Melting Point

aspirin has a melting point of...

135°

[theory of melting point] pure compounds melt within a ______ degree range; for this reason, melting point should more accurately be called .......

1-3; melting point range

Another student reports a melting range of 136°-138°C for the compound in Question 1 and mentions in her notebook that the rate of heating was about 12° per minute. NMR analysis of this student's product does not reveal any impurities. Explain the low melting point.

12 degrees per minute is so fast that the thermometer readings lag behind the actual temperature of the heating block .

A compound melts at 120°-122°C on one apparatus and at 128°-129°C on another. Unfortunately, neither apparatus is calibrated. How might you check the identity of your sample without calibrating either apparatus?

A mixed melting point experiment could be used to determine whether the two compounds have the same identity. Alternatively, the melting point of both compounds could be measured on one apparatus to learn if they are likely to have the same identitywithout calibrating either melting-point apparatus.

A student performs two melting-point determinations on a crystalline product. In one determination, the capillary tube contains a sample about 1-2 mm in height and the melting range is found to be 141°- 142°C. In the other determination, the sample height is 4-5 mm and the melting range is found to be 141°-145°C. Explain the broader melting-point range observed for the second sample. The reported melting point for the compound is 143°C.

A wide melting point range usually indicates that a substance is impure. So, when we are increasing the sample height, the amount of impurities will keep increasing which lends to a broader melting range. Now, if you further increase the width, the range will further increase, say from 140-147.

[melting point range behavior] T or F: sweating, shrinkage, softening, discoloration, decomposition are all observable melting behaviors.

F (these are non-melting behaviors)

Why does sealing the open end of a melting-point capillary tube allow you to measure the melting point of a compound that sublimes?

It traps the gas from sublimination.

[melting point range behavior] T or F: different people may obtain slightly different results (₊/₋ 1-2 degrees) - it is not your goal to match the literature m.pt.!

T

A white crystalline compound melts at 111-112 degrees C and the melting-point capillary is set aside to cool. Repeating the melting-point analysis with the same capillary reveals a much higher melting point of 140 degrees C. Yet repeated crystallization of the original sample yields sharp melting points no higher than 114 degrees C. Explain the behavior of the sample that was cooled and the remelted.

You must always use a fresh sample, because reusing samples and cause errors since some products don't resolidify.

"In order to melt an ionic compound, it is necessary to break ionic bonds. Therefore, ionic compounds usually have [high/low] melting points. To melt a covalent compound, it isn't necessary to break bonds. It is only necessary to overcome the much weaker intermolecular forces that hold the particles together." (please see: https://www.alexandria.k12.mn.us/cms/lib01/MN01000334/Centricity/Domain/202/Covalent_and_Ionic_Properties%20lab.docx)

high

what are the commonly referred to weak inter-molecular forces?

hydrogen-bond dipoles

melting [is/isn't] a reversible phase change from solid to liquid.

is

when a compound contains an impurity: melting point will be __________ and _________ than for the pure compound.

lower; broader

this is used to confirm the identity of a compound

mixture (or mixed) melting point technique

given the knowledge that aspirin has a mpt. of 135° and urea has a mpt. of 133°, would it be safe to assume that your compound, which has a melting point of 133-134°C, is one or the other?

no! you have to use the mixture (or mixed) melting point technique.... this is done by taking your unknown compound and mixing it at a 1:1 ratio with each of the suspected known compounds (which you know have relatively close melting points), and seeing whether the melting point range drops and broadens within each trial. in other words, take a 1:1 mix of aspirin to unknown and measure, take a 1:1 mix of urea to unknown and measure, and then compare results. the one with the little to no change in melting point range is a match for identification.

[theory of melting point] melting point is a _________ property used for identification.

physical

[theory of melting point] melting point is an indication of the ___________ of a solid.

purity

[melting point range behavior] melting ends when...

the entire mass turns into a clear liquid

[melting point range behavior] melting begins when....

the first drop of liquid appears