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Melting Point Determination:

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Organic Chemistry

on 1 December 2013

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Transcript of Melting Point Determination:

Melting Point Experiment Presentation
Zain Aiman, Dominique Coticchio and Anissa Daimally


Melting Point Determination
Purity and Identity of Crystalline Organic Compounds

Reference: Lab Manual H. Hart & L. Craine, Laboratory Manual: Organic Chemistry, A Short Course, 13th E
Melting Point
Temperature at which a transition occurs between solid and liquid phases
The solid and liquid phases are in equilibrium

Physical constant or property
Intrinsic to a given substance in its pure form
Melting Point Range
The difference between the temperature at which the compound begins to melt and the temperature at which the compound is completely melted

Melting Point Range consists of three points:
Onset Point
Meniscus Point
Clear Point

Report the melting point range using the onset point and the clear point
Onset Point
The “official” start of the melting
Liquid appears for the first time as a separate phase in coexistence with the crystals.
Melting Point Uses
Identify unknown compounds

Determine the purity of compounds

Purpose
Determine the melting point of two pure compounds, urea and cinnamic acid

Determine the melting point range of the different mixtures of urea and cinnamic acid

Identify an unknown compound using its melting point
Introduction
Identifying an unknown compound can be a tedious and difficult task. Luckily, chemists can utilize and measure the physical properties of the compound to determine its identity. One such property is the melting point.
Meniscus Point
Liquid meniscus is visible
The Chemistry Behind It
As heat is applied to the substance, the molecules begin to vibrate, increasing its kinetic energy and thus increasing the temperature.

Once the substance reaches the melting point, any heat applied is used to break the intermolecular bonds, thus the temperature does not increase.

There are molecules in the liquid phase, as well as in the solid phase.


Heating Curve
Clear Point
Solid has completely transformed into liquid

Identify Unknown Compounds
Compare the melting point found in the experiment with those found in the CRC Handbook to determine the compound

Use mixture melting point
Determine Purity
Pure crystalline compounds have a sharp melting point range

Impurities
Lower the melting point
Broaden the melting point range

Mixture Melting Point
Melting point of mixture of unknown compound and known compound

If the melting point is the same/similar to the melting point of the unknown compound or the known compound alone, then they are identical.

If the melting point differs significantly from the unknown compound or the known compound alone, they are different.

Compounds Used
Melting Point: 132 - 133 °C

trans-Cinnamic Acid
Melting Point: 132 - 133 °C

Urea
Melting Point: 81 - 82 °C

Vanilin
Melting Point: 113 - 114 °C

Acetanilide
Equipment
Capillary Tubes
Mel-Temp Melting Point Apparatus
Love for Chemistry
Step 1

Pack 1-2mm of urea into a capillary tube
Make sure to tap closed end of tube against a hard surface until all of the substance is compacted into to the bottom of the tube

Known-Substance Procedure
Step 2

Insert capillary tube into heating apparatus



Step 3
Heat apparatus up to 15°C below the known melting point of the compound in order to save time

Step 4
Lower heat at this point so that the temperature only moves
1-2 °C per minute in order to have the most accurate temperature

Step 5
Record temperature where substance begins to melt

Step 6
Record temperature where substance is no longer solid

The two numbers obtained make the melting range


Unknown Procedure
Step 1
Pack 1-2mm of unknown into TWO separate capillary tube

Step 2
Obtain approximate melting point by heating apparatus so that it increases 15-20°C per minute. Observe when substance begins melting.

Step 3
Insert second capillary tube into heat apparatus up to 15°C below the approximate melting point of the compound


Repeat all steps for trans-cinnamic acid
Mixtures
Ratio is Urea: trans-Cinnamic Acid

Repeat all previous steps for a 50-50 mixture of both compounds

Repeat all previous steps for a 75-25 mixture of both compounds

Repeat all previous steps for a 25-75 mixture of both compounds

Step 4
Repeat steps 4-6 from “Known Substance Procedure”

Step 5
Reference to Table of Melting Points to guess unknown compound.

Repeat the “Known Substance Procedure” using a 50-50 mixture of the unknown compound and what you believe it to be the unknown compound in order to definitively identify it.


Mixture Melting Point
Results
Conclusion
The melting point of a substance changes according to changes regarding its purity.

As a substance becomes increasingly impure, its melting point decreases and its melting point range broadens.

As the percentange of the mixture became "more impure," the more the melting point and melting point range changed.
Discussion
The more disproportionate the ratio between chemicals, the more "pure" one chemical can be considered to be

A 50:50 ratio of chemicals allowed for equal contribution of impurity, which created an overall larger impure substance
Melting Point Depression
Cinnamic acid and Urea both have London Dispersion Forces and Hydrogen Bonding ( a strong intermolecular force)

When these chemicals become increasingly impure, their crystal lattice form is disrupted

The disruption weakens the intermolecular bonds, resulting in a lower melting point
Melting Point Range Broadening
At the melting point, not all bonds break at the same time

The bonds in close proximity to the impurity will break fast because they are weaker

The non-disrupted bonds will take longer to break

The different rates of bond breaking within the impure substances is what causes the wider melting point range
Visualization of Impurities
Sources of Error
Unclean lab equipment could have created impurities, resulting in inaccurate results

Impurities among the chemicals used

Observational error

Pure Substances
Exist within a crystal lattice structure

Reliant on intermolecular forces to maintain structure

More resistant to increasing heat, thus resulting in higher melting points
Mixture Results
Unknown Results
Melting Point: 121 - 122 °C
Benzoic Acid
Eutectic System
Eutectic system is a mixture of chemical compounds or elements that has a chemical composition that solidifies at a lower temperature than any other composition
Melting Video
Full transcript