ETSU Organic Chemistry Lab Midterm: Pre-labs and Post-labs

define the eutectic point

the temperature at which the melted compound becomes saturated with impurities; the lowest temperature at which a mixture of two compounds will melt

list three applications that utilize the melting point measurements

1. identifying an unknown compound

2. confirming the identity of a compound

3. evaluating the purity of a solid sample

4. calibrating a melting point device

explain why it is important to calibrate the thermometer and the melting point apparatus

it will help to evade systematic errors like deviations in thermometer readings

explain why a small molecule like sodium chloride melts at 801 degrees C while a much larger molecule like glucose melts at 146 degrees C

sodium chloride is inorganic and has strong ionic bonds, while glucose is organic and covalent in nature; it will take more energy and therefore a higher temperature to disrupt the ionic bonds of sodium chloride

how can you tell if a compound is pure or not from its melting point

a pure compound will typically melt in a range 0.5-1.0 degrees C, while a substance with impurities will have a lowered melting point from its literature melting point as well as a broadened range >2 degrees C

describe and explain how the melting point of the mixture of cinnamic acid and urea differs from that of the pure compounds

name three intermolecular forces in benzoic acid

what is the effect of each of the following on the melting point of cinnamic acid

which of the following is expected to have a higher melting point

a) anthracene or naphthalene

b) benzoic acid or acetanilide

explain the reason

explain "oiling out" phenomenon during recrystallization. when does it happen?

"oiling out" is when the solute precipitates as oil instead of solid crystals when the solution is cooled. it can occur when the recrystallization solvent has a higher boiling point that the melting point of the solute

what is petroleum ether

(C6H14) is a good recrystallization solvent which consists of aliphatic hydrocarbons; it has a boiling point range of 60 degrees C to 95 degreesC; it is lightweight and volatile

is there any danger of using two boiling chips instead of one? explain your answer

one boiling chip is sufficient, so there is no real need for two; there is no immediate danger of using two instead of one

why is it sometimes necessary to filter the hot solution during recrystallization

this will remove any solid, insoluble, impurities; it will also remove charcoal if it is used for decolorization

is boiling the recrystallization solvent for a very long time necessary? does that affect the outcome of the recrystallization? explain

no, it is not necessary; boiling only needs to run for as long as it takes the solid material to dissolve. boiling for a very long time could lead to super-heating, evaporation of the solvent or "oiling out" of the solute

what is the purpose of using charcoal during recrystallization

what is the effect of using an excess of charcoal during recrystallization

why is it important to use boiling chips when boiling solutions

doing so will help the solution to heat evenly throughout and prevent superheating of the liquid

define boiling point

the temperature at which the vapor pressure of a liquid becomes equal to one atmospheric pressure; liquid to vapor transformation

explain why the sidearm of the condenser should be placed deep in the receiving vial during distillation

this will ensure that all matter coming into and out of the condenser is accounted for, not lost, so that results may be true and as accurate as possible

explain why your experimental boiling points (measure at an elevation of 1500 ft) of cyclohexane and toluene are lower that their boiling points reported in literature

this difference can be explained by the fact that at higher elevations there is lower pressure, so a lower temperature is required to create a sufficient vapor pressure match

what is the single most important variable that contributes to an effective separation of liquids by distillation

distillation rate; distillation rate should be kept constantly slow and not faster than 3 drops per minute

explain why, during distillation, the first few drops of distillate are occasionally cloudy

the first few drops are azeotropes; they are a mixture of liquids which boils at a temperature outside the boiling point range of individual liquids; they can act as impurities and water may be present