- to characterize a known compound (if you had several to choose from mp must be different enough)
-record physical characteristic for a new compound
-to establish purity of a known compound (impure products will melt over a wider range)
Why are melting points carried out?
the temperature at which the vapor pressures of the solid and liquid are equal (melting point range w/in 1C)
Definition of melting point of a pure compound
- energy is used to overcome crystal lattice energy to melt a solid
- impurities disrupt crystal lattice so requires less energy to break intermollecular forces
What is the criteria for impurity to depress melting point?
- R and S enantiomers and diastereomers do not suppress melting point
- insoluble impurities
Examples of impurities that don't suppress melting point
- London attractive forces - weakest of all. the larger, the higher the mp
- dipole dipole attractive forces - stronger than previous. increases due to dipole of electronegative atoms. stronger the dipole, bond, and attraction, higher the mp.
- hydrogen b
What forces affect melting point?
the point at which two compounds and their liquid solutions are in equilibrium and lowest mp on the composition graph where a mixture has a very narrow melting range.
Define eutectic point
is the lowest melting point of a mixture of two substances and is very hard to detect and match and gives the mixture of two substances a very narrow mp range.
What are features of a eutectic point?
sample must be crushed and dry. if not, wet sample will act as impurity which depresses the mp and not finely crushed will not pack well and lead to air pockets
How is melting point sample prepared? Why sample should be prepared in a specific manner?
must be dry and finely crushed. Should be packed tightly to prevent air pockets
Criteria for melting point sample before packing and how it should be packed
must be heated slowly. Too fast and temp on thermometer will not be accurate because heat hasnt hit it yet or given it time to register. This will make the melting point of the sample seem very wide and you will miss the exact mp.
How should the melting point sample be heated and why? What happens if you heat the sample too quickly?
to evaluate the approximate melting point of the unknown by itself so that you can compare it to the mp of the mixture
Reasons for doing melting point determination of an unknown species twice
from the eutectic point to when sample is completely liquified
Definition of melting range
because the crystalline structure changes as mp is approached
Why does the melting point sample shrink before melting?
the temperature will be differential throughout the sample
What happens if you have too much melting point sample?
it will cause air pockets to form which will cause slow heat transfer
What happens if melting point sample is coarse (not powdered)?
the sample may decompose, so the second mp will be lower than the first
What might happen if you take the melting point of a sample that already melted and why?
atmospheric pressure doesn't really change mp because the solid/liquid transition doesn't involve a significant molal volume
Effect of changing of atmospheric pressure on melting point and why
- not crushing/packing well
- heating too fast
- product not pure
- product not dry
- pack too much product in capillary tube
Sources of error when taking melting point
melting point range will be very big if there are sources of error
Expected observations of and impact on melting point result when sources of error are in play and when precautions are not adhered to
The student didn't calibrate the MelTemp or crush the sample enough. Maybe heated the sample too quickly
A student measures the melting point of benzil and reports the melting point in the lab notebook as 108�C. Explain what the student did wrong.
vibration and rotation
What are the 2 possible characteristic movements molecules make before melting as heat is added?
a) There are other solids with the same mp
b) mixing unknown with salicylic acid. if the unknown is salicylic acid, the mp should stay the same
You are given a sample of an unknown solid - you have not been given a set of known compounds that this solid might be. You take a melting point of the solid and find that it melts at 158-159�C. You automatically assume that the solid is salicylic acid (m
I would eliminate benzoic acid because:
- too far away from the range
- if unknown isnt pure, the mp would be higher, not lower
You have an unknown sample that has a melting range of 130-132�C. The three possibilities for this unknown are benzoic acid (mp: 122-123�C), benzoin (135-137oC), and trans-cinnamic acid (mp: 133-134�C)
Which of the three standards can you automatically el