Experiment 2 MELTING POINT DETERMINATION

EXPERIMENT 2 MELTING POINT DETERMINATION

GROUP 6

ABO, LYNETTE MAE G. ALVIAR, JOHN PAUL N. GARCIA, MARY GRACE N. SALVACION, PATRCIA ANN SARMIENTO, YVONE

1|ORGA NIC CHEM IST RY LAB ORAT ORY C H M 1 4 1 L – A 0 3

ABSTRACT

The experiment is all about melting point determination of an unknown compound. Melting point is the temperature at which solid melts to turn into a liquid form. This experiment is about to determine the melting points of different organic compounds and to use these to identify unknowns. It is done by heating a solid until it reached a temperature where it melts. The objectives for this experiment are: 1 Identify an unkno wn compound using its experimentally determined melting point. 2.) Summarize the factors that affect the melting point of an organic compound. 3.) Analyze the effects of impurities in the melting point range of a substance.


INTRODUCTION

The melting point or sometimes called liquefaction point, is the temperature at which the substance changes its state from solid to liquid at atmospheric pressure. At the melting point of a substance the solid phase and liquid phase are in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard pressure. The best example of melting point is the disappearance of ice rather than the formation of ice, that is, the melting point. Melting points are often used to distinguish and differentiate organic and inorganic compounds and to measure their purity. The melting point of a pure substance is always higher and has a smaller range than the melting point of an impure substance or of mixtures. The higher the quantity of other components, the lower the melting point. The melting point is sensitive to extremely large changes in pressure, but generally this sensitivity of magnitude is less than that for the boiling point, because the solid-liquid transition represents only a small change in volume. A substance is denser in the solid than in the liquid state, which is why the ice floats on water. The melting point will increase with increases in pressure, making it directly proportional with each other. In this second experiment, there is an unknown substance and determined its melting point. Determining the melting point of a compound is one way to test if the substance is pure. A pure substance generally has a melting range of one or two degrees. Melting range is the difference between the temperature where the samp le starts to melt and the temperature where melting is complete. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample. In this 3|ORGA NIC CHEM IST RY LAB ORAT ORY C H M 1 4 1 L – A 0 3

second experiment, there is an unknown substance and determined its melting point. The objectives of this experiment is to identify an unknown compound using its experimentally determined melting point and summarize the factors that affect the melting point of an organic compound. And also to analyze the effects of impurities in the melting point range of a substance.


PHYSICAL DATA LIST OF CHEMICALS NAME OF

MELTING

COMPOUND

PO I NT, ° C

1. Benzoic Acid

121 - 123

2. Naphthalene

79 - 82

3. Salicylic Acid

157 -159

4. Urea

133 - 135

5. Vanillin

82

6. Fumaric Acid

287

7. Maleic Acid

135

METHODOLOGY

The first part of melting point determination was to prepare a sample solid to melt. An unknown sample was pulverize and packed inside a closed end capillary tube about a 0.5cm.drop repeatedly until it compressed. Two capillary nee d to be filled. Second part was melting it using a hot plate beside an iron stand. A half-filled 50ml beaker mineral oil will be heated and inside there are the thermometer tied with capillary tube using rubber band. The rubber band must be placed above the level of the oil in the beaker. Turning on the hot plate to 300°C monitor the temperature until the first sign of melting and the last sign that was when the crystals are liquefied. After that repeated again for the second trial.


DATA AND OBSERVATIONS Table 1.1 M elti ng point of an un kn own soli d sample

Physical State and Appearance

colorless and odorless solid

Temperature (Initial Melting)

115°C

118°C

Temperature (All Crystals liquefied)

120°C

132°C

Table 1.2 I denti ty of th e Un kn own Compound Identity of Unknown Sample Based on Melting Point (Systematic Name)

Urea Literature Values of the Unknown

Molecular Formula

CH4N2O

Molar Mass

60.06 g/mol

Lewis Structure Physical State and Appearance

white solid

Density

1.32 g/cm³

Dipole Moment

4.56 D

Boiling Point

Decomposes

Melting Point

133 to 135 °C

Solubility in Water Vapour Pressure, kPa at 20°C

107.9 g/100 ml (20 °C) 0.08 kPa

Flash Point

non - flammable

Auto-Ignition Temperature Occupational Exposure Limits (TLV)

non - flammable

Inhalation Risk

15 mg/m3 total dust and 5 mg/m3 respirable dust Over exposure may produce irritation of the mucous membranes, nose, throat, coughing and shortness of breath.


DISCUSSION OF RESULTS

The objective of this experiment is to determine the melting point of an unknown solid sample. It was executed by pulverizing the solid sample and putting it inside a capillary tube. It is important that the solid sample is pressed into the tube. It is then submerged and heated into a mineral oil until the melting point is determined. The group must be mindful and observant on the changes and notice what temperature is the initial melting point — when the sample started to liquefy, and the temperature when all the crystals liquefied. The first trial was very tricky because the group lack prior knowledge about the experiment. The initial melting point obtained was 115°C and the temperature in which all of the sample melted was 120°C. The group was hesitant of the results because there was still a little amount of sample that was not melted and because the hot plate was defective — the electric cord is loose, interrupting the heating process. On the second trial, the group decided that it is best to wait until all of the sample was totally melted. The initial melting point is 118°C and b y 132°C, all the crystals liquefied. Based on the melting point, the unknown sample is Urea. To obtain more accurate results, be sure that the hot plate or any other instruments is not defective. The sample size should be enough to see in the tube. Too much sample will lead to a wide temperature range. Avoid contamination on the sample because this will give an erroneous or lower melting point. Lastly, the sample must be firmly packe d in the bottom of the capillary tube to have an efficient heat transfer.


SUMMARY AND CONCLUSION The objectives of this experiment is to state the factors that affect the melting point of an organic compound, to identify the unknown compound based on its melting point, and to analyze the effects of impurities in the melting point range of a substance. The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure. During the heating, we noted the temperature of the first drop of liquid forms in the sample and the point at which the sample is completely turns to a clear liquid.

In conclusion, melting point is also used to identify the unkno wn compound. The factors that affect the melting point of a sample are the impurities, the force attraction between the molecules and when the unknown solid sample is not pulverize well. The temperature at which all the crystals liquefied was 132°C which is closest to Urea.


REFERENCES

http://www.inchem.org/documents/sids/sids/57136.pdf

http://www.peterschemical.com/urea/msds-sheet-urea/

http://pubchem.ncbi.nlm.nih.gov/compound/urea#section=Top

https://en.wikipedia.org/wiki/Urea


POST – LAB QUESTIONS: 1. Rank the following compounds in decreasing melting point: benzoic acid, naphthalene, salicylic acid, and urea. Justify your rank b y citing the intermolecular forces of attraction present in the particular compound. 1-salicylic acid (159°C) - It is ldf and dipole-dipole and polar 2-urea (133°C) - Hydrogen bonding and London Dispersion forces 3-Benzoic acid (122.41°C) – It is ldf and dipole-dipole and polar 4-naphtalene (80.26°C) - Hydrogen bonding and London Dispersion forces

2. How does molecular geometry affect the melting point of an organic compound? The more symmetrical is the compound, the higher the melting point. If the molecular shape of the molecules allows them to be packed together, the substance has a higher melting point.

3. How do impurities affect the melting point of an o rganic compound? A substance (solid) containing soluble impurities usually melts at a lower temperature than the pure compound, and melts over a wide range of temperatures.

4. In general, what are the factors that affect the melting point of an organic compound? What are the effects? First factor is force of attraction between molecules because stronger intermolecular interactions result in higher melting points. The presence of polarity, or especially hydrogen bonding, generally leads to higher melting point. Second factor is the 10 | O R G A N I C C H E M I S T R Y L A B O R A T O R Y C H M 1 4 1 L – A 0 3

molecular shape. If the shape of the molecules allows them to be packed together, the substance has a higher melting point than compounds with oddly shaped molecules. 5. Before packing into the capillary tube, a solid substance has to be powdered. Why is this done on a watch glass rather than on a piece of paper? It is done on the watch glass because some of the particles stay on the filter paper and other small amounts of paper can mixed up on the substance that can screw up the measurement. The paper can absorb some of the particles in the powdered substance which is important in very small reactions like a capillary tube.

11 | O R G A N I C C H E M I S T R Y L A B O R A T O R Y C H M 1 4 1 L – A 0 3

Experiment 2 MELTING POINT DETERMINATION

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