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
