Melting point Flashcards
what is the purpose of identifying a melting point
it helps us identify the compound and its purity
can melting points tell us if something is absolutely pure?
no, it is possible that we could be observing a eutectic mixture which appears as a sharp range
what do we observe in a phase change
when we reach the initial part of the melting point, all excess energy goes into melting the compound and so the temperature does not change until the phase change is complete
what determines the melting point?
the intermolecular forces interacting with the compounds
if you have a compound with strong IMFs what melting point do we expect?
we expect a high melting point and it is likley that the compound remains a solid at room temp
if you have a compound with intermediate IMFs, what state is most common
it is likely to be a liquid
if a compound has very weak IMFs, what state is usually observed
gaseous
why is the phrase melting “point” a misonomer?
it is not a point and is measured as a range
what is a sharp melting point
a melting point of 2°C or less, this could indicate purity
why do we not retake melting points?
the compound might decompose or change its crystalline structure after reformation, this may change the melting point
what are the 4 types of abnormal melting behavior
decomposition, polymorphs, hydrates, and sublimation
what is decomposition
it is when the solid undergoes some chemical change and becomes dark without liquefying, a sharp range is typically observed
what is a polymorph
compounds may have different crystalline lattice structures that have different intermolecular forces, this can change the melting point
what is a hydrate
a hydrate form of a compound will have a different melting point than a anhydrous
what is sublimation
sublimation is the process when a sample undergoes a phase change from a solid to a gas
what is the effect of impurity on a melting point?
the melting point will be depressed and broadened if impure
what is a eutectic point on a mixed compound
it is a precise mixture that melts at a sharp range that is depressed from the individual melting points of the components
if you have a sharp range, what might that mean about your substance?
it could be pure or it could be eutectic mixture
How do we identify the compound in a mixed solution?
- Take a sample of an authentic and known compound, mix the unknown and the known together. If the melting point remains the same, you have the same compound
- If the melting point depresses (becomes lower) you have two different compounds
If the melting point of a sample has a range of 132.5-133.0 °C, can you conclusively state that it is a pure sample? Explain.
No, it is possible that the compound could be a Eutectic mixture, which has a sharp melting point and could appear to be oure
Why does benzophenone have a lower melting point than benzilic acid
A lower melting point means that it takes less intermolecular forces to break the bonds of benzophenone, therefore the bonds holding benzophenone together are weaker than benzilic acid.
Benzilic acid has O-H groups that allow for hydrogen bonds which are a tip of dipole dipole bond. This can create stronger interactions between other benzophenone molecules, therefore increasing the melting point.
Describe the physical process that occurs when a pure compound melts
Adding heat to a substance weakens the intermolecular forces, which disrupts the crystal lattice and causes a phase change to liquid
What errors could result if you attempt to remelt a sample that has been heated once and recooled
The composition of the substance may have changed into a polymorph which has a different melting point .
Water in the compound may have been evaporated to form an anhydrous solid which has a different melting point than the hydrated form
During heating and cooling, the substance may absorb impurities which affect the melting point
What errors may result if you load more than 1-2 mm of sample in your capillary tube? Explain.
Varying amounts of sample will cause higher temperature ranges than expected
It will take longer to heat a larger sample
Also too much sample could prevent direct contact with the entire heating block of the melting point apparatus
Two students perform melting points of a given pure substance. The first student obtains a melting range from 141-142 °C, while the second student records a melting point of 144-147 °C. Provide two plausible reasons to explain this discrepancy.
- It is likely that the students are assuming “first drop of liquid” differently.
- Adjusting the power settings on the melting apparatus could cause the sample to be heated differently between the two samples. Heating too fast or not allowing it to cool sufficiently could cause an improper, wider range to be reported.
If you find a solid sample in lab that you suspect is benzilic acid, what can you do to confirm the identity
You could do a mixed melting point, if the melting point is low and broad, you have a different compound than benzilic acid
at the melting point, the solid form of a substance is _______ with the liquid phase of the solid
in equilibrium with
what is a wide melting point range indicative of
an impure sample
what is a wide melting point range indicative of
an impure sample
how you prepare a capillary tube for a melting point
tap the open end of the tube into the substance under 1-2mm of powder in the tip, then flip it over and tap the closed end of the tube until it falls to the bottom
why should you use finely ground solid when taking a melting point
air pockets could disrupt heat distribution
uniform small particles heat more consistently through the sample
what are the characteristics of a good solid to take the melting point of
thoroughly dry, solid phase, small particles
what is the order of measurements to report a melting point
a quick measurement for range
a close measurement for accurate range
a slow heating to confirm range
what is eutectic
composition
it is the mixture composition with the lowest melting point
