UCSB CHEM 6AL FINAL EXAM WITH COMPLETE SOLUTIONS!!
LATEST UPDATE ALREADY GRADED A+
Intermolecular forces and how they affect physical properties /purification techniques: dipole-dipole,
electrostatic, hydrogen, ion, london, - (ANSWER)*dipole-dipole*: polar covalent molecules such as
aldehydes and ketones. positive and negative end. polar molecules must be close together for force to
be significant. weaker than ion-dipole forces; increase with polarity of molecule.
*electrostatic*: occur between charged species, cations and anions, and are responsible for very high
MP and BP of ionic compounds and metals
*hydrogen bonding*: hydrogen atom in a polar bond (NOF). considered a dipole-dipole interaction. quite
polar.
*ion-dipole*: interaction betwween a charged ion and apolar molecule. cations are attracted to negative
end of dipole, and anions are attracted to positive end.
*london dispersion*: dependent on surface area and polarizability of the surface of the molecule. only
types of forces NON-polar covalent molecules can experience. result from the movement of electrons in
the molecule, which generates temporary positive and negative regions in the molecule.
melting point and melting point range - (ANSWER)solid has ordered crystal lattice structure. heat makes
this structure move enough to the point where it becomes a liquid. The melting point is how much
energy it takes to make this lattice structre move. Impurities disrupt the crystal lattice, so impurities
make a structure easier to melt. MP also broadens when a compound is impure. If there is water in the
solid, it is impure. solid will stick to side of capillary if still wet.
Best way to determine MP range if you dont know the MP range:
take a quick measurement to get a rough estimate, then take a second, more careful measurement.
limiting reagents and percent yields - (ANSWER)*limiting reagents*: the compound that runs out first.
when limiting reagent runs out, reaction can no longer proceed and other compound is considered in
excess. tell which is limiting reagent by converting grams or mg used to moles using molar mass. when
you have moles, use the stoichimetric ratios to see which is the smaller amount of moles (mole of
reagent under mole produced of product); this is your limiting reagent. could also compare mole ratios
between reagents to see how much you would need of the other reagent to make reaction go to
completion. use limiting reagent to see how much product we will get.
*percent yield*: actual yield/theoretical yield X 100. If you expect 10 g but only get 8.4, your % yield is
84%. get theoretical yield by taking limiting reagent and seeing how much product can be made by LR.
convert g to mol of LR, then convert mol of LR to mol of product; use molar ratio for this. then convert
the mol of product by multiplying molar mass of product.
,UCSB CHEM 6AL FINAL EXAM WITH COMPLETE SOLUTIONS!!
LATEST UPDATE ALREADY GRADED A+
distillation: ideal liquids/Raoults law - (ANSWER)*ideal liquid*: a liquid that in incompressible (density is
constant), irrotational (no turbulence, flow is smooth), and nonviscous (fulid has no internal friction).
they do not exist in nature. Does not change composition by mixing. it obeys Raoult's law for all
compisitions.
*Rauoults Law*: shown in picture. total pressure can be found by adding up partial pressure; partial
pressure can be found with Raoult's law.
to find mol fraction (Xa), divide mols of A over total moles of solution.
assumption of raults law:
1) the liquid is ideal
the contribution of each liquid phase can be found by determining the mole fraction of each in the vapor
phase:
(gama)A = PA/Ptotal
Example: what is the partial pressure of EtOH (Po=45 kPa, MW= 46.07 g/mol) if 3.0 g of it is mixe with
3.0 g of unknown (MW= 32.04)? - (ANSWER)18.5
a compound with high IMF will have a ____ BP than a compound with low IMF; Imagine a solution of
water boiling in a round flask. the vapors above this liquid are ______ the liquid themselves. -
(ANSWER)higher; the same temp as
T or F: higher vp means that a liquid is harder to turn into a vapor - (ANSWER)F its easier!
T or F: molecules with lower IMF exert lower vapor pressures - (ANSWER)F they prodcuce higher VPs!
, UCSB CHEM 6AL FINAL EXAM WITH COMPLETE SOLUTIONS!!
LATEST UPDATE ALREADY GRADED A+
T or F: compounds with a high vp have a low bp - (ANSWER)T :)
How does the vapor composition compare to the liquid composition in a 50:50 mix of two liquids? -
(ANSWER)if we have pentane and hexane, vapor would not be 50:50, it would be more pentane because
pentane has lower BP.
temperature composition diagram - (ANSWER)Lower curve = liquid line
Upper curve = vapor line
can determine the composition of both liquid and gas phases at certain temperatures as well as the
boiling points of certain compositions
Ex: at .8 (point B) decane boiling point is ~100 degrees C and the vapor composition (at point C) is .5
decane
which substance in the distillation mixture will make up more of the vapor composition - (ANSWER)the
substance with the weaker intramolecular forces, lower boiling point, and higher vapor pressure
simple vs fractional distillation - (ANSWER)*simple*: used to purify almost already pure compounds, to
separate liquid from solid impurities, and to remove volatile (readily evaporating) solvents. few cycles,
heat source to heat liquid. have boiling stones in liquid. vapors travel up and condense back down into
different tube. works well if there is a large difference in BP (60-70 degrees C). only does one
evaporation/ condenstion cycle). how can we improve this? by using more theoretical plates (each TP
represents one evap/conden cycle). do this by using fractional distillation
*fractional distillation*: fractionating columns increase Theoretical Plates.
Theoretical plates= surface area where vaporization/ condensation can potential occur
allows for greater separation, very important globally.
LATEST UPDATE ALREADY GRADED A+
Intermolecular forces and how they affect physical properties /purification techniques: dipole-dipole,
electrostatic, hydrogen, ion, london, - (ANSWER)*dipole-dipole*: polar covalent molecules such as
aldehydes and ketones. positive and negative end. polar molecules must be close together for force to
be significant. weaker than ion-dipole forces; increase with polarity of molecule.
*electrostatic*: occur between charged species, cations and anions, and are responsible for very high
MP and BP of ionic compounds and metals
*hydrogen bonding*: hydrogen atom in a polar bond (NOF). considered a dipole-dipole interaction. quite
polar.
*ion-dipole*: interaction betwween a charged ion and apolar molecule. cations are attracted to negative
end of dipole, and anions are attracted to positive end.
*london dispersion*: dependent on surface area and polarizability of the surface of the molecule. only
types of forces NON-polar covalent molecules can experience. result from the movement of electrons in
the molecule, which generates temporary positive and negative regions in the molecule.
melting point and melting point range - (ANSWER)solid has ordered crystal lattice structure. heat makes
this structure move enough to the point where it becomes a liquid. The melting point is how much
energy it takes to make this lattice structre move. Impurities disrupt the crystal lattice, so impurities
make a structure easier to melt. MP also broadens when a compound is impure. If there is water in the
solid, it is impure. solid will stick to side of capillary if still wet.
Best way to determine MP range if you dont know the MP range:
take a quick measurement to get a rough estimate, then take a second, more careful measurement.
limiting reagents and percent yields - (ANSWER)*limiting reagents*: the compound that runs out first.
when limiting reagent runs out, reaction can no longer proceed and other compound is considered in
excess. tell which is limiting reagent by converting grams or mg used to moles using molar mass. when
you have moles, use the stoichimetric ratios to see which is the smaller amount of moles (mole of
reagent under mole produced of product); this is your limiting reagent. could also compare mole ratios
between reagents to see how much you would need of the other reagent to make reaction go to
completion. use limiting reagent to see how much product we will get.
*percent yield*: actual yield/theoretical yield X 100. If you expect 10 g but only get 8.4, your % yield is
84%. get theoretical yield by taking limiting reagent and seeing how much product can be made by LR.
convert g to mol of LR, then convert mol of LR to mol of product; use molar ratio for this. then convert
the mol of product by multiplying molar mass of product.
,UCSB CHEM 6AL FINAL EXAM WITH COMPLETE SOLUTIONS!!
LATEST UPDATE ALREADY GRADED A+
distillation: ideal liquids/Raoults law - (ANSWER)*ideal liquid*: a liquid that in incompressible (density is
constant), irrotational (no turbulence, flow is smooth), and nonviscous (fulid has no internal friction).
they do not exist in nature. Does not change composition by mixing. it obeys Raoult's law for all
compisitions.
*Rauoults Law*: shown in picture. total pressure can be found by adding up partial pressure; partial
pressure can be found with Raoult's law.
to find mol fraction (Xa), divide mols of A over total moles of solution.
assumption of raults law:
1) the liquid is ideal
the contribution of each liquid phase can be found by determining the mole fraction of each in the vapor
phase:
(gama)A = PA/Ptotal
Example: what is the partial pressure of EtOH (Po=45 kPa, MW= 46.07 g/mol) if 3.0 g of it is mixe with
3.0 g of unknown (MW= 32.04)? - (ANSWER)18.5
a compound with high IMF will have a ____ BP than a compound with low IMF; Imagine a solution of
water boiling in a round flask. the vapors above this liquid are ______ the liquid themselves. -
(ANSWER)higher; the same temp as
T or F: higher vp means that a liquid is harder to turn into a vapor - (ANSWER)F its easier!
T or F: molecules with lower IMF exert lower vapor pressures - (ANSWER)F they prodcuce higher VPs!
, UCSB CHEM 6AL FINAL EXAM WITH COMPLETE SOLUTIONS!!
LATEST UPDATE ALREADY GRADED A+
T or F: compounds with a high vp have a low bp - (ANSWER)T :)
How does the vapor composition compare to the liquid composition in a 50:50 mix of two liquids? -
(ANSWER)if we have pentane and hexane, vapor would not be 50:50, it would be more pentane because
pentane has lower BP.
temperature composition diagram - (ANSWER)Lower curve = liquid line
Upper curve = vapor line
can determine the composition of both liquid and gas phases at certain temperatures as well as the
boiling points of certain compositions
Ex: at .8 (point B) decane boiling point is ~100 degrees C and the vapor composition (at point C) is .5
decane
which substance in the distillation mixture will make up more of the vapor composition - (ANSWER)the
substance with the weaker intramolecular forces, lower boiling point, and higher vapor pressure
simple vs fractional distillation - (ANSWER)*simple*: used to purify almost already pure compounds, to
separate liquid from solid impurities, and to remove volatile (readily evaporating) solvents. few cycles,
heat source to heat liquid. have boiling stones in liquid. vapors travel up and condense back down into
different tube. works well if there is a large difference in BP (60-70 degrees C). only does one
evaporation/ condenstion cycle). how can we improve this? by using more theoretical plates (each TP
represents one evap/conden cycle). do this by using fractional distillation
*fractional distillation*: fractionating columns increase Theoretical Plates.
Theoretical plates= surface area where vaporization/ condensation can potential occur
allows for greater separation, very important globally.
