formal report - experiment 5

8/10/2019 Formal Report - Experiment 5

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Chem 31.1 Melting Point and Boiling Point Determination Page 1of 5

Experiment #5: Melting Point and Boiling Point DeterminationDela Cruz, Renzxymon L.; Gabriel, Ezekiel MChem 31.1, HEJ, Ms. Kristine TolentinoOctober 16, 2014

I. AbstractBoiling point and melting point are two of the physical properties of organic compounds that are

determined by intermolecular forces. The experiment made use of quantitative methods to determinesuch properties. Samples in test tubes attached in a thermometer were placed in an oil bath to break thebonds between molecules. It was observed that pure benzoic acid has a higher melting point ascompared to benzoic acid mixed with urea, thus impurities in a compound lowers its melting point due tothe disruption in the packing of benzoic acid. The distillate obtained through simple distillation was foundto have a higher boiling point as compared to the distillate obtained through fractional distillation becausethe first distillate contains more water and was able to increase hydrogen-bond strength between itsmolecules. The objectives of the experiment are to understand the factors affecting physical propertiesand to use these understanding in the industrial level to estimate purity and identify compounds.

II. KeywordsMelting Point, Boiling Point, Intermolecular Forces

III. Introduction

The physical properties according toKlein (2012) of a compound are determined bythe attractive forces between the individualmolecules, called intermolecular forces. Boilingpoint and melting point are physical propertiesexhibited by liquid and solid chemicalsubstances respectively and in this case,organic compounds. Chang (2010) defines theboiling point of a substance as the temperatureat which the vapor pressure of a liquid is equalto the external pressure. The normal boilingpoint of a liquid is the temperature at which it

boils when the external pressure is 1 atm. Themelting point of a solid is the temperature atwhich solid and liquid phases coexist inequilibrium. The normal melting point of asubstance is the temperature at which asubstance melts at 1 atm pressure.

In order for a compound to boil or melt,the forces that hold the individual moleculestogether must be broken. This means that theboiling point and melting point of a compounddepends on the strength of the attractive forcesbetween the individual molecules. If themolecules are held together by stronger forces,

more energy will be needed to pull themolecules away from each other and thecompound will have a higher boiling point andmelting point. In contrast, if the molecules areheld together by weaker forces, fewer amount ofenergy will be needed to pull the moleculesaway from each other and the compound willhave a lower boiling point.

It is difficult to predict the boiling point ormelting point of a compound based only on its

structure but there are certain trends that can beobserved which make it easier to comparecompounds and predict which would have ahigher boiling point or melting point. Thesetrends are the size and weight of the molecule,branching, polarity, the symmetry of themolecule, and the presence of intramolecularhydrogen bonding within the molecule.

The Boiling Point and Melting Point mayalso be affected by pressure and the presenceof impurities within a substance. The purity of animpure substance can therefore be measuredusing the boiling point or melting point given that

the boiling point or melting point of the puresubstance is known. An impure substance willtend to boil or melt over wider range oftemperature as compared to that of puresubstances. Since most organic compoundshave varying boiling points and melting points,we can then use these properties in order to testand identify unknown organic substances.

This experiment seeks to develop the abilityto determine the melting point and boiling pointof substances. It also develops the skill todetermine the purity of a compound using itsintensive properties such as melting and boiling

points. It also provides an understanding on howstructural features of a compound affect itsmelting point or boiling point.

The significance of this experiment lies onthe practical and industrial application of theconcepts governing melting and boiling points.These physical properties can be used toestimate the purity and determine the identity ofan unknown solid or liquid compound.


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IV. Experimental

A. Melting Point Determination of BenzoicAcid and Benzoic Acid-Urea Mixture

In this experiment, the melting point of purebenzoic acid and benzoic acid-urea mixture

were determined by placing 0.5g of theirrespective ground sample in two differentcapillaries sealed in one end occupying 1cm.Each sample was powdered using differentmortar and pestle. These capillaries were thenattached to a thermometer using a rubber band.The thermometer was then place in a beakercontaining enough oil to submerge the 1cmsample. It was then heated and thetemperatures at which the sample starts toliquefy and the temperature at which the solidhas completely melted were recorded as theirrespective ranges of melting point.

B. Boiling Point Determination of Distillate andHexane

The boiling points of hexane, 1st distillate

and 2nd

distillate were determined by introducing10 drops of each sample into a 5mL test tubeattached to a thermometer by a rubber band.Capillaries sealed at one end were place in thetest tube such that the open end is immersed inthe sample being tested. This was thensubjected in an oil bath raising the temperatureuntil a continuous stream of bubbles isobserved. The bath was allowed to cool and thetemperature was noted as the instant bubblescease to come out of the capillary and justbefore the liquid sample enters the capillarytube. Their respective boiling points were thendetermined.

V. Results

A. Melting Point Determination

Table 1. The table shows the ranges of melting point ofbenzoic acid and benzoic acid-urea sample.

Sample Melting Point Range(oC)

Benzoic Acid 110 - 120Benzoic Acid-Urea Mixture

79 - 85

The samples were placed in capillaries andwere placed in an oil bath. The followingexperimental melting points of each sampleyielded. The melting point of pure benzoic acidis greater than the melting point of benzoic acidwith urea as an impurity to the sample. .

B. Boiling Point Determination.

Table 2. The table shows the different experimentalboiling points of each sample.

Sample Boiling Point (oC)

Hexane 69.5

Distillate 1(from simpledistillation)

92

Distillate 2(from fractional

distillation)86

The solutions were again placed in an oilbath to determine their respective boilingpoints. The boiling point of hexane is closed tothe theoretical value which is 68

oC. The first

distillate has a higher boiling point than thesecond and these values were compared tothe results of the previous experiment. Theirboiling points have deviated from thetemperature from which the distillate wasextracted.

VI. DiscussionMelting point and boiling point determination

involves the breaking of bonds so differentattractive forces in the molecule must beconsidered. Ionic compounds tend to havehigher boiling and melting points due to the ionicinteractions between particles which exhibits the

strongest attractive force. However, organiccompounds are covalently bonded so dipoleforces, hydrogen bonding, and Londondispersion forces (which are exhibited by allmolecules) only affect the physical properties ofthe compounds. Of the three forces, hydrogenbonding is the strongest since it is a special typeof dipole-dipole forces involving hydrogencovalently bonded with a very electronegativeatom such as oxygen, nitrogen, and fluorinewhile the weakest is the London dispersionforces.

Factors affecting the boiling point and

melting point of a compound are molecularweight, branching, polarity, intramolecular H-bonding and molecular symmetry.

Larger hydrocarbon chains andcompounds with higher molecular weight havelarger surface area meaning that theyexperience stronger London dispersion forcesand this result in the compounds having higherboiling points and melting points.


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The more branched a molecule is, themore sphere-like it becomes and this in turn,reduces the surface area in which the moleculecould interact which causes a decrease in theboiling point and melting point of a compound.The more rod-like a compound, the easier for itto line up together and interact which gives thecompound a higher boiling point and meltingpoint.

More polar compounds have strongerdipole-dipole interaction and thus increase theboiling point and melting point of the compound.

Symmetrical molecules have a lowerboiling point than asymmetrical moleculesbecause the dipole moments cancel out and themolecule becomes non-polar and wouldtherefore have less interaction. The meltingpoint of symmetrical molecules is influenced bypacking in its crystal lattice and so tighter fitcompounds would require more energy to break

the lattice thus increasing its melting point.Presence of Intramolecular hydrogen

bonding within a compound decreases theboiling point and melting point of the compoundbecause it reduces the interaction betweenmolecules as the polar groups are preventedfrom interacting.

Part A of the experiment was focused ondetermining and comparing the melting points ofa pure benzoic acid and benzoic acid-ureamixture. Conceptually, the amount of sampleplaced in the capillaries does not matterbecause melting point is an intensive property

which means that melting point does not dependon the amount of matter.

Enough heat was applied to break theattractive forces interacting between themolecules of both samples. As expected, purebenzoic acid has a very high melting point. Thisis because carboxylic acids have the ability tofrom two hydrogen bonds per pair of moleculeconsequently requiring huge amount of energyto melt the compound. The benzoic acid-ureamixture has a lower melting point than the purecompound because the urea acts as an impurity.Impurities in compounds disrupt the packing of

the crystal making it disorganize. Thisdisorganization of molecules weakens theattractive forces present in the compound andso less energy is required to break these forcesand lower energy requirement means lowermelting point. Thus, the purer the compound thesharper and the higher its melting point. Samecompounds have the same melting point but themelting point of a mixture of differentcompounds (initially having identical melting

point) will be lower as compared to the meltingpoints of the compounds taken separatelybecause both compounds will act toward eachother as impurities.

In Part B of the experiment, determinationand analysis of the boiling points of hexane,distillate 1 (from simple distillation), and distillate2 (from fractional distillation) were performed.The rationale in setting up the method fordetermining the boiling point of each sample isthe relationship of atmospheric pressure, vaporpressure, and temperature. During the initialheating, the air trapped in the capillary tubeexpands and leaves the tube and vapor from thesample also enters the capillary tube. This givesrise to the initial stream of bubbles. When thetemperature reaches the boiling point, theatmospheric pressure equals the vapor pressureinside the tube. As the temperature rises justabove the boiling point, vapor pressure will start

to escape and this accounts for the second setof bubbles. Once, the heating is stopped, itsvapor pressure will decrease and when thevapor pressure drops below the atmosphericpressure, the liquid will be drawn to the capillarytube forced by the atmospheric pressure and sothe temperature before the sample went up thetube was recorded as the samples boiling point.

The result for the boiling point of hexanewhich is 69.5

oC was close to the theoretical

value of 68oC giving us 2.21% error. The boiling

point of the 1stdistillate (from simple distillation)

is greater than the boiling point of the 2nd

distillate (from fractional distillation). As theprevious experiment implies, fractionaldistillation is more efficient as it greatly extractthe ethanol from water (leaving 95% Ethanoland 5% water in the distillate) than simpledistillation. Hence, the 1

stdistillate contains more

water than the second distillate. The boilingpoint elevation of the 1

st distillate is due to the

formation of hydrogen bonds between ethanoland water. The greater the strength of this forcemeans the higher the boiling point. As comparedto the 2

nddistillate which has purer ethanol, then

it can be expected that its boiling point will be

lower than the boiling point of the 1

st

distillate.The experiment also wants to confirm theboiling point of the distillates via comparing itfrom the temperature from which these distillateswere extracted from the previous experiment.From experiment 4, the temperature from whichdistillate 1 was extracted is 84

oC while distillate

2 is 74oC. The deviation of the results from

experiment 4 and the results from thisexperiment is may be due to different


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atmospheric pressure from which theexperiments were performed because it affectsthe boiling point of a substance.

VII. Conclusions and Recommendations

In this experiment, melting and boiling points

of different samples were determined. Thesephysical properties are affected by differentfactors such as intermolecular forces ofattraction, molecular weight, polarity, branchingand molecular symmetry.

It was noted that impurities in a compoundlowers its melting point and that this physicalproperty can be used to determine the identity ofa compound. The determination of the boilingpoints of the two distillates was performed toconfirm that compounds can be extracted viadistillation by using the knowledge of boilingpoints.

It is recommended that during the procedure,the temperature of the oil bath must be loweredsignificantly before heating the capillary tubescontaining the samples. This is because heatingthe capillary tube in an oil bath with a high initialtemperature would cause an immediateboiling/melting of the sample, resulting in theinaccuracy of temperature reading.

VIII. References

Brown, W.H., Foote, C.S., Iverson, B.L., &

Anslyn, E.V. (2012).Organic

Chemistry (6thEd.).Belmont, CA:

Brooks/Cole Cengage Learning.

Brown, W.H. & Poon, T. (2000).Introduction

To Organic Chemistry(5thed.). New

York, NY: John Wiley & Sons.

Chang, R. (2010). Chemistry (10thed.). New

York, NY: McGraw-Hill Companies,

Inc.

Klein, D.R. (2012). Organic Chemistry. New

York, NY: John Wiley & Sons

IX. Guide Questions1. Give an explanation of the observed melting

points.- The melting point of benzoic acid-urea

mixture is lower than the melting point ofpure benzoic acid because urea is an

impurity. Impurities change the crystallattice of a pure compound therebymaking it poorly arranged or organized.This poor stacking of molecules such thatin benzoic acid-urea mixture will beneeding lesser amount of energy tobreak the existing bonds betweenmolecules than a well organized structureas in the pure benzoic acid. Less energymeans lower melting point while higherenergy requirement to break such bondsmeans higher melting point.

2. Give an explanation of the observed boilingpoints of the two distillates.- The boiling point of the 1

st distillate is

higher than the 2nd

because simpledistillation was used to extract ethanolfrom water. Simple distillation is not thatefficient as compared to fractional

distillation so the distillate coming fromthe simple distillation has more waterthan the second. Since the 1

stdistillate is

mostly water (BP = 100oC), it elevates

the boiling point of pure ethanol alone(BP = 78.37

oC) due to hydrogen bonding

between ethanol and water. Distillate 2has a lower boiling point because it haspurer ethanol.

3. What effect would poor circulation of themelting point bath liquid have on the observedmelting point?

- Poor circulation may cause unevenheating of the sample. This may lead toinaccurate results because the observemelting point will be a little higher thanthe actual value of the melting point ofthe substance being observe.

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

- Incomplete drying of sample maydecrease its observe melting pointbecause incomplete drying may causesome impurties to mix with the pure

compound. Any impurity in the compoundlowers its boiling point due to theunorganized stacking of the molecues.

5. Three test tubes, labeled A, B, and C, containsubstances with approximately the same meltingpoints. How could you prove the test tubescontain three different chemical compounds?

- Combine A and B, B and C, and A and Cin separate test tubes. Test for the


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melting points of the mixtures andsupposing that the three substances aredifferent chemical compounds, then oneof the compounds will act as an impurityto the other compound within each of thetest tubes and so the melting point wouldbe different than previously observed.

6. Which would be expected to have higherboiling point t-butyl alcohol or n-butyl alcohol?Explain.

- n-butyl is expected to have higher boilingpoint than t-butyl. T-butyl is morebranched and spherical so it lessensthe intermolecular forces of attraction(London dispersion force) between itsmolecules. The surface area forinteraction gets shorter and the moleculecannot get close enough for strongerinteraction. The weaker the IFA the lower

the energy required to break theattractive forces, thus lower boiling pointas compared to n-butyl which is linearthat exhibits higher IFA and therefore willhave a higher boiling point.

7. Calculate the vapor pressure of a solutioncontaining 30 mol% hexane and 70 mol%octane at 90

oC assuming Roults Law is obeyed.

(Given vapor pressure of the pure compounds at90

oC: hexane = 1330 torr; octane = 253 torr).

PTotal = XhexPo

hex+ XoctPo

oct

Xhex= 0.3; Xoct= 0.7 : Po

hex = 1330 torr;P

ooct= 253torr

PTotal = (0.3)(1330torr) + (0.7)(253 torr)PTotal = 399 torr + 177.1 torr

PTotal = 576.1 torr

We hereby certify that we have given substantialcontribution to this report

__________________ _______________Dela Cruz, Renzxymon L. Gabriel, Ezekiel M.

formal report - experiment 5

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