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Chem 31.1 Experiment 5

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Nathea Valdez

on 10 May 2013

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Transcript of Chem 31.1 Experiment 5

Introduction Boiling and melting points are physical properties of liquids and solids that can be used to estimate the purity and determine the identity of the liquid or solid. Impure liquids and solids tend to have lesser boiling point and wide melting point ranges compared to pure samples. Results Answers to
Guide Questions: Methodology Experiment No. 5: Boiling Point and Melting Point Determination AB2
Group 5/7
Milla, Valdez, Verdera Objectives: learn the methods used to get the
melting point and boiling point determine the effect of impurities in the melting and boiling point of substances Boiling Point - temperature at which the liquid turns into gas Melting Point - temperature at which a solid is converted into liquid involves breaking of bonds IMFA
Boiling and Melting Points Factors that affect boiling and melting points: molecular size molecular symmetry branching polarity intramolecular H-bonding recognize how structure affects
a compound’s physical and chemical
properties study the factors that affect
boiling and melting points Melting Point benzoic
acid benzoic
acid
+ urea NOTE: CAPILLARY TUBES WERE USED. Boiling Point Figure 1: Melting Point Apparatus 10 drops of hexane into a 5mL test tube immerse open end of the capillary tube into the test tube repeat with distillates and ethane DISCUSSION Liquid molecules are farther than solid molecules and closer than gaseous molecules. This allows the molecules to interact
via non-covalent interactions. It permits movement although the molecules are still held together in a definite volume. Structural features that
affect boiling point: DISCUSSION Melting Point 1. Polarity - direct relationship
2. Molecular Weight - direct relationship
3. Branching - inverse relationship
4. Molecular Symmetry - inverse relationship
5. Intramolecular H-bonding - direct relationship Boiling Point Bubbles due to:
-replacement of air trapped in capillary
tube by vapor of liquid Liquid in capillary tube due to:
-pushed inside via higher atmospheric
pressure Theoretically... b.p of distillate from f.d < b.p of distillate from s.d WHY?
-obtained higher percentage of ethanol since f.d is more efficient
-ethanol is an impurity -> lower b.p
-ethanol is volatile so it is easily vaporized -enough heat was applied to overcome the interactions between molecules
-criteria for purity and identification of org. compounds
--as compound is purified, m.p become sharper and higher
--indicators that m.p has been reached:
--shrinking, slumping, or softening structural features that
affect melting point: 1. Polarity - direct relationship
2. Molecular Weight - direct relationship
3. Branching - direct relationship
4. Molecular Symmetry - direct relationship
5. Intramolecular H-bonding - direct relationship What are Mixed Melting Points? Two sample of the same compound, having the same melting point separately, should have the same melting point when they are mixed together. Also, in general, sample of two different compounds having the same or similar melting points separately, will act towards each other as impurities when they are mixed together; hence, such mixtures will have broadened and depressed melting point ranges. CONCLUSION SUCCESS! [X] learned boiling point and melting point determination methods
[X] determined effects of impurities
[X] recognized effect of structures
[X] studied factors of boiling point and melting point RECOMMENDATION Be careful with oil baths as this frequently results to glassware breakages. 1. Give an explanation of the observed melting points. The benzoic acid-urea mixture has a melting point range lower and larger in the ranges of values than the benzoic acid alone. This is brought about by the impurity of the mixture. Impurities change the crystal lattice of a pure compound making it poorly organized. Since melting point is dependent on the high symmetry and organization of molecular structures, any change in the latter will cause deviation from the former. Bruice, Paula Yurkanis. Organic Chemistry 4th Edition. (Prentice Hall,2004)

Brown, T. L., Lemay Jr. H. E., Bursten, B. E. Chemistry: The central Science (9th ed.). Singapore: Pearson Education (Asia) Pte Ltd.

Committee on General Chemistry (2010). Laboratory Manual In General Chemistry (2010 ed.). Manila, Philippines: University of the Philippines Manila

Experiment 4: MELTING POINTS OF ORGANIC COMPOUNDS. OChemOnline. <http://ochemonline.pbworks.com/f/04_mp.pdf>

Organic Laboratory Technique 5: Boiling Point Determination. University of Calgary, Faculty of Science, Department of Chemistry. May 8, 2013.http://www.chem.ucalgary.ca/courses/351/laboratory/boilingpoint.pdf 2. Give an explanation of the observed boiling points of the two distillates. Whatever method of preparation is used, the ethanol is initially obtained in a mixture with water. Although the boiling point of ethanol, based on our experiment, 86 °C, is significantly lower than the boiling point of water, 100°C, these materials cannot be separated completely by distillation. Instead, an azeotropic mixture, a mixture of two or more substances that has a constant boiling point and cannot be separated by fractional distillation, (i.e. amixture of 95% ethanol and 5% water) is obtained, and the boiling point of the azeotrope, again, based on our experiment, is 84°C. In a distillation, the most volatile material, the material that has the lowest boiling point, is the first material to distill from the distillation flask, and this material is the azeotrope of 95% ethanol which has the lowest boiling point. If an efficient fractionating column is used, 95% alcohol is obtained first, then a small intermediate fraction of lower concentration, and then water. But no matter how efficient the fractionating column used, 95% alcohol cannot be further concentrated by distillation.

The separation of a mixture by fractional distillation occurs because the vapour has a different composition from the liquid from which it distils (i.e. the vapour is richer in the more volatile component). We cannot separate 95% alcohol into its components by distillation, because here the vapour has exactly the same composition as the liquid; towards distillation, then, 95% alcohol behaves exactly like a pure compound.

Poor circulation can cause uneven heating of a sample. This may lead to inaccurate results. Observed melting point may be unusually higher than the actual. 3. What effect would poor circulation of the melting point bath liquid have on the observed melting point? 4. What effect would the incomplete drying of a sample have on the melting point? Incomplete drying of a sample may cause some depression of the melting point. This is caused by the impurities that are left in the sample. Any impurity of a substance lowers its melting point. 5. Three test tubes, labeled A, B, and C, contain substances with
approximately the same melting points. How could you prove the test tubes contain three different compounds? First, determine the respective melting points of the unknown compounds. If they are approximately the same, mix the substances in test tubes A, B, and C. Determine the melting point exhibited once again. If the melting point of the mixture depresses from the individual melting points of the substances, therefore, the unknown compounds are different. 6. Which would be expected to have higher boiling point –
t-butyl alcohol or n-butyl alcohol? Explain. n-butyl alcohol is expected to have a higher
boiling point than t-butyl alcohol because the
previous has a more linear structure compared to
tert-butyl alcohol which exhibits branching.
'This decreases the strength of the intermolecular forces, thus, also reduces the boiling point. 7. Calculate the vapor pressure of a solution containing 30mol % hexane and 70mol % octane at 90 °C assuming Raoult’s Law is obeyed. (Given: vapor pressure of the pure compounds at °C: hexane=1330 torr; octane=253 torr). Phexane=P°hexaneXhexane = (1330 torr)(0.3)
= 399 torr
Poctane=P°octaneXoctane = (253 torr)(0.7) = 177.1 torr
Ptotal=Phexane+ Poctane=399 + 177.1 = 576.1torr REFERENCES Applications product characterization and purity determination
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