Melting Point Lab

one of the characteristic physical properties of a solid

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)

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.

Intermolecular forces (ionic attractions, VdW forces, dipole-dipole interactions, and hydrogen bonds)

How molecules stack (shape of molecule)

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.

Mel-Temp melting point apparatus

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?

Moisture from solvents like water and other solvents will depress the melting point like an impurity.

If sample is in chunks, sample will not pack well, causing air pockets that slow heat transfer. Air pockets are not good head conductors

To avoid air pockets. By rapping sharply on a hard surface

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.

Sample in capillary tube is placed in sample holder of Mel-Temp. Record melting range

temperature from when the first drop of liquid is observed (in the sample) to the temperature when sample is completely liquefied.

from eutectic point to when sample is completely liquefied

shrinking, sagging, color change, texture changes and sweating (sweating is where droplets appear on the outside surface of sample due to inefficient drying)

The sample may decompose, so second mp will be lower than the first

Change of color is an indicator of decomposing. Ionic organic molecules and compounds having many strong hydrogen bonds i.e. sucrose

Sample has sublimed and the remedy would be to use sealed capillaries

The solid/liquid transition does not involve a significant change in molal volume

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.

Lower

two samples of the pure unknown because 1st; fast determination to ascertain approx mp 2nd: to mix with standard to identify unknown

melting point composition diagram

mixture has a lower vapor pressure than the vapor pressure of the pure compound

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

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

Must be soluble in compound. Can be solid or liquid. Disrupt the crystal lattice of the pure substance, making the substance weaker

Thus takes less energy to break the bonds (lowers mp). Also melts over a wider range of temps

narrow range and lower than pure mp

If A and B are diff compounds, then mixture of A + B will have a lower mp than 120 degrees

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.

Using too much sample in capillary tube

Using fully powdered sample

Contamination and the thermometer of the Mel-temp not being calibrated correctly

Larger molecular weight (or more van der waals interactions)-because the bonds are all the same

It would not depress the melting point

It has to be soluble within the compound

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.

It would be difficult for the sample to be packed properly

To give time for heat from the apparatus or mel-temp to be transferred equally to the thermometer and sample

Air pockets would cause a poor heat transfer

Keep it at the boiling point longer so that excess solvent may evaporate to point of saturation

To allow enough time to approach the m.p. slowly

To check the purity of the compound

The observed m.p. will be lower than the true m.p. if it is heated to rapidly the melting range will be narrow

To characterize (or provide physical characteristic) of the new compound

To pack the sample tightly

It is necessary for the heat to be transferred equally to a sample, than a glass and a thermometer

* Ionic molecules often decompose before melting as do compounds having strong hydrogen bonds.
* Due to crystal structure changes

Sand & charcoal are insoluble impurities & will not depress melting point, but widen MR

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

• 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