What is Melting Point?
one of the characteristic physical properties of a solid
What is Melting Point used for?
to characterize a known compound (if you had to choose between several known compounds)
to record a physical characteristic for a new compound (important for comparison between compounds)
to establish purity of a known compound (if an experiment reliably forms this known compound OR if you are taking a melting point of a known compound)
What kind of compounds will melt over a wider range of temperatures and at a lower mp than original and why?
Impure compounds because energy is used to overcome crystal lattice energy to melt a solid.
Impurities disrupt crystal lattice, so requires less energy to break intermolecular forces.
What are factors that affect melting point?
Intermolecular forces (ionic attractions, VdW forces, dipole-dipole interactions, and hydrogen bonds)
How molecules stack (shape of molecule)
Which packs better, saturated or unsaturated and why?
saturated (butter) packs better as it has a higher melting point than unsaturated fat (olive oil) which has a lower melting point.
Efficient stacking is the ability of molecules to pack tightly into a crystal lattice maximizing van der waals forces.
Melting Point Apparatus
Mel-Temp melting point apparatus
Filling Melting Point Capillaries Criteria
1. Sample must be dry
2. Sample must be finely powdered
3. Sample must be packed tightly
4. Sample must not be more than 2-3 mm high?
Why does a sample need to be dry for filling the mp capillary
Moisture from solvents like water and other solvents will depress the melting point like an impurity.
Why does a sample need to be finely powdered?
If sample is in chunks, sample will not pack well, causing air pockets that slow heat transfer. Air pockets are not good head conductors
Why must the sample be packed tightly?And how is it accomplished?
To avoid air pockets. By rapping sharply on a hard surface
Why does a sample have to be no more than 2-3 mm high?
If too much in capillary, there will be a differential in temperature throughout sample. Steps 2-4 is to ensure efficient transfer of heat throughout sample. If sample is pure and dry and steps 2-4 are not adhered to then sample will appear to have a wider range and upper melting point of range will eventually match true mp.
Determination of Melting Point
Sample in capillary tube is placed in sample holder of Mel-Temp. Record melting range
Observed Melting range
temperature from when the first drop of liquid is observed (in the sample) to the temperature when sample is completely liquefied.
Real Melting range
from eutectic point to when sample is completely liquefied
What is not considered melting?
shrinking, sagging, color change, texture changes and sweating (sweating is where droplets appear on the outside surface of sample due to inefficient drying)
Why can you not use a sample that you had already melted to run another melting point determination?
The sample may decompose, so second mp will be lower than the first
What kind of molecules decompose before melting?
Change of color is an indicator of decomposing. Ionic organic molecules and compounds having many strong hydrogen bonds i.e. sucrose
What if you find that the sample disappears as you heat it?
Sample has sublimed and the remedy would be to use sealed capillaries
Why doesn’t a change in atmospheric pressure change mp by much?
The solid/liquid transition does not involve a significant change in molal volume
Why must determination be slow?
To give enough time for heat to be transferred equally from heating block to sample to the thermometer. If you go too fast, temperature at sample is very different than temperature at thermometer. And sample appears to have a wide range.
Noting that the heat reaches the sample before the thermometer, when you heat a sample too quickly, do you expect the initial observed mp temp to be higher or lower than expected temperature?
When you take mp of unknowns, in the procedure it is suggested that you make
two samples of the pure unknown because 1st; fast determination to ascertain approx mp 2nd: to mix with standard to identify unknown
What do use to explain the behavior of impure compounds
melting point composition diagram
A change in what characteristic results in a lower melting point?
mixture has a lower vapor pressure than the vapor pressure of the pure compound
Melting vs. Dissolving
Melting is disrupting/breaking intermolecular forces within molecule
Dissolving is disrupting attractive forces of two molecules and getting them to interact with each other.
the lowest temperature at which a mixture at a certain concentration will start melting
Lowest point on melting point composition diagram where a mixture has a very narrow melting range
Point at which the two solids and their liquid solutions are in equilibrium
Why would a eutectic point or temperature be difficult to detect?
Impurities could cause the mixture to begin melting at a temperature lower than the eutectic point.
Because it represents the point at which an infinitesimal amount of solid mixture has begun to melt
What criteria must an impurity satisfy in order for it to depress a melting point?
Must be soluble in compound. Can be solid or liquid. Disrupt the crystal lattice of the pure substance, making the substance weaker
Insoluble impurities are poor heat conductors and act just like air pockets. Effect on mp and melting range?
Thus takes less energy to break the bonds (lowers mp). Also melts over a wider range of temps
What are similarities and differences between a eutectic point and mp of a pure compound?
narrow range and lower than pure mp
What if Compound A (mp 120 degrees) and Compound B (mp 120 degrees) are mixed together?
If A and B are diff compounds, then mixture of A + B will have a lower mp than 120 degrees
Mixed melting point? How is it carried out?
a way to confirm the identity of an unknown. Mix unknown with a small quantity of known compound with similar mp. If mp stays the same then the unknown is the known compound. If mp goes down then the unknown is not the known compound.
What mistake made during the preparation of a melting point sample may cause a differential of temperature throughout the sample?
Using too much sample in capillary tube
How can air pockets in a melting point sample be avoided?
Using fully powdered sample
If you can rule out moisture in the air as a factor, what two other reasons would there be for melting point samples of pure compounds not agreeing with literature values if sample fulfilled all the necessary criteria for filling the melting point capillary AND the determination of the melting point was carried out correctly?
Contamination and the thermometer of the Mel-temp not being calibrated correctly
Anthracene (look up structure) has a much higher melting point than naphthalene (look up structure). What reason would be the main reason for the higher melting point?
Larger molecular weight (or more van der waals interactions)-because the bonds are all the same
How would the presence of an insoluble impurity affect the melting point of a compound?
It would not depress the melting point
What criteria must an impurity satisfy in order for it to depress a melting point?
It has to be soluble within the compound
How is a mixed melting point experiment carried out? (Note: you just have to describe the procedure briefly. You don’t have to give possible results)
The unknown’s melting range is found. Then it is mixed with the possible standard it could be (has to have a melting range close to it) and then that melting range is found.
Why would air pockets in a melting point sample cause inefficient heat transfer throughout the sample?
It would be difficult for the sample to be packed properly
Why do you need to heat a sample slowly to get an accurate reading for a melting point?
To give time for heat from the apparatus or mel-temp to be transferred equally to the thermometer and sample
Why would coarse particles in a melting point sample cause an inaccurate melting point to be observed?
Air pockets would cause a poor heat transfer
What is the remedy if too much solvent is used in the recrystallization process and no material crystallizes?
Keep it at the boiling point longer so that excess solvent may evaporate to point of saturation
In the hints of preparation, you were instructed to make sure that the temp of the mel-temp apparatus was 80 C or lower before inserting a sample of pure urea (mp 132.5 C-133C) into the slot in the mel-temp. Why was it important that the mel-temp temp be so much lower that the sample’s melting point to get an accurate melting point?
To allow enough time to approach the m.p. slowly
Among structural isomers, the more symmetrical will have the higher melting point.
You have carried out an experiment that reliably produced a known characteristic compound x. which of the 3 reasons for a melting point determination would best describe why you would take the melting point of compound x?
To check the purity of the compound
How will the observed m.p. compare to the true melting point if the m.p. apparatus is heated too rapidly?
The observed m.p. will be lower than the true m.p. if it is heated to rapidly the melting range will be narrow
What is the purpose of taking the m.p. of a new compound?
To characterize (or provide physical characteristic) of the new compound
What is the specific purpose of rapping a mp capillary after it is filled?
To pack the sample tightly
The melting point of a pure compound is where the vapor pressure of the solid and liquid are equal
Why is it necessary that the rate of heating be no greater than 1 C a minutes when determining the mp of a sample?
It is necessary for the heat to be transferred equally to a sample, than a glass and a thermometer
Why does the sample shrink before melting?
* Ionic molecules often decompose before melting as do compounds having strong hydrogen bonds.
* Due to crystal structure changes
Examples of impurities that don’t depress melting point.
Sand & charcoal are insoluble impurities & will not depress melting point, but widen MR
What forces affect melting point? (Also, consider the effect on the melting point of larger molecules, symmetrical molecules and R, S isomers, and hydrogen bonding molecules)
a) more symmetrical structural isomer has higher melting point
b) Enantiomers have same melting point, but racemate & diastereomers have
c) different melting point
d) The larger a hydrocarbon, the ↑ the London/Van der Waals forces, the ↑ the melting point
e) The ↑ the dipole-dipole forces (II), the ↑ the melting point
f) The more hydrogen bonds (III), the higher the MP
g) Ionic substances have high melting points due to strong ionic attractive forces
h) (IV) between positive & negative ions
Sources of error when taking a melting point
• Moisture in air absorbed by sample
• Contamination of sample
• Thermometer in Mel-Temp not calibrated (only instance mp could be higher than expected)
• Determination made too fast
• Criteria for filling melting point capillaries not adhered to