question

melting point for a pure substance

answer

sharp range: 1-2°C

question

melting point for an impure substance

answer

broad range: 5-15°C

question

what happens to melting points for mixed substances?

answer

it's lowered and broaden because the foreign substance disrupts the crystal lattice of the other one, which lowers the energy required to melt the solid

question

eutectic point

answer

the limit to how far the melting point can be depressed for a mixed substance

question

melting point apparatus

answer

MEL-TEMP

question

how much substance should be put in a capillary tube when taking a melting point?

answer

1-2 mm

question

determining the melting point of an unknown using mixed melting points

answer

perform a fast and slow melting point in order to pinpoint the range and compare it to the known melting point range of one of the components. if the melting point range doesn't get lowered or broadened, then can identify it based on the unknown.

question

distillation

answer

purification technique based on the boiling points of liquids. a liquid is vaporized to a gas, which condenses back into a liquid called the distillate.

question

distillate

answer

the liquid that forms during distillation after the original liquid is vaporized and then condensed, which is collected and measured

question

how does the composition of a liquid mixture compare to the composition of its vapor?

answer

the vapor pressure will be richer in the substance with the lower b.p. than the one with the higher b.p., but still contains a significant amount of the higher b.p. substance. this makes complete separation of a liquid by simple distillation impossible

question

when is it possible to get a fair result from simple distillation?

answer

when the substances' b.p. differ by more than 110°C

question

fractional distillation

answer

uses multiple vaporization/condensation cycles to purify a liquid. -insert an air condenser column packed with stainless steel sponge (many vaporization/condensation cycles occur going up the column) -spike in graph signifies the substance with the lower b.p. is removed from the vapor -obtains a better result than simple distillation

question

how many samples are usually collected for distillation?

answer

10-15 in order to make a smooth graph of temperature vs. cumulative volume

question

refractive index (RI)

answer

temperature and wavelength dependent, which quantifies the fact that light travels at different velocities in air and water.

question

refractometer

answer

used to measure the refractive index, R(f). light is sent through a prism that's in contact with the sample, which influences how much the light is refracted. a sensor detects the refraction and obtains an R(f) value to help identify a substance based on known R(f)

question

Height of a Column Equivalent to one Theoretical Plate (HETP)

answer

reports the column efficiency in terms of the number of vaporization/condensation cycles for fractional distillation (cm/plate) -low HETP means a more efficient column

question

what is one Theoretical Plate?

answer

1 vaporization/condensation cycle (longer columns have more TP's)

question

calculating % of a substance present from RI data

answer

obtain RI values through normalized sodium D line. (your fraction RI - known RI of other substance) ÷ (known RI of substance - known RI of other substance)

question

definition of boiling

answer

the vapor pressure equals the ambient pressure (atm)

question

Raoult's Law

answer

applies to miscible solutions that do not interact P(tot)=X(a)P(a) +X(b)P(b)

question

vigreux column

answer

modification of the air condensing column with downward pointing indentations to increase the surface area without increasing the column length

question

azeotropes

answer

mixtures with constant b.p. that can't be separated. behavior is attributed to slight attractions and repulsions between molecules. vapor pressure is higher or lower than predicted by Raoult's Law -minimum b.p. diagram: shows repulsion -maximum b.p. diagram: shows attraction

question

molecular modeling (computational chemistry)

answer

calculates and models 3D properties of molecules by derivations: 1. quantum mechanics 2. molecular modeling

question

molecular mechanics

answer

uses empirical force field models (mathematical functions that describe the potential energy of a system) and is based on Newtonian mechanics. 1. inexpensive 2. scalable 3. can model large systems

question

molecular mechanics equation

answer

E(strain)=E(stretch)+E(angle)+E(torsion)+E(oop)+E(VdW)+E(dipole) -E(stretch), E(angle), E(torsion), E(VdW) affect it the most -want to minimize E(strain)

question

E(strain)

answer

energy calculated from mechanical force fields

question

E(angle)

answer

energy calculated from expanding or compressing bond angles away from the most stable

question

E(torsion)

answer

energy calculated from the staggered derivations throughout ring flipping

question

E(VdW)

answer

energy calculated from electron repulsions that are due to the overlap of nonbonding atoms in close proximity to each other

question

major limitation of molecular mechanics

answer

a local minimum might be found instead of a global minimum which can be seen in rotational energy (ROT-E) projections

question

rotational energy (ROT-E) projections

answer

represent the rotations of a molecule graphically (energy vs. angle) -global minimum: lowest energy conformation -local minimum: depends on where the starting structure corresponds to the graph

question

why was molecular mechanics developed

answer

in order to find the lowest energy conformation of a given molecule or to compare the energies of several conformations of the same molecule -doesn't calc thermodynamic energy, electron distribution, dipole moments, or molecular orbitals

question

quantum mechanics

answer

calculates and models properties of molecules using complex derivations (e.g. Schrödinger's equation) 1. expensive 2. not scalable 3. breaks down for larger molecules

question

what is a force field?

answer

an empirical mathematical function used to describe the potential energy of a system of molecular particles

question

molecular mechanics computer program

answer

PCModel

question

crystallization

answer

dissolving a solute into a hot solvent (must be miscible; s→l→c) and then observing the solution as it cools to see if it separates back out of solution -wasteful and never meets the theoretical yield -successful only when minimal amounts of impurities

question

solute

answer

a dissolved substance

question

solvent

answer

the dissolving medium

question

mother liquor

answer

the resulting solution from filtration in recrystallization which contains the solvent and any impurities

question

an ideal solute for recrystallization

answer

low solubility at room temperature and high solubility at the solvent's b.p. -makes the solubility curve steep -follow \"like dissolves like\"

question

step in picking a solute for recrystallization

answer

-difference in solubility in cold and hot solvent -easily removed by drying -not toxic or flammable (safety) -not reactive with the solvent -liquid range near room temperature

question

how to remove an impurity in recrystallization that's soluble at all temperatures

answer

don't have to worry because it goes into the mother liquor after filtration

question

how to remove an impurity in recrystallization that's insoluble at all temperatures

answer

can be removed with hot filtration using fluted filter paper, which traps the impurities while allowing the pure solute to pass through. then, vacuum filtration is performed to dry out the remaining crystals.

question

how to remove an impurity in recrystallization that has similar solubility as the solvent

answer

the ease of removing it depends on the amount

question

how to remove an impurity in recrystallization that's colored

answer

can be removed using charcoal (Norit)

question

why is recrystallization employed?

answer

doing two or more crystallizations to improve the purity of the obtained crystals or when there is a lot of impurity relative to the solvent

question

what happens if you cool the solution down too quickly during recrystallization?

answer

the impurities might get trapped in solution because it will become supersaturated

question

semi-microscale recrystallization

answer

main method used for crystallization. an Erlenmeyer flask is used to dissolve material and a vacuum filtration is used to filter crystals. It is used when the weight of the solid is more than 0.1 g

question

microscale recrystallization

answer

used when the amount of product is less than 0.1 g. a craig tube is used instead of an Erlenmeyer flask and a centrifugation filters the solution which minimizes the number of transfers. this results in a greater yield of crystals.

question

what would happen if a hot saturated solution were filtered by vacuum filtration using a Hirsch funnel?

answer

some of the crystals would precipitate out and crystallize which would cause the funnel to get clogged. the rest of the solution would not be able to get filtered

question

what to do if no crystals form

answer

1. induce crystallization by scratching the glass (provides a spot for them to form) or using seed crystals 2. the material might have separated out as a liquid if there b.p. of the solvent > the m.p. of the solute.

question

how to fix oiling out during recrystallization

answer

use a solvent with a lower b.p. or add more solvent

question

liquid-liquid extraction

answer

transferring a solute from one solvent into another -done with a separatory funnel -recrystallization can remove small amounts of impurities

question

ideal extraction system for liquid-liquid extractions

answer

substances should: 1. be unreactive to each other 2. be virtually immiscible 3. differ in density so gravity can separate them 4. be non-flammable 5. differ in polarity distinctly so they dissolve in a series of dissimilar substances 6. be non-toxic 7. one should be volatile so it can be easily removed by boiling

question

what can be done to help purify an organic substance during liquid-liquid extraction?

answer

a water wash which uses water to extract water soluble and low molecular weight impurities to help purify the substance

question

partition coefficient (K)

answer

the solute distributes itself between the two solvents in liquid-liquid extraction and K is the ratio of the concentration of the solute in each of the solvents K=C₂/C₁

question

what does a large K mean?

answer

that multiple extractions don't need to be done, but it's always better to use several small partitions of the second solvent than to perform one large one

question

what happens if the substances are mixed too vigorously during liquid-liquid extraction?

answer

CO₂ pressure increases sharply and when venting, some of the liquid will shoot out of the separatory funnel

question

solid phase extraction (SPE)

answer

used to perform separatory reactions in which new compounds are prepared.

question

SPE column

answer

constructed from the body of a plastic syringe that is packed with a sorbent. a frit is inserted below and above the sorbent to support it in the column

question

sorbent

answer

materials that can both absorb (attract to its surface by physical attraction) or absorb (penetrate into the material like a sponge)

question

frit

answer

a substance used to make porcelain or glazes (used in SPE columns)

question

C₁₈ column

answer

SPE column used in lab. the sorbent has a 18 C chain branching off of an silica molecule to make it non polar. -stationary phase: C₁₈ reversed-phased silica -mobile phase: brewed tea

question

reverse phase chromatography

answer

the polar substances come off first and the relatively non polar substances will be retained (i.e. caffeine) in an SPE column -condition the column with polar substances -pass caffeine through -flush the caffeine from the column with EtOAc

question

sublimation

answer

purifies the leftover solid caffeine following reverse phase chromatography -caffeine is warmed enough until the vapor pressure is high enough for vaporization and then condensation occurs on a cooled surface.

question

why is the sublimation step in the SPE lab hooked up to a vacuum?

answer

because many solids do not develop enough vapor pressure at sea level to be purified. it also prevents thermal decomposition of the caffeine.

question

sublimation vs. crystallization purification

answer

sublimation is a faster technique, but it's not as selective, which is why crystallization is more widely used.

question

different ways to decaffeinate coffee

answer

1. extract with CH₂Cl₂: CH₂Cl₂ is somewhat toxic 2. use supercritical CO₂: high temp and pressure 3. solid phase extraction (SPE): green chemistry

question

thin layer chromatography (TLC)

answer

the separation of nonvolatile compounds of a mixture by differential elution on an absorbent -stationary phase: TLC plate (polar silica and glass) -mobile phase: solvent

question

eluent

answer

the chamber containing the solvent with the filter paper in TLC

question

what does running or developing the TLC plate mean?

answer

when the solvent ascends the TLC plate by capillary action, which partitions the sample between the mobile and stationary phase

question

rate of elution R(f)

answer

helps determine the relative polarities of the solutes (should always be below 1) R(f) = solute (spot) distance / solvent distance

question

do polar substances travel more or less than non polar substances on a TLC plate?

answer

less because they are attracted to the polar silica. non polar substances have less affinity for the plate and dissolve into the solvent more readily.

question

different ways to visualize a developed TLC plate

answer

1. UV indicator 2. iodine or vanillin 4. special sprays (free amines)

question

dielectric constant

answer

measures how well solvents separate charges 1. methanol (32.7) 2. acetone (21) 3. ethyl acetate (6.0) 4. hexane (1.9)

question

how to choose a mixed solvent for TLC?

answer

pick ones that distinctly separate one solute from the others. then choose which applies to the unknown. -also fluorenone was yellow so picked it based on that too

question

what can TLC show?

answer

if all the reactants are used up and how much longer to run the reaction -can't give % yield

question

why should the filter paper be shorter than the spot plate in the eluent?

answer

so the solvent only ascends the plate (i.e. doesn't come from both sides)

question

one method to prepare alkenes

answer

dehydrating 2° or 3° alcohols (1° alcohols are much harder)

question

alcohol dehydration

answer

an acid-catalyzed reaction by strong, concentrated acids (e.g. phosphoric or sulfuric acid). the acid protonates the alcohol to form an alkene. -alcohol is the limiting reagent since the acid is a catalyst

question

how to isolate a high laboratory yield of alkenes?

answer

distilling the product of alcohol dehydration, which shifts the equilibrium to the right

question

what reactions can determine the presence of an alkene?

answer

alkenes react with: 1. bromine solution (red) to decolorize it 2. potassium permanganate (purple) to produce a managense oxide (brown)

question

Zaitzev's Rule

answer

the more highly substituted alkene is formed in a reaction

question

what is the main limitation of alcohol dehydration?

answer

the carbocation intermediates can rearrange which results in a complex mixture of many isomers. -utility limited to degenerate rearrangements (the most stable products without rearrangements)

question

gas chromatography's role in alcohol dehydration

answer

determines the percent of each product formed (first peak is an air peak)

question

gas chromatography (GC)

answer

analyzes mixtures of organic compounds. the apparatus is called a gas chromatograph. as the sample passes through the column, it separates into its components and a computer measures the presence of each. -stationary phase: organic dehydration solution -mobile phase: He gas -TCD allows for theoretical recovery and FID destroys the sample

question

why did we use a GC column of 5% phenyl substitution of methyl?

answer

because it lowers the b.p. to make the mobile phase travel faster, which means there is less retention time and more accurate results.

question

TCD vs. FID in GC

answer

TCD analyzes in terms of conductivity difference between the carrier gas and He, while FID destroys the sample during analysis

question

steam distillation

answer

(discovered around 1000 AD) the codistillation of plant essential oils with water. doesn't follows Raoult's Law because between immiscible liquids in a heterogeneous mixture). b.p. will always be below the b.p. of water. two methods: -direct steam method -live steam method

question

what does the composition of the vapor pressure in steam distillation depend on?

answer

the vapor pressure of each components because all gases mix even if the liquid is immiscible. the mole fraction is not needed because the components are not soluble -P(tot)=P(a) + P(b)

question

why are the varying b.p. of water and limonene significant during steam distillation?

answer

because limonene will have a lower vapor pressure at 100°C

question

direct steam distillation method

answer

generates steam by heating a distillation flask containing the compound and water (method we used)

question

why do heterogenous mixtures boil at lower temperatures than predicted by Raoult's Law?

answer

because they behave like azeotropes (constant boiling mixtures that cannot be altered by simple or fractional distillation). this incompatibility leads to a higher than expected vapor pressure.

question

live steam distillation method

answer

generates steam outside and passes it through the distillation flask using an inlet tube

question

TLC solutes relative polarities

answer

in order of increasing: 1. benzoic acid 2. 9-flourenone 3. trans-stilbene

question

TLC solvents relative polarities

answer

in order of increasing polarity: 1. acetone 2. ethyl acetate 3. methanol 4. hexane

question

binary system phase diagram

answer

below of curve is solid and above is liquid. both solid and liquid present inside the curve

question

fractional distillation hold-up

answer

the liquid that cannot be distilled because it condensed on the surface of the column

question

when and why do you use a saturated NaCl (brine) solution?

answer

used when there's an emulsion to decrease the solubility of the organic layer in the aqueous layer, which destroys the emulsion (i.e. separates). -increases the surface tension of the droplets -increases the density of the aqueous layer (diethyl ether solvents)

question

common drying agents

answer

CaCl₂ (no go for hydroxyl, amino and carbonyl), Na₂SO₄ (wet solutions), CaSO₄ (no go for wet solutions), and MgSO₄ (works well with diethyl ether)

question

drying agents

answer

dry isolated organic compounds because usually they are contaminated with traces of water. in aqueous extractions, the water is partially miscible with the organic layer.

question

emulsion

answer

stable dispersion of one liquid in a second immiscible liquid (b, which delays the separation (problem) of the two liquids. -caused by shaking (e.g. vinaigrette)

question

crystallization vs. precipitation

answer

crystallization: -crystals are produced -takes more time and is harder due to ordered crystal structure precipitation: -amorphous solids are produced

question

recrystallization second crop and third crop

answer

further improves the recovery of material by harvesting another round using the mother liquor (won't be as pure as the original crop because the mother liquor's impurities is alway increasing)

question

recrystallization solubility gradient

answer

the change in solubility with the change in temperature -water and alcohols show the greatest gradients

question

why is fluted filter paper used in gravity filtration?

answer

to increase the surface area of the paper to make the filtration more efficient

question

why are many small extractions better than one large one?

answer

because you are increasing the surface area between the organic and aqueous layer to separate them more thoroughly

question

three factors affecting crystallization

answer

1. concentration of the solute 2. what solvent is used 3. temperature of the solvent

question

gas chromatograph

answer

has a flow controller after the He gas chamber. then the sample is injected into the column and a computer detects the separation of substances. wastes exits

question

polarimeter

answer

measures the extent to which a substance interacts with polarized light. optically active substances rotate the plane of polarization to some angle α.

question

plane polarized light

answer

light that passes through a prism

question

specific rotation

answer

[α] = α / c⋅l -c: g/mL -l: dm (1 dm=10cm)

question

polarimetry

answer

analyzes the purity of the substance produced (e.g. limonene)

Organic Chemistry 1 Lab Final

Unlock all answers in this set

Haven't found what you were looking for?

Search for samples, answers to your questions and flashcards