Why are melting points carried out?
Three reasons
1. if compound is a known one, to characterize a sample
2. if compound is an unknown one, to allow future characterization by others
3. range of the melting point is indicative of the purity of the compound (impure compounds melt over a wide
What is the criteria for impurity to depress melting point? What would one observe when taking a
melting point of an impure sample?
Insoluble impurities are poor heat conductors and are just like air pockets
-must be soluble in the compound
-can be solid or liquid
-disrupt the crystal lattice of the pure substance, making the substance weaker. Thus, takes less energy to break the bond
Examples of impurities that don't depress melting point and their effect on melting point
sand and charcoal, because they are insoluble impurities they have no effect on the melting point
The impact of rate on taking melting point - how to take melting point accurately
The temperature increase should not be greater than 1 degree C per minute. Melting points will not be accurate if ther ate of heating is too fast as you are likely to MISS the end MP range.
Mixed melting points experiment - procedure, meaning of results
Mixed melting point experiments can be used to confirm the identity of the unknown. If compounds X and Y are identical, then they will have the same melting point, and not depress one or the other.
How sample is prepared? Why sample should be prepared in a specific manner?
The sample must be finely crushed into a powder and packted tightly within the capillary. Air pockets slow heat transfer as they are not good conductors of heat. Sample will appear to have large melting point.
Criteria for sample before packing and how it should be packed and why it should be packed that
way. - expected observations if criteria not adhered to compared to ideally packed samplec
Sample must be dry- Because moistrue from solvents such as water and other solvents will depress MP like an impurity would.
Sample must be finely powdered- Because if a sample is in chunks the sample will not pack well and cause air pockets that slow heat
Impact on experiment and observations if criteria not adhered to
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Impact on melting point and range if criteria not adhered to
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How should sample be heated and why? What may happen if you heat the sample too quickly?
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Details on when to slow down heating and why\
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What happens if you have too much sample
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What happens if sample is coarse (not powdered)
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Sources of error when taking a melting point\
Moisture in the air- a wet sample will result in a lower MP and larger range
Sample is contaminated- perhaps the wrong spatula was used to scoop the sample- result in an impure sample, lowering MP
Thermometer in melt-temp not calibrated- samples may be co
Expected observations when sources of error are in play
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