matter. Physical properties are used to observe and describe matter. Often, a compound may be identified simply by determining a number of its physical properties.
Physical properties include appearance, texture, color, odor, melting point, boiling point, density, solubility, polarity, and many others. Two of the most commonly recognize physical properties of a compound are boiling point and melting point.
The three states of matter are solid, liquid, and gas. The melting point and boiling point are related to changes of the state of matter. All matter may exist in any of three physical states of matter.
The Melting point is the temperature at which the solid phase changes over to the liquid phase. Melting point, unlike boiling point, is a solid state property and hence is influenced by properties of solids such as amorphous or crystalline nature, allotropy, polymorphism, molecular symmetry, as additional and more important factors than intermolecular forces.
The boiling points of organic compounds can give important clues to other physical properties and structural characteristics. A liquid boils when its vapor pressure is equal to the atmospheric pressure. Vapor pressure is determined by the kinetic energy of molecules. When the temperature reaches the boiling point, the average kinetic energy of the liquid particles is sufficient to overcome the forces of attraction that hold molecules in the liquid state.
The physical properties of a compound are determined by the attractive forces between the individual molecules, called Intermolecular Forces. It is often difficult to predict a precise melting pint or boiling pint or biling pint for a compound. All intermolecular forces are electrostatic, that is, these forces occur as a result of the attraction between opposite charges. There are the four types of intermolecular forces: ion-ion, ion-dipole, dipole-dipole, and London dispersion forces or van der Waals forces. Hydrogen bonding is under dipole-dipole branch.
Ion-ion interactions are the strongest. Since a large amount of heat energy must be provided to disrupt these forces, ionic compounds typically have high melting and boiling points. Ion-dipole interaction is the force of attraction between an ion and a polar molecule. Dipole-dipole interactions are the electrostatic attractions between polar molecules, which align in such a way that the opposite poles are in proximity. London forces between essentially non-polar molecules are the weakest of all intermolecular forces. "Temporary dipoles" are formed by the shifting of electron clouds within molecules. These temporary dipoles attract or repel the electron clouds of nearby non-polar molecules. The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine.
Strength of IMFs: London Forces < Dipole Forces < Hydrogen bonds < Ionic Forces
Stereochemistry is always encountered throughout organic chemistry. Most geometrical isomers result from cylic systems or restricted rotation about double bonds. Stereo isomers are molecules with the same chemical formula and the same molecular structure, but different orientation of the atoms in space.
The existence of geometric or cis-trans isomers is the consequences of the lack of rotation about the double bonds. Maleic acud and fumaric acid are geometric isomers of butenedioic, where maleic acid is cis-butenedioic and fumaric acid is trans-butenedioic. Fumaric acid, trans-butenedioic | Maleic acid, cis-butenedioic |

Maleic acid is more soluble in water than fumaric acid. When maleic acid is heated with hydrochloric acid, an acid-catalyst, maleic acid will be converted t fumaric acid. Impurities cause a melting point range to be observed, rather than a sharp transition from solid t liquid, with the size of the range often indicating the amount of impurity. The presence of even a small amount of impurity will lower the melting point by a few degrees and broaden the melting pint temperature range. Because the impurity causes defect is the crystalline lattice, it is easier to overcome the intermolecular interactions between molecules.
III. EXPERIMENTAL RESULTS I. Melting Point A. Structural Effect a. Intermolecular Forces of Attraction COMPOUND | T1 (OC) | T2 (OC) | MELTING POINT | Benzoic acid | 122.6 | 132.5 | 127.55 | Benzoin | 133.3 | 138.1 | 135.7 | Naphthalene | 75.8 | 80.6 | 78.2 | Salicylic acid | 152.5 | 157.1 | 154.8 | Urea | 127 | 132.2 | 129.6 | b. \Geometric Isomers of Substitent COMPOUND | T1 (OC) | T2 (OC) | MELTING POINT | Maleic acid | 122.3 | 131.8 | 127.05 | Fumaric acid | 278 | 286 | 282 |

B. Effect of Purity on Melting Point Range COMPOUND | T1 (OC) | T2 (OC) | MELTING POINT | Pure urea | 127 | 132.2 | 129.6 | Impure urea | 98.5 | 128.5 | 113.5 |

II. Boiling Point C. Structural effect c. Intermolecular Forces of Attraction COMPOUND | T1 (OC) | T2 (OC) | BOILING POINT | n-Butanol | 120 | 136 | 128 | 2-Butanone | 80 | 86 | 83 | n-Heptane | 142 | 151 | 146.5 | Propanoic acid | 138 | 142 | 140 | n-Hexane | 110 | 115 | 112.5 |

d. Branching COMPOUND | T1 (OC) | T2 (OC) | BOILING POINT | n-butyl alcohol | 120 | 136 | 128 | sec-butyl alcohol | 110 | 112 | 111 | tert-butyl alcohol | 102 | 108 | 105 |

IV. EXPERIMENTAL SECTION
Reagents:
Naphthalene Benzoic acid
Salycylic acid Benzoin Urea Maleic acid
Fumaric acid n-Butanol
2-Butanone n-Hexane n-Petane Propanoic acid sec-Butanol Tert-butanol
Glycerol Silicone fluid
Apparatus:
Thomas-Hoover Melting Point Apparatus
Thiele tube Hot plate Thermometer Micro test tubes Capillary tubes Procedures: I. Melting Point A. Structural Effect a. Intermolecular Forces of Attraction
Test Compounds : Finelt fround naphthalene, benzoic acid, salicylic acid, benzoin and urea.
Procedure
1. Get capillary tubes and seal one end of each tube using heat. These tubes will serve as the sample holders. 2. Get a small amount of the slid sample and press it into the open end of a capillary tube until enough samples are inside the tube. Tapthe closed end of the capillary tube on the table in order to move the sample to the bottom. If the sample remains on the upper portion of the tube, drop the capillary tube inside a one meter glass tubing in which one end is placed on the floor. The sample should be at least 5-6 mm in height and must be tightly packed (figure 4.1) 3. Place the capillary tube containing the sample inside the sample holder of the Thomas Hoover apparatus (Figure 4.2) for melting point determination. 4. Switch on the apparatus. Adjust the heat and turn on the stirrer. Record the temperatures at which the compounds start to liquefy and at which it has completely liquefied as the melting-point range. 5. Repeat the above procedures until all the samples were tested.
Note: Allow the silicone fluid to cool down first before performing another test. b. Geometric Isomers
Test compounds: Finely ground maleic acid and fumaric acid
Procedure: Follow te same procedure as of part I.A.a

B. Effect of Purity on Melting Point Range Test Compounds: Finely ground urea and prepared impure urea Procedure: Follow te same procedure as of part I.A.a II. Boiling Point C. Structural effect c. Intermolecular Forces of Attraction
Test compounds: n-Butanol, 2-Butanone, n-Hexane, n-Heptane, Propanoic acid
Procedure:
1. Fill up the Thiele tube with glycerol up to the level slightly above its arm and assemble the set-up shown in figure 4.3. 2. Get a capillary tube and heat it at the middle until the tube become completely tisted. 3. Place the capillary tube inside the micro test tube containing two to three drops of the test compounds. 4. Place the micro test tube containing the sample and the capillary tube side by side with the thermometer. Use a 2-mm wide rubber tubing to hold the capillary tube. (figure 4.4) 5. Immerse the thermometer in the glycerol (bath liquid) ensuring that the rubber tubing is not immersed in the liquid. 6. Heat the oil bath rapidly until a flow of bubbles comes out of the capillary tube. 7. Remove the flame and allow the oil to cool down. As the temperature drops, the bubbling will slowly cease and the liquid will start to enter the capillary tube. Record this temperature as the boiling point of the test compound. 8. To obtain the boiling point range of the test compound, reheat the oil slowly, note the temperature when the liquid comes out of the capillary tube. The temperature previously recorded and this current temperature is the boiling point range. 9. Repeat the above procedures until all the sample are tested.
Note: Allow the glycerol to cool down to room temperature before reusing it. d. Branching
Test compound: n-Butyl alcohol, sec-Butyl alcohol, tert-Butyl alcohol
Procedure: Follow the same procedure as of Part II.A.a

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4
V. INTERPRETATION
In this experiment, the intermolecular forces of attraction play important role on determining the physical properties, namely boiling point and melting point. From the introduction, it is stated that the strength of attraction is directly proportion with the melting point and the boiling point. As comparison, of course the hydrogen bond is stronger than London forces and dipole-dipole force. There are no ionic forces in this experiment. In the first part of the experiment, Salicylic acid has the highest melting point between the rest of the sample with the range 152.5 to 157.1°C. Naphthalene has melted fastest as it melted at range 75.8 to 80.6°. Look at the structure of this two structure, it can be identified why the result is as stated before.

The melting point of fumaric acid is between 278-286°C which is higher than Maleic acid with 122.3-131.8°C. Meanwhile, the pure urea has 127-132.2°C melting point which is higher than impure urea with 98.5-128.5°C. Meanwhile, Propanoic acid has the highest boiling point of 138-142°C and 2-Butanone act as the lowest boiling point with 80-86°C. The tert-butanol (most branched molecule) has the lowest boiling point of 102-108°C and n-butyl which has straight chains molecule has the highest with 120-136°C boiling point. In other words, pure substance has higher melting point than the impure. The trans-isomers (fumaric acid) is also have higher melting point due to its greater stability compared to cis-isomers (maleic acid). Branching of organic compound will decrease boiling point as branched molecules have weaker dispersion forces, unlike the straight chains. Therefore, straight chains have higher boiling point since more energy is needed to break the intermolecular attraction. Compound | Forces of attraction | Melting point oC | Benzoic acid | Dispersion ForceDipole BondHydrogen Bond | 127.55 | Benzoin | Dispersion ForceDipole BondHydrogen Bond | 135.7 | Naphthalene | Dispersion Forces | 78.2 | Salicylic acid | Dispersion ForceDipole BondHydrogen Bond | 154.8 | Urea | Dispersion ForceDipole BondHydrogen Bond | 129.6 | From the table of above, we can know that all of the compounds have through dispersion force, dipole and hydrogen bonding except naphthalene. Naphthalene has very symmetric structure that is why it is easier to melt and it doesn’t have hydrogen bonding as hydrogen bonding only happen between hydrogen with nitrogen, oxygen and fluorine. In naphthalene, it has only bonds between carbon and hydrogen. However, benzoin has a higher melting point than benzoic acid because it has bigger molecular weight. Due to bonding between hydrogen and nitrogen in urea, it become the second highest as it has hydrogen bonding. Salicylic has the highest melting point due to its very polar structure and the hydrogen bonding between oxygen and hydrogen. Compound | Geometric isomers | Melting Point | Maleic acid | Cis-isomer | 127.05 | Fumaric acid | Trans-isomer | 282 | Maleic acid and fumaric acid are both geometric isomers of butenedioic acid. They are two different compound when it comes to the geometric isomers although they have the same molecular weight and composition. Their difference is due to their different orientation of the carboxylic group in the double bond. Fumaric acid, a trans-isomer is more stable than the cis-isomer, maleic acid. The more stable the compound the higher its melting point, because they need more energy to break the bond. So, Maleic acid has a lower melting point than fumaric acid. Compound | Crystalline lattice | Melting point | Pure urea | | 129.6 | Impure urea | | 113.5 | Due to impurities of then second sample, it has lower melting point than the pure compound. When there is impurity on a compound, the melting point will be lower than the pure one, will broader and takes less energy to disrupt crystal lattice. The pure solid has a very tight crystal lattice and will have a sharp melting point. As heat is added to a solid sample, the energy of the molecules becomes sufficient to overcome the intermolecular forces which had held the molecules in the solid matrix. Impurities in the lattice structure weaken the lattice, which results in less energy needed to overcome the intermolecular forces. The disruption of the lattice is non-uniform, however. The molecules closest to the impurity feel a greater effect; further away, the lattice is relatively undisturbed. This effect causes the melting range of an impure sample to broaden, as the lattice structure near the impurity breaks down at a lower temperature than the remaining sample. Compound | Boiling point | n-butanol | 128 | 2-butanone | 83 | n-heptane | 146.5 | Propanoic acid | 140 | n-hexane | 112.5 | The boiling point of an organic compound is different from each other. Boiling point, is depend on the forces that is bind the components in the molecules. Usually, the greater molecular weight of a molecule, the boiling pint will increase, too. But still it depends on the branching of the molecule. More branch means the boiling point will decrease. So, normally straight chain will have higher boiling point than the branched one. In the result we can see that the propanoic acid has the highest boiling point. Due to the error, we get the n-heptane as the highest point. Propanoic acid has the highest boiling point because of the polarity and the forces between them. Again because of the error in doing the experiment, we got 2-butanone as the lowest boiling point. Actually n-hexane will have the lowest boiling point as 2-butanone has hydrogen bonding in their structure, and n-hexane only a straight chain that is non-polar molecule. Compound | Boiling point | n-Butyl alcohol | 128 | sec-Butyl alcohol | 111 | tert-Butyl alcohol | 105 | The experiment above describe where the boiling point of an organic compound is affected by the way they are branched. n-Butyl alcohol has a straight chain structure compared to tert-Butyl alcohol which has the most branch structure. Branching affects the boiling point of a compound in the way that branch molecules have weaker dispersion forces and it also decreases the surface area of the compound. VI. CONCLUSION

Each compound has different boiling point and melting point. Boiling point and melting point is two of the most important of physical properties to determine the factors or the structure inside of the compounds. The factors itself is intermolecular forces, geometric isomers, purity, polarity, branching and molecular weight.

The greater the intermolecular forces attraction of a compound the higher the melting point and boiling point are needed to break the attraction or force.
Geometric isomers define the stability of a compound. trans-isomers are more stable than cis-isomers, therefore, from the experiment we know that the more stable of a compound the higher melting point and boiling point they need.

Purity is also another factor affecting the melting point. Impurities in the sample lowers the melting point and it impure samples have broader range of the melting point.

Branching and polarity also affect the boiling point. The more branch the compound the lower its surface area, resulting to a lower boiling point. Non-polar has lower boiling point than the one has polar in the compound.

VII. REFRENCES 1. Brook and Consiglio. Intermolecular Forces. http://www.haspi.org/curriculum-library/MedicalChemistry/03%20Standard%202%20Chemical%20Bonds/Labs%20and%20Activities/IntermolecularForces.pdf 2. Klein. Organic Chemistry, 1st edition, John Wiley & Sons, Inc, United States 3. Chang, R. 2010. General Chemistry, 10th edition, McGraw Hill, New York. 4. Klein. Organic Chemistry, 1st edition, John Wiley & Sons, Inc, United States. 5. Molina and Ruiz. Boiling Point and Melting Point Determination. http://www.scribd.com/doc/79069710/Experiment-5-Ppt 6. http://www.ciens.ucv.ve/quimicaorg/PRACTICAS/practica%203.pdf 7. http://survival-training.info/Library/Chemistry/Chemistry%20-%20Geometric%20Isomers%20-%20Unknown.pdf 8. http://www.elmhurst.edu/~chm/vchembook/104Aphysprop.html 9. http://faculty.swosu.edu/william.kelly/mp_pl.pp 10. http://www.123helpme.com/view.asp?id=148415 11. http://faculty.coloradomtn.edu/jeschofnig/class/class_jeschof/ch1-lb10.htm

VIII. GUIDE QUESTION 1. Rank the following compounds in decreasing melting point : Benoic acid, Benzoin, Naphthalene, Salicylic acid and urea. The following information are based on the true value of the melting point of different organic compounds Naphthalene (78.2) Dispersion Force Benzoic Acid (127.5)
Dispersion Force
Dipole Bond Hydrogen Bond (1 O-H) Urea (129.6)
Dispersion Force
Dipole Bond Hydrogen Bond (2 N-H) Benzoin (135.7)
Dispersion Force
Dipole Bond (More Polarized) Hydrogen Bond (1 O-H) Salicylic Acid (154.8)
Dispersion Force
Dipole Bond Hydrogen Bond (2 O-H)

2. How does the molecular geometry affect the melting point of an organic compound? Molecular geometry has some properties itself, for example is the bond angles and polarity. The bigger the bond angle the bigger the bond length. If two molecules are close to each other, it means that its intermolecular force is strong that lead the compound to have higher melting point. The more polar of a compound, the higher melting point that will be resulted. 3. How do impurities affect the melting point of an organic compound? Impurity of organic compounds will affect the melting point as the impurity will lower the melting point and make broader the melting point range. This is because impurities disrupt the crystalline lattice arrangement of the molecules resulting in randomly oriented and weaker bonds, thus, lower temperature is needed for the substance to melt. 4. In general, what are the factors that affect the melting point of an organic compound? What are the effects? There are three factors that can affect the melting point of an organic compound, namely intermolecular force of attraction, geometric isomers/substituent and purity. Intermolecular force
The stronger the force of attraction, the more energy is needed to break the attraction resulting to higher melting point. Geometric isomers/ substituent: trans-isomers are more stable than cis isomers. A more stable the compound means it has a greater molecular force causing higher melting point. Purity
Pure substance have higher melting point than impure ones because impurities alter the crystalline lattice of the compound.

5. Consider the boiling points of n-Butanol, 2-Butanone, n-Heptane, and Propanoic acid. How does polarity affect the boiling point of organic compounds? * n-Butanol (120) has a high boiling point due to its polar end and hydrogen bonding.\ * 2-Butanone (83) is polar because of O double bond * Propanoic Acid (146.5) has the highest boiling point among the four because it is a highly polar compound and it has a hydrogen bond. * n-Heptane (90) is a non-polar organic compound which has 0 polarity has the lowest boiling point. 6. Compare the melting points of n-Pentane and n-Hexane. Which has the higher boiling point? Justify your answer. n-hexane has higher boiling point than n-Pentane. Both n-Hexane and n-Pentane has the straight chain. However, n-hexane has a higher molecular weight. Due to this factor more energy is needed in order to break the molecules resulting to higher boiling point. 7. Compare the boiling points of n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol. What makes the difference? n-butyl alcohol, sec-butyl alcohol and tert-butyl alcohol have the same numbers of carbon, oxygen and hydrogen atoms. However they are different from each other wjen it comes to their structures. n-butyl alcohol is a straight chain, sec-butyl alcohol has its OH molecule attached to the second carbon atom. tert-butyl alcohol has more complex structure than the rest. These compounds have different boiling point due to branching. A more branched compound has a lower boiling point compared to the straight chains. 8. In general, what are the factors that affect the boiling point of organic compounds? What are the effects of each factor? There are four factors affecting the boiling point of an organic compound: intermolecular forces of attraction, branching, branching and molecular weight. Intermolecular force: the stronger the force of attraction, the more energy is needed to break the attraction resulting to higher boiling point. Branching: more branches will lead to lower boiling point. Polarity: the more polar the compound the higher the boiling point. Molecular weight: the greater molecular weight the higher boiling point. IX. COPY OF GRADED PDS…...

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Does Salt Effect the Boiling Point of Water

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