Melting Points Lab Report, Exercises for Organic Chemistry
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Melting Points Lab Report, Exercises for Organic Chemistry

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Melting Point

Lexa Marchese

7/25/17

Chem 315-01

I Objectives

• Obtain melting point of Cinnamic acid and Urea

• Obtain melting point of a 50:50 solution of Cinnamic acid and Urea

• Obtain the melting point of 2 unknown substances

• Determine what the two unknown substances are

II Theory

Melting point is the temperature at which a solid melts to become a liquid.1 Melting point is a

physical property that can be used to identity a substance and its general purity. Purity is the degree to

which a substance is undiluted or unmixed with extraneous material, typically expressed as a

percentage.2 We can tell if there is impurities by how wide the range is in a melting point, the larger the

range the more impurity there is. Melting point is used for identification because most compounds

usually do not have the same melting point. Melting point is like a finger print, each compound has a

different one that can make them easily identifiable.A more specific definition for melting point is the

temperature at which the solid exists in equilibrium with its liquid under an external pressure of one

atmosphere.3

Factors that affect melting points are size of a molecule, shape of molecule, and intermolecular

attractions. The size of a molecule affects the melting point because when molecules are smaller and

polar they melt at lower temperatures because there bonds are not as strong. Also the melting point gets

lower as the polar molecule gets larger because its bonds are not as stable, more compact bonds are

harder to break and the larger the molecule the less compact. However, if ionic compounds are present

in a smaller molecule it makes its melting point higher than a large molecule with ionic bonds because

the smaller molecule can get more compact and form more stable bonds. The shape affects the melting

point because if the molecule allows itself to be very tightly compacted it will have a higher melting

point. The less compact a molecule, the lower the melting point because it is not as stable so the bonds

are easier to break. This is why symmetrical compounds usually have a high melting point because they

are more stable and compact. Lastly, intermolecular attractions affect the melting point. Covalent bonds

are weaker than ionic bonds, so compounds with covalent bonds usually have a lower melting point

than those with ionic bonds. But in other cases if the compound that has covalent bonds has polar

bonds as well the electronegativity of the atoms then causes the melting point to be higher than

compounds with nonpolar molecules.

Hydrogen bonds which are hydrogens bonded with high electronegative atoms, are the strongest

bonds we focus on in organic. Since the bonds they form are strong, it causes the melting point to rise

because it takes more energy to break them apart. Dipole-dipole is another attraction that forms by the

positive end of one polar molecule and the negative end of a neighboring molecule. Dipole-dipole is a

fairly weak bond that raises the melting point because of its polar bonds. The last prominent bond we

discuss in organic that affects the melting point is London dispersion forces. The way you tell if

London forces raise or lower the melting point is by surface area. The more surface area a molecule has

the higher the indivudual molecules ability to attract eachother.4

III Reagant5

Name Structure Molecular Weight

Density Experimental Melting Point

Boiling Point

Melting Point

Precaution

Acetanilide CH3CONHC6H5

135.17 g/mol

1.22 g/cm3

105o-125o 304oC 114.3oC Toxic, irritant

Caffeine C8H10N4O2

194.19 g/ mol

1.23 g/cm3

210o-243o 365oC 235oC Flammabl e, irritant

Cinnamic Acid C9H8O2

148.16 g/ mol

1.25 g/cm3

134o-139o 300oC 133oC Flammabl e, irritant

Urea NH2CONH2

60.06 g/mol

1.32 g/cm3

135o-139o N/A 133oC Highly toxic, flammable

IV Procedure

Reference CHEM 315- Organic Chemistry I Lab Laboratory Manuel, Fall 2017, Dr. John K Berch, Jr.

Lab 2- Melting Point

V Observations

• The tubes/substances looked like they were sweating which showed that its melting point began

• All substances melted into a clear liquid

• Cinnamic acid and Urea have a range and melting points that are close

• the 50:50 mixture has the most range in melting points which shows impurities

VI Data/Results

Compound Observed Melting Point Actual Melting Point

Cinnamic Acid 134oC-139oC 133oC

Urea 135oC-139oC 133oC

50:50 Mixture 102oC-115oC -

Unknown 1 (Acetanilide) 105oC-125oC 114.3oC

Unknown 2 (Caffeine) 210oC-243oC 235oC

VII Calculations

No calculations were made during this lab

VIII Conclusions

After completeing the lab, melting points for all four substances were found. Cinnamic Acid

and Urea both had a melting point range from 134oC-139oC. Both substances began to sweat at 134oC

beginning there melting point process and both completely melting at around 139oC. The 50:50 mixture

of Cinnamic Acid and Urea had a melting point range of 102oC-115oC. It began melting at 102oC and

had completely melted by 115oC. Unknown 1 had a range of 105oC-125oC. It began to sweat at 105oC

and completely melted at 125oC. Unknown 2 had a range of 210oC-243oC. It began to melt at 210oC

and was completely melted by 243oC.

Cinnamic Acid and Urea melting points found in the lab were a little above there actually

melting points showing some lab error. The samples were first put into bowls before the lab so they

could have absorbed moisture causing some impurities. This could also be the case for the two

unknowns. Since there melting point ranges were very spread out it shows impurities due to lab error.

Another error in lab that could have occurred was someone not watching the sample every second so it

began to melt before someone caught it.

The 50:50 mixture of Cinnamic Acid's and Urea's range was signicantly lower than the melting

points of the two indivual substances which shows the mixture has impurities. But since it was a

mixture of the two substances it was expected that it would melt at a lower temperture and have a larger

range. The difference in melting point and range shows how sensitive melting points are and how easily

they change when exposed to impurities.

When indentifying the 2 unknown substances we had to compare melting points and the known

boiling points found in research done for this report against the 4 possible substances: Acetanilide,

Caffeine, Cinnamic Acid, and Urea. When looking at the melting point for unknown 1, which was

105oC-125oC, we can confer that it must be Acetanilide because its actually melting point is 114.3oC

and its actual melting point is the closest in range to Acetanilide. The other substances melting point go

to high for it to be them. Unknown 2 having an experiemental melting point 210oC-243oC shows to be

Caffeine because Caffeine has an actual melting point of 235oC. None of the other substances even

come close to having such a high melting point, so ruling them out was the easiest. Though the melting

points differ, we can conclude that unknown 1 is Acetanilide and unknown 2 is Caffeine because there

actual melting points are closest in range compared to the other substances. Since Caffeine's range is so

wide you can tell it has impurities. The varity in the ranges and actuals in these 2 unknowns shows lab

error. The wide range in the 2 unknown also shows a lot of impurities. The samples could have

absorbed moisture or when the melting point began and the substances began to sweat could have been

missed by a second or more

IX References

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch14/melting.php1

http://amrita.olabs.edu.in/?sub=73&brch=7&sim=33&cnt=12

http://medical-dictionary.thefreedictionary.com/chemical+purity3

https://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/4

http://www.sigmaaldrich.com/catalog/substance/acetanilide1351610384411?lang=en&region=US5

http://www.sigmaaldrich.com/catalog/substance/caffeine194195808211?lang=en&region=US5

http://www.sigmaaldrich.com/catalog/substance/urea60065713611?lang=en&region=US5

http://www.sigmaaldrich.com/catalog/substance/transcinnamicacid1481614010311?

lang=en&region=US5

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