chem 31.1 ex. 5 sc final

Powerpoint Templates


Boiling Point & Melting PointDeterminationGrp. 5 Suzuki-Cardona


Introduction

• Three physical properties:– Solubility– Melting point– Boiling point


Introduction

Boiling point• the temperature at which the vapor

pressure of the substance is slightly greater than the pressure of the surface of the liquid

• vapor forms and rises from the body of the liquid, causing the turbulent reaction


Introduction

Melting point• the temperature at which a solid is converted

to its liquid phase in less than 1 atmosphere of pressure


Introduction

Significance of Melting Point and Boiling Point Determination

• To identify or characterize a compound• To determine the purity of a compound


Objectives

• To determine the melting point ranges of benzoic acid and benzoic acid-urea mixture

• To determine the boiling points of hexane and ethane


methodology

Melting point determination


methodology

Boiling point determination


Discussion

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Intermolecular forces

IONIC

ION-DIPOLE

HYDROGEN BOND

LONDON DISPERSION FORCES

DIPOLE

STRENGTH


Discussion

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Polarity•Presence of Hydrogen bonds and dipole-dipole contribute to the polarity of a molecule. •A polar compound has a higher boiling point than molecules which exhibit covalent interactions.

Molecular Weight & Size

•The larger the molecule, the more interactions required to break in order for phase transformation to occur.


Discussion

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Branching

•Branching of molecules prevent closed-packing of atoms which then reduces the interactions between them.

Symmetry

•Symmetrical molecules tend to have a lower boiling point because they have less surface area. •Close packing in a solid state gives the compound a higher melting point.


Discussion

Benzoic acid

•is a colorless crystalline solid and the simplest aromatic carboxylic acid.

•exhibits LDF and Hydrogen bonding which makes it a very strong organic compound.

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Discussion

Melting point range

Benzoic Acid

Experimental: 120 °C - 124 °C

•Pure samples have 1-2 °C in their melting point range while impure samples have a broader ranger larger than 5°C or the lower value has decreased

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Started to liquefy

Completely melted


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SampleMelting Point Range ( )Experimental Theoretical

Benzoic Acid 120 - 124 121-122

Benzoic Acid-Urea Mixture

96 - 116 -

A pure compound is a structure of high molecular symmetry.

Urea an impurity that dismantles the crystal lattice of the benzoic acid which lead to the disorientation and weakening of the bonds.

Weakening of the bonds -> less energy needed to break the molecule

Ornithuric acid


Discussion

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Atmospheric pressure pushes the liquid into the tube

Cooling: Vapor pressure lowers

•The bubbles that came out of the capillary tube is the air replaced by the vapor of the heated hexane or of the distillates.


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SampleBoiling Point ( )Experimental Theoretical

Hexane 100 69

Distillate 1 109 78.15

Distillate 2 125 78.3

Distillate 2 has a larger boiling point because the percentage of water is bigger than the percentage of water in the 1st distillate’s composition.


Discussion

Ethanol-water distillates •cannot be separated and distinguished from each other through simple distillation

•An azeotrope is a mixture where the ratio of the substances in a vaporized form is equivalent to percent composition of the mixture in its liquid form.

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TemperatureMiscibility of ethanol in water

Boiling Point


Discussion

Ethanol-water distillates •Dipoles on the ethanol and water molecules allow the formation of weak hydrogen bonds (Brucken-bindungen) between negative and positive charges on the different molecules.

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TemperatureMiscibility of ethanol in water

Boiling Point


CONCLUSION

Boiling point and melting point

•may vary accordingly with changes in structures and temperature

•These are Intrinsic properties

Impurities

•disrupt the symmetrical lattice of a molecule and weaken the atomic interactions will lower the boiling and melting point of the substance.


recommendations

•Use other substances that may serve as an impurity with benzoic acid

•Conduct the melting point and boiling point determinations with other samples


GUIDE QUESTIONS & ANSWERS

1.Give an explanation of the observed boiling points.

The benzoic acid-urea mixture has a lower boiling point because compared to the pure benzoic acid sample, the urea acts as a contaminant. Urea disturbs the crystal lattice of the benzoic acid, requiring less energy (in this case, energy in the form of heat) in order for the bonds to break.



2. Give an explanation of the observed boiling points of the two distillates.

Although distillation did not completely separate ethanol from water because of their nature as azeotropic mixtures, fractional distillation was able to obtain a greater ethanol component rather than the sample obtained from simple distillation, thus the distillate from the fractional distillation achieved a boiling point closer to the boiling point of the alcohol.



3.What effect would poor circulation of the melting point bath liquid have on the observed melting point?

The poor circulation of the melting point bath liquid will cause the uneven heating of the sample, leading to the inaccuracy of the results.



4.What effect would the incomplete drying of a sample have on the melting point?

The incomplete drying of the sample will lead for the substance obtained to have a lower melting point. The solvent that would be left over will act as a contaminant, which will cause the depression in the melting point range of the sample.



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 compounds in the test tubes. If they are approximately the same, combine the substances in test tubes A, B, and C. If either A and B consist of the same compounds as the one in C then, the mixed boiling point of C+A or C+B will not be depressed, therefore C=A OR C=B. However if both the melting points of C+A and C+B are depressed then neither of them is equal to C.



6. Which would be expected to have a higher boiling point – t-butyl alcohol or n-butyl alcohol? Explain.

N-butyl is expected to have a higher boiling point because its structure is not branched, instead it is linear which then allows more room for molecular interactions. While T-butyl prevent the closed packing of molecules because it is branched. The greater the surface are for interaction, the higher 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).


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 (2006).Laboratory manual in general chemistry (2006 ed.). Manila, Philippines: University of the Philippines Manila

Boiling Point and Distillation. (2011).http://eee.uci.edu/programs/hongchem/RDGbpdistill.pdf

Microsoft ® Encarta ® 2007. © 1993-2006 Microsoft Corporation. All rights reserved.

References:

chem 31.1 ex. 5 sc final

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