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Chem 2242 Final
Terms in this set (93)
Melting Point range
the difference between the temperature when the sample begins to melt and the temperature when the sample is completely melted.
Two step determination of melting point
1. determine rough melting point by quickly heating a sample
2. Careful determination by gradually heating a sample at the rate of 1-2 degrees C per second
What are the advantages to the two step melting point
what two effects do impurities have on melting point of an organic compound
lower melting point
broader range of MP
Why is it important to pack a sample tightly in a tube
firm packing ensures efficient heat transfer
why is it important to heat the sample slowly and steadily as the MP temperature is approached
slow heating allows you to find a more accurate melting point range
What is the effect of using too large a sample
larger amounts will cause slower heating, and the MP range will be inaccurately large
What purpose does the second substance serve during codetermination of MP
if the MP range of the known substance is incorrect, it would indicate and error in the apparatus
For what two purposes is MP routinely used
1. identifying a compound
2. checking the purity of a compound
Three test tubes contain white crystalline organic solids A, B, and C, each of which melts at 149-150 degrees C. A 50-50 mixture of A and B melts at 130-139 degrees C. A 50-50 mixture of A and C melts at 149-150 degrees C. In what range would a 50-50 mixture of B and C probably melt? What can you say about the identities of A, B, and C?
A 50-50 mixture of B and C would probably melt in the range of 130-139 degrees C. A and C are likely the same compound because they maintain their melting point ranges when combined. When mixed with B, they have a lower melting point range because it is an impurity.
How can MP be used to identify substances
if two substances are mixed and they have the same melting point, they are the same.
To accurately determine MP, a substance must be,
dry, crushed into a fine powder and firmly packed into a capillary tube
Effect of impurities on water
impurities lower the melting point of water
impure compounds melt over a __________ temperature range
pure compounds melt over a _________ temperature range
Why is it necessary to use only a minimum amount of the required solvent for recrystallization
using the minimum amount minimizes the amount of material lost by retention in the solvent
why is it necessary to carry out the recrystallization at or near the boiling point of the solvent used?
The solute has increased solubility at higher temperatures, so less is needed.
How are insoluble impurities removed during crystallization
The compound ismixed with a solvent, the solution is filtered to remove the insoluble impurities and the solvent is evaporated to leave a pure compound.
What purpose does the addition of finely added charcoal serve during the recrystallization of impure compounds?
Charcoal is a selectie absorbent, which removed traces of color or residual impurities
what is the purpose of recrystallization
to isolate pure compounds and to filter out impurities
how are impurities removed during recrystallization
soluble impurities will dissolve in a solvent, leaving behind crystals of a pure compound.
How does scratching the inside of the vessel with a glass rod induce crystallization
scratching releases microscopic pieces of glass that acts as a seed to begin crystallization
tendency to vaporize
what are two reasons why the vacuum filtration is sometimes preferable to gravity filtrtion
vaccum filtration is faster and provides a dryer product in a less amount of time
why is water a good solvent for the recrystallization of acetanilide?
acetandilide readily dissolves in hot water, but is insoluble at low temps. It dissolves in hot water but crystallized easily when cool
Why must a funnel be heated before filtration of a heated substance during recrystallization
a cool funnel could cause the solution to crystallize prematurely inside the funnel
what properties are desirable in a recrystallization solvent
low boiling point
high solubility when hot
low solubility when cold
why can we use a water/ethanol mixture but not a water/hexane mixture as a recrystallization solvent
water and ethanol are miscible, water and hydrocarbon are not
2. filter out insoluble compounds, add water
3. gravity filtration
4. chill, crystal formation
5. evaopration and separation
6. dry crystals
7. use melting point to check purity
What are four ways to induce crystallization
agitation, scratching the inside of the flask, addition of a seed, flash freezing
how does polarity affect crystallization
compounds of middle polarity tend to recrystallize by highly polar or non polar solvents. polar compounds can't be crystallized by non polar solvents, non polar and polar solvents can be mixed to adjust polarity
solids get _________ liquids get __________
3 types of impurities
soluble, insoluble, partly soluble
why do we have to use a cold solvent to rinse the crystals in recrystallization
so the crystals don't dissolve
the conversion of a compound from the liquid phase to the gas phase, followed by condensation of gas, leading to the isolation of a purer liquid compound.
What happens to the MP of water if you add antifreeze
it goes down
what happens to the BP of water if you add antifreeze
it goes up
a packed column contains
steel wool or mesh
column with wavy insides
which column has the greater hold up? why?
packed column. Larger surface area
which column would be used to separate a mixture of Pentane (BP 36) and Octane (BP 39)
Which column would be used to separate a mixture of methyl alcohol (BP 65) and water (BP 100)
water outlet is at the ____
water inlet is at the _______
Why is cold water used i the column?
to condense the vapor
What are the benefits of carrying out distillation under reduced pressure
reduction in pressure causes a reduction in boiling point, meaning faster boiling. This is useful for distilling liquids with high boiling points
Why is it necessary to add boiling stones during distillation
Boiling stones prevent bumping due to super heating
the pressure created by vapor molecules striking the walls of the container they occupy
temperaturea t which the vapor pressure of the liquid becomes equal to the atomospheric pressure
why does the vapor pressure of a liquid increase with temperature
as temperature increases, molecules escape from the vapor, until equilibrium is reestablished.
Why is fractional distillation more effective at separating the mixture into its components
the iron wool in the column causes the liquid to reconvenes, thus preventing less volatile liquid from evaporating
why does a pure liquid have a constant boiling point, but a liquid with a constant boiling point is not necessarily pure?
in some mixtures the vapor has the same portion f constituents as the unboiled mixture. Azeotropes are mixtures of compounts with the same boiling point, thus, a fixed boiling point.
What effect does a reduction of atmospheric pressure have on the boiling point of a liquid
when pressure is reduced, boiling point is also reduced, BP can differ by location depending on pressure
why doesnt a pure liquid in a distilling flask vaporize all at once when the boiling temperature is reached?
additional heat must be supplied for a phase change to occur
why is it dangerous to heat a liquid in a distiilling apparatus that is closed tightly at every joint and has no vent to the atmosphere
the increase in heat and pressure inside the apparatus will increase and possibly explode the distillation equipment.
what is the purpose of distillation
to purify liquids, separation is based on boiling point
is preformed when trace impurities need to be removed from a relatively pure compound, or when a mixture of compounds with significantly different boiling points needs to be separated.
s used when the difference in boiling points of the components in the liquid mixture is not large. the column is packed with an appropriate material which results in the liquid mixture being subjected to many vaporization/condensation cycles as it moves up the column. Composition of the vapor gradually becomes enriched in the lower boiling component. Near the top of the column, nearly pure vapor reaches the condenser and condenses back to liquid.
How do impurities affect boiling point
impurities raise boiling point
liquids that dissolve freely in one another in any proportion
what is the best type of solvent to use in extraction
one that doesn't react with the solute
the higher density liquid is located on the _________ of the extraction flask
the lower density liquid is located on the _________ of the extraction flask
drying agent, removing water from organic layer
What safety precautions should one take when using chlorinated solvents for extractions?
The lab should be well ventilated, there should be no flames, and gloves should be worn.
What criteria must be considered when selecting a solvent for extraction?
The solvent must dissolve the substance being extracted and be sparingly soluble in the substance that is being extracted from.
What differences in the chemical properties of carboxylic acid and phenol make extraction with aqueous sodium bicarbonate and an organic solvent possible?
Phenol remains in the aqueous layer and the salt formed with carboxylic acid dissolves in the aqueous layer.
What are the advantages of using dichloromethane (higher density) over diethyl ether (lower density) for extracting organic compounds from an aqueous solution using a separatory funnel?
Because dichloromethane is denser than water, it can be drained off of the aqueous layer allowing a second layer of solvent to be used in the next extraction. This reduces the product lost in the transfer between containers.
K = C2/C1, where C1 and C2 are the concentrations at equilibrium of solute A in solvent 1 and in solvent 2, respectively.
Why are multiple extractions more efficient?
Larger volume determined by the partition coefficient-determines what percentage of the material is going to diffuse into the organic layer vs. the aqueous layer.
Why must the stopper of the separatory funnel be removed before liquid can be withdrawn through the stopcock?
The stopper must be removed so that the air pressure will help push the liquid out of the funnel. If not, the flow of liquid is obstructed.
What are the advantages of having a solvent be denser than water when extracting an organic compound from an aqueous solution in a separatory funnel?
Two distinct layers form, allowing for easy separation.
What are the advantages and disadvantages of using ether as an extraction solvent?
1. It is a great solvent. It can dissolve most of the organic compounds.
2. It has low boiling point. It can be distilled over easily
3. It is chemically inert.
1. Highly flammable
most common form of initial purification from extraction
lowest MP for a mixture
In IR spec. the lighter the atom the ____________ the frequency
in IR spec. the stronger the bond the _______ the frequency
Strong IR signal depends on
how polar the bond is
What is the purpose of missing sulfuric acid and nitric acid?
to make NO2
what is the role of sulfuric acid in the electrophilic aromatic substitution lab
protonates HNO3 to make NO2
in IR spec. what happens to C=O if there is resonance
there is lower frequency if resonance
1st step of reductive amination
2nd step of reductive amination
3red step of reductive amination
role of NaBH4 in reduction step
role of CH3CH2OH in reduction step
solvent that protonates hydrogen
How many moles of Hydride ions (H:-) can 1 mole of NaBH4 provide?
Ser 195 forms a covalent bond with
5) Which of these compounds is the hemiacetal that forms when ethanol reacts with propanal?
If aspirin is dissolved in ethyl acetate and 10mL of 2M HCl is added, which layer will be the organic phase?
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