Briefly explain or indentify a) two useful functions served by knowing the melti

Question

Briefly explain or indentify

a) two useful functions served by knowing the melting point of an organic compounds

b) why a finely powered sample should be used in a melting point measurement

c) why is it so important to heat a sample slowly to obtain an accurate melting point

d0 two reasons why is it sometimes difficult to measure the tempeature at which the crystals first begin to liquefy

e) what two effects a soluble impurity usually has on the melting point of a compound

F) what occured when crystals began to disappear from the bottom of the capillary tube rather than turning to a liquid

Explanation / Answer

a) The two useful functions would be that it certifies the purity of a compound and helps to identify and confirm identity as well, chemical and physical properties.

b) if sample is finely powdered, it makes the melting point reading more accurate because more surface area is exposed and heat is distributed evenly.

c) So the observer can correctly recognize the exact point at which it melts and its melting point range. Boiling it may toss the substance around before it melts as well.

d) The impurity is too small. Heating the sample too rapidly, stick to 1-2 degrees because the range at which crystals first transfer is a small range and can be missed if not closely paid attention to.

e) An impurity causes the melting point to deviate from the standard. Actually, it makes the magnitude of the melting point lower. For example, the melting point of benzoic acid is 122.4 degree Celcius. When we did this in the laboratory, we got 120 degree Celcius because of the impurity present which is sodium sulfate.

An impurity also causes the melting point to become a melting range. The melting point is not anymore sharp, but a range of values. The more impurities present, the greater the range of values. For example, when we purified crude acetanilide via recrystallization, we obtained a melting range of 105-116 degree Celcius. By the way, the standard melting point of acetanilide is 113-115 degree Celcius.

So basically, those are the two effects that impurities have on melting point iof a pre compound. I learned that it is because the impurities "defect" the crystal lattice of the compound. A pure compound is a structure of high molecular symmetry. An impurity is rather assymetrical, or is a poorly organized structure. If the impurites are present with the pure compound, the melting point will deviate because melting point is also dependent on the high symmetry and organization of molecular structures, just as the vapor pressure. This is the principle involved in Sublimation and Melting point determination.

f) Sublimation.

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