Chem Lab Exam

Hydrogen bonding

strongest IMF, sharing hydrogen by strong dipole-dipole interactions, OH and NH bond

dipole-dipole

weaker than hydrogen-bonding, only involves electropositive atoms connected to electronegative atoms, attraction of negative pole on one molecule to the positive dipole on the other molecule

London dispersion

Weakest attractive force, short-term dipole-dipole, temporary dipoles due to asymmetrical distribution of electron density, no permanent dipoles

greater surface area

increases number of London dispersion force interactions, linear molecules stack better

relationship between bp and imf

more imf —> higher boiling point; more energy needed to break imfs

branched compounds have fewer IMFs so

lower bp

miscible

full mix in all proportions

immiscible

never fully mix in any proportions

molecules that share intermolecular force interactions (similar functional groups) will likely be 

miscible

hydrophobic

non-polar molecules, water-fearing, immiscible with polar molecules, repulsion with water leads to separation by layer

hydrophilic

polar molecules, water-loving, immiscible with non-polar molecules (long-chain hydrocarbons)

bond length/motions influenced by

bond order (single, double, triple)

size (mass) of atoms bound

hybridization/geometry (sp3, sp2, sp)

electronegativity

bond strength (IR)

the stronger the bond, the higher the frequency, or energy of absorption band

atom size (IR)

larger atoms lead to absorption bands with lower energy (lower frequency)

bond order (IR)

shorter bonds are stronger bonds (higher frequency)

vapor pressure, Pvap

pressure of evaporating liquid pushing against pressure of the atmosphere

boiling point, BP

temperature at which Pvap = Patm

volatile liquid

high Pvap at a given temperature, less heat input is needed to boil, low BP

non-volatile liquid

low Pvap at given temperature, more heat input needed to boil, high BP

strong intermolecular forces (H bonds, dipole) lead to

higher boiling points (low vapor pressure, non-volatile)

weak intermolecular forces (london dispersion forces, no dipole) lead to

low boiling point (high vapor pressure, volatile)

boiling point changes

with atmospheric pressure

distillation

separation of liquids with different boiling points, two (or more) liquids with significantly different boiling points can be separated

conformations

different arrangements of atoms that are interconverted by rotation about single bonds

staggered conformations

more stable (lower in energy) 

torsional strain

increase in energy caused by eclipsing interactions

steric strain

increase in energy when atoms are forced too close to one another, in addition to torsional strain

achiral

molecule or object that is superimposable on its mirror image

chiral

not superimposable

ion-dipole interactions

allow previously insoluble compounds to dissolve in water

density differences

enable liquid-liquid extractions

melting

process of a solid undergoes a phase change to a liquid, weakning IMFs

melting point 

temperature at which melting and freezing processes are at equilibrium

melting point of pure substance

narrow range (1-2 degrees C)

impurities

disrupt IMFs of solid and lower/broaden melting point

recystallization

purification technique for solid compounds 

volatile liquids have a high vapor pressure (boil at low temperatures)

true

boiling point of a liquid is the temperature at which the vapor pressure is equal to the atmospheric pressure

true

strong intermolecular forces (such as hydrogen bonds, dipole-dipole interactions) lead to low boiling points

false

the boiling point of water (temperature at which water boils) is the same at all altitudes 

false

london dispersion forces are an example of weak intermolecular forces

true

miscibility describes the ability of two solids to dissolve in one another

false

density describes the mass per unit volume of a substance

true

water is more dense than diethyl ether and will therefore sink to the bottom during a liquid-liquid extraction

true

water is more dense than dichloromethane and will therefore sink to the bottom during a liquid-liquid extraction

false

melting point can help inform you if your compound contains impurities

true

impurities cause the melting point of a solid to

lower

the lowest melting point possible for a theoretical mixture describes the _____ for that mixture.

eutectic point

eutectic point

composition at which the melting point is depressed to it’s lowest temperature

a good recrystallization solvent is one at which the material that is being recrystallized completely dissolves at low temperatures

false

impurities raise the melting temperature in a crystalline solid by disrupting intermolecular forces (h bonding, dipole-dipole, london dispersion forces etc)

false

ethanol is miscible with hexane because they share which intermolecular forces

london dispersion forces

hexane only has LDF as the IMF, so by default this is the only one they can share. Ethanol also can do H-bonding and dipole-dipole but hexane cannot.

a compound with a low vapor pressure has a _____ boiling point?

high

which of the following can make liquids boil more easily?

increase temperature in flask, decrease atmospheric pressure

pentane has only LDF as its primary IMF. Would you expect it to have a low or high boiling point because of this?

low, weaker IMF = lower boiling point

high energy vibrational bands in an IR appear on the _____ side of an IR spectrum

left

miscibility describes the ability of a solid to dissolve in a liquid

false, liquid dissolving in liquid

an IR peak resulting form a highly polarized bond should be _____

strong, large dipole moment increases IR signal intensity

during distillation, a thermometer is placed in the _____ to measure boiling point

distillation head

good recrystallization solvent displays _____ solubility toward the solid at low temperatures

low, needs to not dissolve at low (like room temp or lower) temp but does dissolve when hot

impurities is compounds lead to a(n) _____ in melting point

decrease, disrupt packing of a pure crystalline lattice