Melting point

Question 1

what is the purpose of identifying a melting point

Answer 1

it helps us identify the compound and its purity

Question 2

can melting points tell us if something is absolutely pure?

Answer 2

no, it is possible that we could be observing a eutectic mixture which appears as a sharp range

Question 3

what do we observe in a phase change

Answer 3

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

Question 4

what determines the melting point?

Answer 4

the intermolecular forces interacting with the compounds

Question 5

if you have a compound with strong IMFs what melting point do we expect?

Answer 5

we expect a high melting point and it is likley that the compound remains a solid at room temp

Question 6

if you have a compound with intermediate IMFs, what state is most common

Answer 6

it is likely to be a liquid

Question 7

if a compound has very weak IMFs, what state is usually observed

Answer 7

gaseous

Question 8

why is the phrase melting “point” a misonomer?

Answer 8

it is not a point and is measured as a range

Question 9

what is a sharp melting point

Answer 9

a melting point of 2°C or less, this could indicate purity

Question 10

why do we not retake melting points?

Answer 10

the compound might decompose or change its crystalline structure after reformation, this may change the melting point

Question 11

what are the 4 types of abnormal melting behavior

Answer 11

decomposition, polymorphs, hydrates, and sublimation

Question 12

what is decomposition

Answer 12

it is when the solid undergoes some chemical change and becomes dark without liquefying, a sharp range is typically observed

Question 13

what is a polymorph

Answer 13

compounds may have different crystalline lattice structures that have different intermolecular forces, this can change the melting point

Question 14

what is a hydrate

Answer 14

a hydrate form of a compound will have a different melting point than a anhydrous

Question 15

what is sublimation

Answer 15

sublimation is the process when a sample undergoes a phase change from a solid to a gas

Question 16

what is the effect of impurity on a melting point?

Answer 16

the melting point will be depressed and broadened if impure

Question 17

what is a eutectic point on a mixed compound

Answer 17

it is a precise mixture that melts at a sharp range that is depressed from the individual melting points of the components

Question 18

if you have a sharp range, what might that mean about your substance?

Answer 18

it could be pure or it could be eutectic mixture

Question 19

How do we identify the compound in a mixed solution?

Answer 19

• 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

Question 21

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.

Answer 21

No, it is possible that the compound could be a Eutectic mixture, which has a sharp melting point and could appear to be oure

Question 22

Why does benzophenone have a lower melting point than benzilic acid

Answer 22

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.

Question 23

Describe the physical process that occurs when a pure compound melts

Answer 23

Adding heat to a substance weakens the intermolecular forces, which disrupts the crystal lattice and causes a phase change to liquid

Question 24

What errors could result if you attempt to remelt a sample that has been heated once and recooled

Answer 24

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

Question 25

What errors may result if you load more than 1-2 mm of sample in your capillary tube? Explain.

Answer 25

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

Question 26

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.

Answer 26

1. It is likely that the students are assuming “first drop of liquid” differently.
2. 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.

Question 27

If you find a solid sample in lab that you suspect is benzilic acid, what can you do to confirm the identity

Answer 27

You could do a mixed melting point, if the melting point is low and broad, you have a different compound than benzilic acid

Question 28

at the melting point, the solid form of a substance is _______ with the liquid phase of the solid

Answer 28

in equilibrium with

Question 29

what is a wide melting point range indicative of

Answer 29

an impure sample

Question 30

what is a wide melting point range indicative of

Answer 30

an impure sample

Question 31

how you prepare a capillary tube for a melting point

Answer 31

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

Question 32

why should you use finely ground solid when taking a melting point

Answer 32

air pockets could disrupt heat distribution

uniform small particles heat more consistently through the sample

Question 33

what are the characteristics of a good solid to take the melting point of

Answer 33

thoroughly dry, solid phase, small particles

Question 34

what is the order of measurements to report a melting point

Answer 34

a quick measurement for range

a close measurement for accurate range

a slow heating to confirm range

Question 35

what is eutectic
composition

Answer 35

it is the mixture composition with the lowest melting point