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Melting Point Determination:
Transcript of Melting Point Determination:
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
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:
Report the melting point range using the onset point and the clear 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
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
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.
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.
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
Pure crystalline compounds have a sharp melting point range
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.
Melting Point: 132 - 133 °C
Melting Point: 132 - 133 °C
Melting Point: 81 - 82 °C
Melting Point: 113 - 114 °C
Mel-Temp Melting Point Apparatus
Love for Chemistry
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
Insert capillary tube into heating apparatus
Heat apparatus up to 15°C below the known melting point of the compound in order to save time
Lower heat at this point so that the temperature only moves
1-2 °C per minute in order to have the most accurate temperature
Record temperature where substance begins to melt
Record temperature where substance is no longer solid
The two numbers obtained make the melting range
Pack 1-2mm of unknown into TWO separate capillary tube
Obtain approximate melting point by heating apparatus so that it increases 15-20°C per minute. Observe when substance begins melting.
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
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
Repeat steps 4-6 from “Known Substance Procedure”
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
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.
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
Exist within a crystal lattice structure
Reliant on intermolecular forces to maintain structure
More resistant to increasing heat, thus resulting in higher melting points
Melting Point: 121 - 122 °C
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